Configuring Modem Support and Asynchronous Devices

This chapter describes how to configure the Cisco IOS software for line, terminal, and modem connections. Cisco devices have four types of lines: console, auxiliary, asynchronous, and virtual terminal lines. Different routers have different numbers of these line types. Refer to the hardware or software configuration guide that shipped with your device for exact line numbering schemes.

For a complete description of the commands in this chapter, refer to the "Modem Support and Asynchronous Device Commands" chapter of the Dial Solutions Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

The following sections are provided in this chapter:

General Concepts

Asynchronous ports can be on the outside of your access server (for example, the Cisco AS2511-RJ) or on the inside of your device (for example, the Cisco AS5200). In both cases, external or internal modems connect to these interfaces. The remote clients referenced in this chapter are making analog calls in to the network via asynchronous ports.

To enable clients to dial in, you configure two components of each asynchronous port: lines and interfaces. Asynchronous interfaces correspond to physical terminal (TTY) lines. For example, asynchronous interface 1 corresponds to TTY line 1.

Generally, commands entered in asynchronous interface mode enable you to configure protocol-specific parameters for asynchronous interfaces, whereas commands entered in line configuration mode permit you to configure the physical aspects for the same port. In Figure 35, which shows the show line output on a Cisco 2511, TTY line 1 corresponds with asynchronous interface 1, TTY line 16 corresponds with asynchronous interface 16, and so on.

Figure 35: Show Line Output--Comparing TTY Lines to Asynchronous Interfaces

Asynchronous Modem Lines

Asynchronous line configuration commands configure ports for the following options:

Physical layer options (such as modem configuration)
Security for EXEC mode
ARA protocol configuration (PPP is configured in interface configuration mode)
Autoselect to detect incoming protocols (ARA and PPP)

To enter line configuration mode, first connect to the console port of the access server and enter privileged EXEC mode. Then enter global configuration mode and finally enter line configuration mode for the asynchronous lines that you want to configure. The following example shows the process of entering line configuration mode for lines 1 through 16:

router> enable
router# configure terminal 
router(config)# line 1 16
router(config-line)#

Asynchronous Interfaces

Generally, interfaces enable the Cisco IOS software to use routing functions. Specifically, you configure asynchronous interfaces to support PPP connections. You configure interfaces on an access server for the following functions:

Network protocol support (such as IP, IPX, or AppleTalk)
Encapsulation support (such as PPP)
IP client addressing options (default and/or dynamic)
IPX network addressing options
PPP authentication
ISDN BRI and PRI configuration

Line Numbering Issues

The TTY line numbering scheme used by your access server or router is specific to your product and its hardware configuration. Refer to the product-specific documentation that came with your product for line numbering scheme information.

For example the Cisco AS5200 access server has TTY lines that map directly to integrated modems as shown in Table 5. The TTY lines 1 through 24 directly connect to modems 1/0 through 1/23, which are installed in the first chassis slot in this example. The TTY lines 25 through 48 directly connect to modems 2/0 through 2/23, which are installed in the second chassis slot. For more information about the Cisco AS5200's slot numbering scheme, refer to the Cisco AS5200 Universal Access Server Software Configuration Guide.

**Table 5: TTY Lines Associated to Integrated AS5200 Modems**
TTY Line	Slot/ Modem Number	TTY Line	Slot/ Modem Number
1	1/0	25	2/0
2	1/1	26	2/1
3	1/2	27	2/2
4	1/3	28	2/3
5	1/4	29	2/4
6	1/5	30	2/5
7	1/6	31	2/6
8	1/7	32	2/7
9	1/8	33	2/8
10	1/9	34	2/9
11	1/10	35	2/10
12	1/11	36	2/11
13	1/12	37	2/12
14	1/13	38	2/13
15	1/14	39	2/14
16	1/15	40	2/15
17	1/16	41	2/16
18	1/17	42	2/17
19	1/18	43	2/18
20	1/19	44	2/19
21	1/20	45	2/20
22	1/21	46	2/21
23	1/22	47	2/22
24	1/23	48	2/23

Group Asynchronous Interfaces

To configure multiple asynchronous interfaces at the same time (with the same parameters), you can assign each asynchronous interface to a group and then configure the group. Configurations throughout this guide configure group asynchronous interfaces, rather than configuring each interface separately.

Note After assigning asynchronous interfaces to a group, you cannot configure these interfaces separately. If you want to configure different attributes on different asynchronous interfaces, do not assign them to the group or assign different interfaces to different groups. For example, on a Cisco 2511 access server, you could assign asynchronous interfaces 1 to 8 as part of one group (such as group-async1) and asynchronous interfaces 9 to 16 as part of another group (group-async2).

To configure a group asynchronous interface, specify the group async number (an arbitrary number) and the group range (beginning and ending asynchronous interface number). The following example shows the process of creating and configuring a group asynchronous interface for asynchronous interfaces 1 through 48 on a Cisco AS5200 access server, which is loaded with 48 V.34 Microcom modems:

router(config)# interface group-async 1
router(config-if)# ip unnumbered ethernet 0
router(config-if)# encapsulation ppp
router(config-if)# async mode interactive
router(config-if)# ppp authentication chap pap
router(config-if)# peer default ip address pool default
router(config-if)# group-range 1 48 
Building configuration...
router(config-if)#

Modem Support Configuration Task List

You can perform the tasks in the following sections to set up externally or internally connected modems. The first three tasks are required. The last task is optional.

Attach the Modems

Before configuring lines on a Cisco access server, you must attach your modems to the asynchronous ports. To attach a modem, refer to the user guide or installation guide that accompanied your access server.

Some Cisco access servers (such as the Cisco AS5200 universal access server), have integrated modems and do not require that you attach an external modem. If you have an access server with integrated modems, you can skip this section and proceed to the next section "Configure the Modem Lines."

Configure the Modem Lines

You must configure the lines to which you attach modems and allow dialin access. To configure lines, enter line configuration mode for the specific lines you need to configure. The following example shows lines 1 through 16 being configured on a Cisco 2511 access server (remember, bold screen font indicates what you type):

router> enable
Password: 
router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
router(config)# line 1 X (X = the highest number of lines the router has; 8 or 16)
router(config-line)#

After you enter line configuration mode for the lines to which your modems are attached, configure the lines using the required commands listed in the following task table. These are the most common commands necessary for basic modem communications.

Task	Command
Step 1 Sets line speed to the highest common speed for the modem and the access server port. See Table 6 for a list of modem transmission rates and line speeds you should set on the access server.¹	speed 115200 or speed 57600 or speed 38400
Step 2 Sets RTS/CTS flow control on the line.	flowcontrol hardware
Step 3 Configures the line to drop the connection when the Carrier Detect (CD) signal is lost (cycle DTR to close the connection). The modem dialin command can be used instead of the modem inout command to enhance security because the modem dialin command restricts outgoing connections from the access server to the modem. Use the modem inout command during setup so that you can configure the modem from the access server, and change to the modem dialin command when testing is completed. Use the modem host command to attach or connect to a printer or host device.	modem inout

Task

Command

Step 1 Sets line speed to the highest common speed for the modem and the access server port. See Table 6 for a list of modem transmission rates and line speeds you should set on the access server.¹

speed 115200

speed 57600

speed 38400

Step 2 Sets RTS/CTS flow control on the line.

flowcontrol hardware

Step 3 Configures the line to drop the connection when the Carrier Detect (CD) signal is lost (cycle DTR to close the connection). The modem dialin command can be used instead of the modem inout command to enhance security because the modem dialin command restricts outgoing connections from the access server to the modem. Use the modem inout command during setup so that you can configure the modem from the access server, and change to the modem dialin command when testing is completed. Use the modem host command to attach or connect to a printer or host device.

modem inout

¹ Cisco Systems assumes that your modem supports flow control. Refer to your modem documentation to learn whether your modem supports flow control, and to learn the maximum transmission speed of your modem.

**Table 6: Matching the Access Server Line Speed to Your Modem Speed**
Modem Transmission Rate	Line Speed on the Access Server
9600	38400
14400	57600
28800	115200

Configure the Modems

This section describes the tasks required to configure modems that are externally attached to the access server:

When you configure modems to function with your access server, you must provide initialization strings and other settings on the modem to tell it how to function with the access server.

For modem configuration information about specific access servers that have integrated modems, refer to the section "Checking Other Modem Settings" later in this chapter.

Communicating with the Modem

This section assumes you have already physically attached the modem to the access server. If not, refer to the user guide or installation and configuration guide for your access server for information about attaching modems.

Before you can configure the modem, you must establish communication with it, which requires terminal access to the modem's command environment. The process of manually configuring a modem consists of the following tasks:

Establishing a Direct Telnet Session to the Modem

You communicate with the modem by establishing a direct Telnet session from the access server's asynchronous line, which is connected to the modem.

Note This process is also referred to as reverse Telnet. Performing a reverse Telnet means that you are initiating a Telnet session out the asynchronous line, instead of accepting a connection into the line (called a forward connection).

This section explains how to establish a direct Telnet session with an external modem (existing outside the router) that is connected through an asynchronous port. Internal or integrated modems, such as used with the Cisco AS5200, are connected to through a direct connect session issued from the Cisco IOS software, which is described later in this section.

To establish a direct Telnet session to an external modem, determine the IP address of your LAN (Ethernet) interface, then enter a Telnet command to port 2000 + n on the access server, where n is the line number to which the modem is connected. For example, to connect to the modem attached to line 1, enter the following command from an EXEC session on the access server:

router# telnet 172.16.1.10 2001
Trying 172.16.1.10, 2001 ... Open

This example enables you to communicate with the modem on line 1 using the AT (attention) command set defined by the modem vendor.

Time Saver Use the ip host configuration command to simplify direct Telnet sessions with modems. The ip host command maps an IP address of a port to a device name. For example, the modem1 2001 172.16.1.10 command enables you to enter modem1 to initiate a connection with the modem, instead of repeatedly entering telnet 172.16.1.10 2001 each time you want to communicate with the modem.

If you are unable to connect to the modem, check the following:

Issue the show users EXEC command. It should not indicate the line is in use.
Verify that the line is configured for modem inout.
Issue the show line EXEC command. The output should contain the following two lines:

Modem state: Idle         
Modem hardware state: CTS noDSR  DTR RTS

Check to see if the virtual terminal connections to lines in the access server require passwords.
Check to see if the speed between the modem and the access server are the same. They are likely to be different. If they are different, switch off the modem, then switch it back on. This should match the speed of the modem with the speed of the access server.

Testing the Modem Connection

After you make a direct Telnet connection to the modem, you need to test the connection. Send the modem the AT command to request its attention. It should respond with OK. For example:

at 
OK

If the modem does not reply to the AT command, check the following:

Look at the output of the show line 1 command. If it displays "no CTS" for the modem hardware state, the modem is not connected, powered on, and waiting for data, or the modem might not be configured for hardware flow control.
Check your cabling (for modems connected externally only) and the modem configuration (echo or result codes might be off). Enter the appropriate AT modem command to view the modem configuration, or enter the command at&f to return to factory defaults. Refer to your modem documentation to learn the appropriate AT command to view your modem configuration.

Suspending and Terminating Telnet Sessions

If you are connected to an external modem, the direct Telnet session must be terminated before the line can accept incoming calls. If you do not terminate the session, it will be indicated in the output of the show users command when it returns a modem state of ready if the line is still in use. If the line is no longer in use, the output of the show line value command will return a state of idle. For routers that support integrated modems (for example, the Cisco AS5200), calls will not be interrupted unless you enter an AT command that requires extra processing power from the modem's CPU, such as the ATH command.

Terminating the Telnet session requires first suspending it, then disconnecting it. To suspend a Telnet session, enter the escape sequence Ctrl-Shift-6 x (press Control-Shift-6, let go, then press x). Enter the disconnect EXEC command to terminate the Telnet session.

Note Ensure that you can reliably issue the escape sequence to suspend a Telnet session. Some terminal emulation packages have difficulty sending the Ctrl-Shift-6 x sequence. Refer to your terminal emulation documentation for more information about escape sequences.

To suspend and then disconnect a Telnet session, perform the following steps:

Step 1 Enter Ctrl-Shift-6 x to suspend the Telnet session:

- suspend keystroke -

router#

Step 2 Enter the where EXEC command to check for open sessions:

router# where

Conn Host Address Byte Idle Conn Name

* 1 172.16.1.10 172.16.1.10 0 0 172.16.1.10

2 172.16.1.11 172.16.1.11 0 12 modem2

Step 3 After suspending a session with one modem, you can connect to another modem (then suspend it):

router# telnet modem2

Trying modem2 (172.16.1.11, 2002) ... Open

- suspend keystroke -

router#

Step 4 To disconnect (completely close) a session, issue the disconnect EXEC command:

router# disconnect line 1

Closing connection to 172.16.1.10 [confirm] y

router# disconnect line 2

Closing connection to 172.16.1.11 [confirm] y

router#

Note Before attempting to allow inbound connections, make sure you close all open connections to the modems attached to the access server. If you have a modem port in use, the modem will not accept a call properly.

After you have established and tested the connection to the modem, you can proceed with the next section "Automatically Configuring Your External Modem."

Automatically Configuring Your External Modem

The Cisco IOS software can issue initialization strings automatically for most types of modems externally attached to the access server. A modem initialization string is a series of parameter settings that are sent to your modem to configure it to interact with the access server in a specified way. The Cisco IOS software defines seven initialization strings that have been found to properly initialize most modems so that the modems function properly with Cisco access servers. These initialization strings have the following names:

Codex_3260
Usr_courier
Usr_sportster
Hayes_optima
Global_village
Viva
Telebit_t3000

Note Internal or integrated modems, such as used by the Cisco AS5200, are preconfigured by Cisco Systems and do not need to be initialized.

If you do not know which of these modem strings is appropriate for your modems, issue the modem autoconfigure discovery line configuration command, as shown in the following example:

router# configure terminal 
router(config)# line 1 16
router(config-line)# modem autoconfigure discovery 
router(config-line)# Ctrl-Z
router# copy running-config startup-config

The Cisco IOS software first tries the first of these strings to see if the modem initializes properly. If not, the Cisco IOS software cycles to the next string and repeats the process until the appropriate string is found. If none of the strings properly initializes the modem, you must manually configure the modem (refer to "Manually Configuring Your External Modem" later in this chapter).

If you know that your modem can be configured using an initialization string from one of these scripts, you can issue the modem autoconfigure type type command, where type is one of the strings in the preceding list. If you list a specific modem type, initialization proceeds more quickly.

To display the list of modems for which the router has modem string entries, issue the show modemcap command. You can change a modem value that was returned from the show modemcap EXEC command. For example, you might want to add the factory default, &F, entry to the configuration file. To do this, enter the modemcap edit modem-name attribute value line configuration command. Configure one attribute of one modem at a time.

The following example shows how to enter line configuration mode and issue the modem autoconfigure type type command for a US Robotics Sportster modem:

router(config-line)# modem autoconfigure type usr_sportster

For more information about the recommended strings for any type of modem, refer to the section "Sample Modem Strings" in the appendix "Configuring Modem Support and Chat Scripts" in the Dial Solutions Command Reference. If you have access to Cisco Connection Online (CCO), you can also access the following URL for more information:

http://www.cisco.com/warp/customer/76/4.html

Note Note that this URL is subject to change without notice.

Manually Configuring Your External Modem

If you cannot configure your modem automatically, you must configure it manually. The following sections describe how to configure your externally attached modem manually:

Configuring Modem Initialization Strings

This section describes how to determine and issue the correct initialization string for your modem and configure your modem with it.

Modem command sets vary widely. Although most modems use the Hayes command set (prefixing commands with AT), Hayes-compatible modems do not use identical AT command sets.

Refer to your modem manufacturer's documentation to learn how to examine the current and stored configuration of the modem you are using. Generally, you enter AT commands such as &v, i4, or *o to view, inspect, or observe the settings.

Note You must first create a direct Telnet or connection session to the modem before you can send an initialization string. You can use AT&F as a basic modem initialization string in most cases.

A sample modem initialization string for a US Robotics Courier modem is as follows:

&b1&h1&r2&c1&d3&m4&k1s0=1

Modem initialization strings enable the following functions:

Locks the speed of the modem to the speed of the serial port on the access server
Sets Hardware Flow Control (RTS/CTS)
Ensures Correct DCD Operation
Ensures Proper DTR Interpretation
Answers Calls on the First Ring

Time Saver Initialization strings for other modems are listed in the appendix "Configuring Modem Strings and Chat Scripts" in the Dial Solutions Command Reference and on Cisco Connection Online (CCO), in Tech Tips (http://www.cisco.com). Note that these URLs are subject to change without notice.

Note Make sure to turn off automatic baud rate detection because the modem speeds must be set to a fixed value.

The port speed must not change when a session is negotiated with a remote modem. If the speed of the port on the access server is changed, you must establish a direct Telnet session to the modem and send an AT command so that the modem can learn the new speed.

Modems differ in the method they use to lock the EIA/TIA-232 (serial) port speed. In the modem documentation, vendors use terms such as, port-rate adjust, speed conversion, or buffered mode. Enabling error correction often puts the modem in the buffered mode. Refer to your modem documentation to see how your modem locks speed (check the settings &b, \j, &q, \n, or s-register settings).

Ready-To-Send (RTS) and Clear-To-Send (CTS) signals must be used between the modem and the access server to control the flow of data. Misconfiguring flow control for software or setting no flow control can result in hung sessions and loss of data. Modems differ in the method they use to enable hardware flow control. Refer to your modem documentation to see how to enable hardware flow control (check the settings &e, &k, &h, &r, or s-register).

The modem must use the data carrier detect (DCD) wire to indicate to the access server when a session has been negotiated and is established with a remote modem. Most modems use the setting &c1. Refer to your modem documentation for the DCD settings used with your modem.

The modem must interpret a toggle of the Data Terminal Ready (DTR) signal as a command to drop any active call and return to the stored settings. Most modems use the settings &d2 or &d3. Refer to your modem documentation for the DTR settings used with your modem.

If a modem is used to service incoming calls, it must be configured to answer a call after a specific number of rings. Most modems use the setting s0=1 to answer the call after one ring. Refer to your modem documentation for the settings used with your modem.

Checking Other Modem Settings

This section defines other settings that might be needed or desirable depending on your modem.

Error correction can be negotiated between two modems to ensure a reliable data link. Error correction standards include LAPM and MNP4. V.42 error correction allows either LAPM or MNP4 error correction to be negotiated. Modems differ in the way they enable error correction. Refer to your modem documentation for the error correction methods used with your modem.

Data compression can be negotiated between two modems to allow for greater data throughput. Data compression standards include V.42 bis and MNP5. Modems differ in the way they enable data compression. Refer to your modem documentation for the data compression settings used with your modem.

To establish a direct connect session to an internal or integrated modem (existing inside the router), such as required for Microcom modems in the Cisco AS5200, first open a directly connected session with the modem at-mode command then send an AT command to the specified modem. For example, the following example sends the AT command at\s to modem 1/1:

AS5200# modem at-mode 1/1
You are now entering AT command mode on modem (slot 1 / port 1).
Please type CTRL-C to exit AT command mode.
at%v
 
MNP Class 10 V.34/V.FC Modem Rev 1.0/85
 
OK
at\s
 
IDLE           000:00:00
LAST DIAL      
 
NET ADDR:      FFFFFFFFFFFF
MODEM HW: SA 2W United States
4 RTS 5 CTS 6 DSR - CD 20 DTR - RI 
MODULATION     IDLE
MODEM BPS      28800  AT%G0   
MODEM FLOW     OFF    AT\G0
MODEM MODE     AUT    AT\N3
V.23 OPR.      OFF    AT%F0
AUTO ANS.      ON     ATS0=1
SERIAL BPS     115200 AT%U0   
BPS ADJUST     OFF    AT\J0
SPT BPS ADJ.   0      AT\W0
ANSWER MESSGS  ON     ATQ0   
SERIAL FLOW    BHW    AT\Q3
PASS XON/XOFF  OFF    AT\X0
PARITY         8N     AT

Initializing the Modem

Refer to this section if you could not or chose not to initialize your modems automatically, as described in the "Automatically Configuring Your External Modem" section earlier in this chapter.

After the modem initialization string has been determined, perform the following steps to configure the modem. This example configures a U.S. Robotics Courier modem on line 1 (decimal number 2000 + line number 1 = 2001):

Step 1 Map a host name to a decimal port. The port number is 200x, plus the number of the TTY line. The following example maps port 2001 to the IP address of the Ethernet0 interface on the access server (172.16.1.10):

router(config)# ip host modem1 2001 172.16.1.10

router(config)# exit

router#

Step 2 Establish a direct Telnet session to the modem:

router# telnet modem1

Trying modem1 (172.16.1.10, 2001)... Open

Step 3 Return the modem to its factory defaults (this step is optional):

at&f

OK

Step 4 Configure the modem with an initialization string. The following example string is for a U.S. Robotics Courier modem:

at&b1&h1&r2&c1&d3&m4&k1s0=1

OK

Step 5 Store the modem settings in NVRAM on the modem:

at&w

OK

Note

Step 6 Suspend and disconnect your Telnet session:

- suspend keystroke -
router# disconnect
Closing connection to modem1 [confirm] y
router#

Time Saver The script-reset line configuration command can automate the configuration of your modems. See the "Technical Tips" section on CCO for more information.

Testing the Dial-In Connection

The access server and modem are now correctly configured for dial-in access. Before configuring any additional protocols for the line (such as SLIP, PPP, or ARA), test the dial-in connection.

Note The same configuration issues exist between the client data terminal equipment (DTE) and client modem. Make sure you have the correct EIA/TIA-232 cabling and modem initialization string for your client modem.

The following is an example of a successful connection from a PC using a U.S. Robotics Courier modem to dial in to a Cisco 2500 series access server:

at&f&c1&d3&h1&r2&b1&m4&k1&w
OK
atdt9,5551234
CONNECT 14400/ARQ/V32/LAPM/V42BIS
User Access Verification
Username: janedoe 
Password:
router>

Configure Additional Modem Control Parameters

Cisco routers use six EIA/TIA-232 signals for each port, so one 50-pin telco, RJ-11, or RJ-45 connector can support eight sessions. The router can support the most popular forms of modem control and hardware flow control, as well as high-speed dial-up modems.

The EIA/TIA-232 output signals are Transmit Data (TXDATA), Data Terminal Ready (DTR), and Ready To Send (RTS, 2500 only). The input signals are Receive Data (RXDATA), Clear to Send (CTS), and RING. The sixth signal is ground. Depending on the type of modem control your modem uses, these names may or may not correspond to the standard EIA/TIA-232 signals.

Dial-up modems that operate over normal telephone lines at speeds of 28800 bits per second (bps) use hardware flow control to stop the data from reaching the host by toggling an EIA/TIA-232 signal when their limit is reached.

In addition to hardware flow control, modems require special software configuring. For example, they must be configured to create an EXEC session when a user dials in and to hang up when the user exits the EXEC. These modems also must be configured to close any existing network connections if the telephone line hangs up in the middle of a session.

The Cisco IOS software supports hardware flow control on its CTS input signal, which is also used by the normal modem handshake.

The following modem line characteristics and modem features are discussed in the following sections:

Signal and line state diagrams accompany some of the tasks in the following sections to illustrate how the modem control works. The diagrams show two processes:

The "create daemon" process creates a TTY daemon that handles the incoming network connection.
The "create EXEC" process creates the process that interprets user commands. (Refer to
Figure 36 through Figure 41.)

In the diagrams, the current signal state and the signal the line is watching are listed inside each box. The state of the line (as displayed by the show line EXEC command) is listed next to the box. Events that change that state appear in italics along the event path, and actions that the software performs are described within the ovals.

Figure 36 illustrates line states when no modem control is set. The DTR output is always high, and CTS and RING are completely ignored. The Cisco IOS software starts an EXEC session when the user types the activation character. Incoming TCP connections occur instantly if the line is not in use and can be closed only by the remote host.

Figure 36: EXEC and Daemon Creation on a Line with No Modem Control

Configure Automatic Dialing

With the dial-up capability, you can set a modem to dial the phone number of a remote router automatically. This feature offers cost savings because phone line connections are made only when they are needed--you only pay for using the phone line when there is data to be received or sent. To configure a line for automatic dialing, perform the following task in line configuration mode:

Task	Command
Configure a line to initiate automatic dialing.	modem dtr-active

Using the modem dtr-active command causes a line to raise DTR signal only when there is an outgoing connection (such as reverse Telnet, NASI, or DDR), rather than leave DTR raised all the time. When raised, DTR potentially tells the modem that the router is ready to accept a call.

Automatically Answer a Modem

You can configure a line to answer a modem automatically. You also can configure the modem to answer the telephone on its own (as long as DTR is high), drop connections when DTR is low, and use its Carrier Detect (CD) signal to accurately reflect the presence of carrier. (Configuring the modem is a modem-dependent process.) Wire the modem's CD signal (generally pin-8) to the router's RING input (pin-22), and perform the following task in line configuration mode:

Task	Command
Configure a line to automatically answer a modem.	modem dialin

You can turn on the modem's hardware flow control independently to respond to the status of the router's CTS input. Wire CTS to whatever signal the modem uses for hardware flow control. If the modem expects to control hardware flow in both directions, you might also need to wire the modem's flow control input to some other signal that the router always has high (such as the DTR signal).

Figure 37 illustrates the modem dialin process with a high-speed dial-up modem. When the Cisco IOS software detects a signal on the RING input of an idle line, it starts an EXEC or autobaud process on that line. If the RING signal disappears on an active line, the Cisco IOS software closes any open network connections and terminates the EXEC facility. If the user exits the EXEC or the software terminates because of no user input, the line makes the modem hang up by lowering the DTR signal for five seconds. After five seconds, the modem is ready to accept another call.

Figure 37: EXEC Creation on a Line Configured for a High-Speed Modem

Support Dial-In and Dial-Out Modems

You can configure a line for both incoming and outgoing calls by performing the following task in line configuration mode:

Task	Command
Configure a line for both incoming and outgoing calls.	modem inout

Figure 38 illustrates the modem inout command. If the line is activated by raising the data set ready (DSR) signal, it functions exactly as a line configured with the modem dialin line configuration command described in the "Automatically Answer a Modem" section earlier in this chapter. If the line is activated by an incoming TCP connection, the line functions similarly to lines not used with modems.

Figure 38: EXEC and Daemon Creation for Incoming and Outgoing Calls

Note If your system incorporates dial-out modems, consider using access lists to prevent unauthorized use.

Configure a Line Timeout Interval

You can change the interval that the Cisco IOS software waits for the CTS signal after raising the DTR signal in response to the DSR (the default is 15 seconds). To do so, perform the following task in line configuration mode. The timeout applies to the modem callin command only.

Task	Command
Configure modem line timing.	modem answer-timeout seconds

Note The DSR signal is called RING on older ASM-style chassis.

Close Modem Connections

You can configure a line to close connections from a user's terminal when the terminal is turned off and prevent inbound connections to devices that are out of service. To do so, perform the following task in line configuration mode:

Task	Command
Configure a line to close connections.	modem cts-required

Figure 39 illustrates the modem cts-required command operating in the context of a continuous CTS signal. This form of modem control requires that the CTS signal be high for the entire session. If CTS is not high, the user's input is ignored and incoming connections are refused (or sent to the next line in a rotary group).

Note For the Cisco IOS software to reliably detect a CTS signal change, the CTS signal must remain in the new state for at least one full second.

Figure 39: EXEC and Daemon Creation on a Line Configured for Continuous CTS

Configure a Line to Automatically Disconnect

You can configure automatic line disconnect by performing the following task in line configuration mode:

Task	Command
Configure automatic line disconnect.	autohangup

The autohangup command causes the EXEC facility to issue the exit command when the last connection closes. This feature is useful for UNIX-to-UNIX copy program (UUCP) applications because UUCP scripts cannot issue a command to hang up the telephone. This feature is not often used.

Support Old-Style Dial-In Modems

The Cisco IOS software supports dial-in modems that use DTR to control the off-hook status of the telephone line. This feature is supported primarily on old-style modems, especially those in Europe. To configure the line to support this feature, perform the following task in line configuration mode:

Task	Command
Configure a line for a dial-in modem.	modem callin

Figure 40 illustrates the modem callin command. When a modem dialing line is idle, it has its DTR signal at a low state and waits for a transition to occur on the DSR (RING) input. This transition causes the line to raise the DTR signal and start watching the CTS signal from the modem. After the modem raises CTS, the Cisco IOS software creates an EXEC session on the line. If the timeout interval (set with the modem answer-timeout command) passes before the modem raises the CTS signal, the line lowers the DTR signal and returns to the idle state.

Figure 40: EXEC Creation on a Line Configured for Modem Call-In

Note The modem callin and modem cts-required line configuration commands are useful for SLIP operation. These commands ensure that when the line is hung up or the CTS signal drops, the line reverts from SLIP mode to normal interactive mode. These commands do not work if you put the line in network mode permanently.

Although you can use the modem callin line configuration command with newer modems, the modem dialin line configuration command described in this section is more appropriate. The modem dialin command frees up CTS input for hardware flow control. Modern modems do not require the assertion of DTR to answer a phone line (that is, to take the line off-hook).

Support Reverse Modem Connections and Prevent Incoming Calls

In addition to initiating connections, the Cisco IOS software can receive incoming connections. This capability allows you to attach serial and parallel printers, modems, and other shared peripherals to the router or access server and drive them remotely from other modem-connected systems. The Cisco IOS software supports reverse TCP, XRemote, and LAT connections.

The specific TCP port or socket, to which you attach the device determines the type of service that the Cisco IOS software provides on a line. When you attach the serial lines of a computer system or a data terminal switch to the serial lines of the access server, the access server can act as a network front-end device for a host that does not support the TCP/IP protocols. This arrangement is sometimes called front-ending, or reverse connection mode.

The Cisco IOS software supports ports connected to computers that are connected to modems. You can configure the Cisco IOS software to function somewhat like a modem by performing the following task in line configuration mode. This command also prevents incoming calls.

Task	Command
Configure a line for reverse connections and prevent incoming calls.	modem callout

Figure 41 illustrates the modem callout process. When the Cisco IOS software receives an incoming connection, it raises the DTR signal and waits to see if the CTS signal is raised to indicate that the host has noticed the router's DTR signal. If the host does not respond within the interval set by the modem answer-timeout line configuration command, the software lowers the DTR signal and drops the connection.

Figure 41: Daemon Creation on a Line Configured for Modem Call-out

Asynchronous Configuration Task List

This section describes how to configure basic functionality on asynchronous interfaces, and then customize the interfaces for your environment. Basic configuration tasks include the following:

If you want to call back a PPP client requesting asynchronous callback, refer to the chapter "Configuring Asynchronous Callback" in this publication.

See the "Asynchronous Configuration Examples" section at the end of this chapter for examples of asynchronous configuration files. Tasks are performed in global configuration mode unless otherwise specified.

Specify an Asynchronous Interface

On an access server, you can configure asynchronous interfaces. The auxiliary port (labeled AUX on the back of the product) can also be configured as an asynchronous serial interface, although performance on the AUX port is much slower than on standard asynchronous interfaces and does not support some features. Table 7 illustrates why asynchronous interfaces permit substantially better performance than AUX ports configured as asynchronous interfaces.

Table 7: Differences between Auxiliary (AUX) Port and Asynchronous Port

Feature	Asynchronous Interface	Auxiliary Port
Maximum speed	115200 kbps	38400 kbps
Supports DMA buffering¹	Yes	No
PPP framing on chip²	Yes	No
IP fast switching³	Yes	No

¹ Direct Memory Access (DMA) buffering moves data packets directly to and from system memory without interrupting the main CPU. This process removes overhead from the CPU and increases overall system performance.
² PPP framing on a hardware chip removes overhead from the router's CPU, which enables the router to sustain 115.2-kbps throughput on all asynchronous ports simultaneously.
³ After the destination of the first IP packet is added to the fast switching cache, it is fast switched to and from other interfaces with minimal involvement from the main processor.

Use the line command with the appropriate line configuration commands for modem control, such as speed. Perform the following task in global configuration mode to specify a port as an asynchronous interface:

Task	Command
Specify an asynchronous serial interface.	interface async port-number

Configure Group and Member Asynchronous Interfaces

You can create an asynchronous interface to be used as a group interface, which can be associated with other, member asynchronous interfaces.

This association allows you to configure the group interface and all of its member interfaces with a single command entered at the asynchronous group interface command line. You can have more than one group interface on a device; however, a member interface can be associated with only one group.

See the "Group and Member Asynchronous Interfaces Examples" section later in this chapter for an example of group and member interfaces.

Figure 42 illustrates the group-member interface concept.

Figure 42: Group-Member Association on Asynchronous Interfaces

Create the Group Interface and the Member Interfaces

To create an asynchronous group interface and associate member interfaces to this group interface, perform the following commands starting in global configuration mode:

Task	Command
Create an asynchronous group interface.	interface group-async unit-number
Associate one or more asynchronous interfaces (members) to the group interface so that all associated interfaces can be configured through the group interface.	group-range low-end-of-range high-end-of-range

Refer to the "Group and Member Asynchronous Interfaces Examples" section in this chapter for an example configuration.

Define Member Asynchronous Interface Characteristics

Member interfaces can have certain interface configurations that differ from their group. The following are valid interface configuration commands:

async default ip address
description

To configure a member with two or more interface configurations that are different from its group, enter the following command in interface configuration mode, where interface-command is one of the commands listed in the preceding list:

Task	Command
Configure a member to have specific differences from its group.	member interface-number interface-command

Configuring Dual-Purpose Ports

To specify the mode of a low-speed serial interface as either synchronous or asynchronous, perform the following task in interface configuration mode:

Task	Command
Specify the mode of a low-speed interface as either synchronous or asynchronous.	physical-layer {sync \| async}

This command applies only to low-speed serial interfaces available on Cisco 2520 through Cisco 2523 routers.

In synchronous mode, low-speed serial interfaces support all interface configuration commands available for high-speed serial interfaces, except the following two commands:

sdlc cts-delay
sdlc rts-timeout

When placed in asynchronous mode, low-speed serial interfaces support all commands available for standard asynchronous interfaces. The default is synchronous mode.

Note that when you enter this command, it does not appear in the output of show running config and show startup config command, because the command is a physical-layer command.

Assign IP Addresses for Local Devices

The local address is set using the ip address or ip unnumbered command.

IP addresses identify locations to which IP datagrams can be sent. You must assign each router interface an IP address. See the publication Network Protocols Configuration Guide, Part 1 for detailed information on IP addresses.

To assign an IP address to a network interface, perform the following task in interface configuration mode:

Task	Command
Set an IP address for an interface.	ip address address mask [secondary]

A subnet mask identifies the subnet field of a network address.

Conserve Network Addresses

When asynchronous routing is enabled, you might find it necessary to conserve network addresses by configuring the asynchronous interfaces as unnumbered. An unnumbered interface does not have an address. Network resources are therefore conserved because fewer network numbers are used and routing tables are smaller.

To configure an unnumbered interface, perform the following task in interface configuration mode:

Task	Command
Configure the asynchronous interface to be unnumbered.	ip unnumbered type number

Whenever the unnumbered interface generates a packet (for example, a routing update), it uses the address of the specified interface as the source address of the IP packet. It also uses the address of the specified interface to determine which routing processes are sending updates over the unnumbered interface.

You can use the IP unnumbered feature even if the system on the other end of the asynchronous link does not support it. The IP unnumbered feature is transparent to the other end of the link because each system bases its routing activities on information in the routing updates it receives and on its own interface address.

Configure the Interface Addressing Method for Remote Devices

You can control whether addressing is dynamic (the user specifies the address at the EXEC level when making the connection), or whether default addressing is used (the address is forced by the system). If you specify dynamic addressing, the router must be in interactive mode and the user will enter the address at the EXEC level.

It is common to configure an asynchronous interface to have a default address and to allow dynamic addressing. With this configuration, the choice between the default address or a dynamic addressing is made by the users when they enter the slip or ppp EXEC command. If the user enters an address, it is used, and if the user enters the default keyword, the default address is used.

This section describes the following tasks:

Assign a default asynchronous address.
Allow an asynchronous address to be assigned dynamically.

Assign a Default Asynchronous Address

Perform the following task in interface configuration mode to assign a permanent default asynchronous address:

Task	Command
Assign a default IP address to an asynchronous interface.	peer default ip address address

Use the no form of this command to disable the default address. If the server has been configured to authenticate asynchronous connections, you are prompted for a password after entering the slip default or ppp default EXEC command before the line is placed into asynchronous mode.

The assigned default address is implemented when the user enters the slip default or ppp default EXEC command. The transaction is validated by the Terminal Access Controller Access System (TACACS) server, when enabled, and the line is put into network mode using the address that is in the configuration file.

Configuring a default address is useful when the user is not required to know the IP address to gain access to a system (for example, users of a server that is available to many students on a campus). Instead of requiring each user to know an IP address, they only need to enter the slip default or ppp default EXEC command and let the server select the address to use. See the chapter "Making Connections to Network Devices" in this book for more information about the slip and ppp EXEC commands.

Allow an Asynchronous Address to Be Assigned Dynamically

When a line is configured for dynamic assignment of asynchronous addresses, the user enters the slip or ppp EXEC command and is prompted for an address or logical host name. The address is validated by TACACS, when enabled, and the line is assigned the given address and put into asynchronous mode. Assigning asynchronous addresses dynamically is also useful when you want to assign set addresses to users. For example, an application on a personal computer that automatically dials in using SLIP and polls for electronic mail messages can be set up to dial in periodically and enter the required IP address and password.

To assign asynchronous addresses dynamically, perform the following task in interface configuration mode:

Task	Command
Allow the IP address to be assigned when the protocol is initiated.	async dynamic address

The dynamic addressing features of the internetwork allow packets to get to their destination and back regardless of the access server, router, or network they are sent from. For example, if a host such as a laptop computer moves from place to place it can keep the same address no matter where it is dialing in from.

Logical host names are first converted to uppercase and then sent to the TACACS server for authentication.

Configure Asynchronous Serial Encapsulation

To configure an asynchronous serial interface on a routing device, you must set up the interface to send SLIP or PPP packets. PPP and SLIP define methods of sending Internet packets over a standard EIA-232 asynchronous serial line. PPP also defines methods for sending IPX and ARA packets during PPP sessions.

There are two asynchronous serial encapsulation methods:

Serial Line Internet Protocol (SLIP)
Point-to-Point Protocol (PPP)

Only IP packets can be sent across lines configured for SLIP. PPP supports transmission of IP, IPX, and ARA packets on an asynchronous serial interface.

For information about configuring PPP and SLIP, refer to the chapter "Configuring Asynchronous PPP and SLIP" in this publication.

Configure Dedicated or Interactive PPP and SLIP Sessions

You can configure one or more asynchronous interfaces on your access server (and one on a router) to be in dedicated network interface mode. In dedicated mode, an interface is automatically configured for SLIP or PPP connections. There is no user prompt or EXEC level, and no end-user commands are required to initiate remote-node connections. If you want a line to be used only for SLIP or PPP connections, configure the line for dedicated mode.

In interactive mode, a line can be used to make any type of connection, depending on the EXEC command entered by the user. For example, depending on its configuration, the line could be used for Telnet or XRemote connections, or SLIP or PPP encapsulation. The user is prompted for an EXEC command before a connection is initiated.

Configure Dedicated Network Mode or Interactive Mode

You can configure an asynchronous interface to be in dedicated network mode. When the interface is configured for dedicated mode, the end user cannot change the encapsulation method, address, or other parameters.

To configure an interface for dedicated network mode or return it back to interactive mode, perform one of the following tasks in interface configuration mode.

Task	Command
Place the line into dedicated asynchronous network mode.	async mode dedicated
Return the line to interactive mode.	async mode interactive

By default, no asynchronous mode is configured. In this state, the line is not available for inbound networking because the SLIP and PPP connections are disabled.

Enable Routing on Asynchronous Interfaces

To route IP packets on an asynchronous interface, perform one of the following tasks in interface configuration mode:

Task	Command
Configure an asynchronous interface for dynamic routing.	async dynamic routing
Automatically configure an asynchronous interface for routing.	async default routing

This async dynamic routing command routes IP packets on an asynchronous interface, which permits you to enable the IGRP, RIP, and OSPF routing protocols for use when the user makes a connection using the ppp or slip EXEC commands. The user must, however, specify the /routing keyword at the SLIP or PPP command line.

For asynchronous interfaces in interactive mode, the async default routing command causes the ppp and slip EXEC commands to be interpreted as though the /route switch had been included in the command. For asynchronous interfaces in dedicated mode, the async dynamic routing command enables routing protocols to be used on the line. Without the async default routing command, there is no way to enable the use of routing protocols automatically on a dedicated asynchronous interface.

Configure Support for Extended BOOTP Requests

To configure Cisco IOS software to respond to BOOTP requests from client machines, perform the following task in global configuration mode:

Task	Command
Specify the router network information that is sent in response to BOOTP requests.	async-bootp tag [:hostname] data

Monitor and Maintain Asynchronous Devices

This section describes the following monitoring and maintenance tasks that you can perform on asynchronous interfaces:

Monitor and maintain asynchronous activity
Debug asynchronous interfaces
Debug PPP

To monitor and maintain asynchronous activity, perform one or more of the following tasks in privileged EXEC mode:

Task	Command
Return a line to its idle state.	clear line line-number
Display parameters that have been set for extended BOOTP requests.	show async bootp
Display statistics for asynchronous interface activity.	show async status
Display the status of asynchronous line connections.	show line [line-number]

To debug asynchronous interfaces, perform the following task in privileged EXEC mode:

Task	Command
Displays errors, changes in interface state, and log input and output.	debug async {framing \| state \| packets}

To debug PPP links, perform the following tasks in privileged EXEC mode:

Task	Command
Enable debugging of PPP protocol negotiation process.	debug ppp negotiation
Display PPP protocol errors.	debug ppp error
Display PPP packets sent and received.	debug ppp packet
Display errors encountered during remote or local system authentication.	debug ppp chap

Configure Chat Scripts for Asynchronous Lines

Chat scripts are strings of text used to send commands for modem dialing, logging onto remote systems, and initializing asynchronous devices connected to an asynchronous line. On a router, chat scripts can be configured on the auxiliary port only. A chat script must be configured to dial out on asynchronous lines. You also can configure chat scripts so that they are executed automatically for other specific events on a line, or so that they are executed manually. Each chat script is defined for a different event. These events can include the following:

Line activation
Incoming connection initiation
Asynchronous dial-on-demand routing
Line resets
Startup

To use a chat script, perform the following steps:

Step 1 Define the chat script in global configuration mode using the chat-script command.

Step 2 Configure the line so that a chat script is activated when a specific event occurs (using the script line configuration command), or start a chat script manually (using the start-chat privileged EXEC command).

Create a Chat Script

To define a chat script, perform the following task in global configuration mode:

Task	Command
Create a script that will place a call on a modem, log on to a remote system, or initialize an asynchronous device on a line.	chat-script script-name expect send...

A limited list of keywords are supported, along with expect/send pairs. Send strings can have special escape modifiers.

Cisco recommends that one chat script (a "modem" chat script) be written for placing a call and another chat script (a "system" or "login" chat script) be written to log onto remote systems, where required.

Chat scripts are not supported on lines where modem control is set for inbound activity that only uses the modem dialin command.

Suggested Chat Script Naming Conventions (for Dial Scripts only)

When you create a script name, include the modem vendor, type, and modulation, separated by hyphens. For example, if you have a Telebit t3000 modem that uses V.32bis modulation, your script name would be telebit-t3000-v32bis.

A suggested naming convention for chat scripts used to dial is as follows:

vendor-type-modulation

In other words, the syntax of the chat-script command becomes the following:

chat-script vendor-type-modulation expect send...

For example, if you have a Telebit t3000 modem that uses V.32bis modulation, you would name your chat script as follows:

telebit-t3000-v32bis

The chat-script command could become the following:

router(config)# chat-script telebit-t3000-v32bis ABORT ERROR ABORT BUSY ABORT
"NO ANSWER" "" "ATH" OK "ATDT\T" TIMEOUT 30 CONNECT

Adhering to this naming convention allows you to specify a range of chat scripts using partial chat script names with regular expressions. This is particularly useful for dialer rotary groups.

Configure the Line to Activate Chat Scripts

Chat scripts can be activated by any of five events, each corresponding to a different version of the script line configuration command. To start a chat script manually at any point, refer to the following section, "Start a Chat Script Manually on an Asynchronous Line."

To define a chat script to start automatically when a specific event occurs, perform the following tasks in line configuration mode:

Task	Command
Start a chat script on a line when the line is activated (every time a command EXEC is started on the line).	script activation regexp¹
Start a chat script on a line when a network connection is made to the line.	script connection regexp
Specify a modem script for DDR on a line.	script dialer regexp
Start a chat script on a line whenever the line is reset.	script reset regexp
Start a chat script on a line whenever the system is started up.	script startup regexp

¹ The argument regexp is a regular expression that is matched to a script name that has already been defined using the chat-script command.

Note Outbound chat scripts are not supported on lines where modem control is set for inbound activity only (using the modem dialin command).

Start a Chat Script Manually on an Asynchronous Line

You can start a chat script manually on any line that is currently not active by performing the following task in privileged EXEC mode:

Task	Command
Start a chat script manually on any asynchronous line.	start-chat regexp [line-number [dialer-string]]

If you do not specify the line number, the script runs on the current line. If the line specified is already in use, you cannot start the chat script. A message appears indicating that the line is already in use.

Configure Additional Asynchronous Features

You can configure the following asynchronous features:

Establish and Control the EXEC Process

By default, the Cisco IOS software starts an EXEC process on all lines. However, you can control EXEC processes, as follows:

Turn the EXEC process on or off.

: A serial printer, for example, should not have an EXEC session started.

Set the idle terminal timeout interval.

The EXEC command interpreter waits for a specified amount of time to receive user input. If no input is detected, the EXEC facility resumes the current connection. If no connections exist, it returns the terminal to the idle state and disconnects the incoming connection. To control the EXEC process, perform the following tasks in line configuration mode:

Task	Command
Turn on EXEC processes.	exec
Set the idle terminal timeout interval.	exec-timeout minutes [seconds]

Configure the Auxiliary (AUX) Port

The AUX port is typically configured as an asynchronous serial interface on routers without built-in asynchronous interfaces. To configure the AUX port as an asynchronous interface, configure it first as an auxiliary line with the line aux 1 global configuration command.

The AUX port sends a DTR signal only when a Telnet connection is established. The auxiliary port does not use Ready to Send/Clear to Send (RTS/CTS) handshaking for flow control. To understand the differences between standard asynchronous interfaces and AUX ports configured as an asynchronous interface, refer to Table 8. To enable the auxiliary port, perform the following task:

Task	Command
Enable the auxiliary serial DTE port.	line aux line-number

You cannot use the auxiliary (AUX) port as a second console port. To use the AUX port as a console port, you must order a special cable from your technical support personnel.

On an access server, you can configure any of the available asynchronous interfaces (1 through 8, 16, or 48). The auxiliary port (labeled AUX on the back of the product) can also be configured as an asynchronous serial interface, although performance on the AUX port is much slower than on standard asynchronous interfaces and does not support some features. Table 8 illustrates why asynchronous interfaces permit substantially better performance than AUX ports configured as asynchronous interfaces.

**Table 8: Differences between the Auxiliary (AUX) Port and the Asynchronous Port**
Feature	Asynchronous Interface	Auxiliary Port
Maximum speed	115200 kbps	38400 kbps
Supports DMA buffering¹	Yes	No
PPP framing on chip²	Yes	No
IP fast switching³	Yes	No

¹ Direct Memory Access (DMA) buffering moves data packets directly to and from system memory without interrupting the main central processing unit (CPU). This process removes overhead from the CPU and increases overall system performance.
² PPP framing on a hardware chip removes overhead from the router's CPU, which enables the router to sustain 115.2 kbps throughput on all asynchronous ports simultaneously.
³ After the destination of the first IP packet is added to the fast switching cache, it is fast switched to and from other interfaces with minimal involvement from the main processor.

On routers without built-in asynchronous interfaces, only the AUX port can be configured as an asynchronous serial interface. To configure the AUX port as an asynchronous interface, you must also configure it as an auxiliary line with the line aux 1 command. Access servers do not have this restriction. Use the line command with the appropriate line configuration commands for modem control, such as speed.

Only IP packets can be sent across lines configured for SLIP. PPP supports transmission of IP, IPX, and AppleTalk packets on an asynchronous serial interface.

Configure the Line to Automatically Detect the Incoming Protocol and Start a Session

To configure the Cisco IOS software to allow an AppleTalk Remote Access (ARA), Point-to-Point Protocol (PPP), or Serial Line Internet Protocol (SLIP) session to start automatically, perform the following task in line configuration mode:

Task	Command
Configure a line to automatically start an ARA, PPP, or SLIP session.	autoselect {arap \| ppp \| slip \| during login}

The autoselect command enables the Cisco IOS software to start a process automatically when a start character is received. The Cisco IOS software detects either a Return character (which is the start character for an EXEC session, or the start character for the ARA protocol).

The autoselect command bypasses the login prompt and enables the specified session to begin automatically. However, by entering the autoselect command with the during login keyword, the username or password prompt appears without pressing the Return key. While the username or password prompt is displayed, you can choose either to answer these prompts or to send packets from an autoselected protocol.

Note When you use the autoselect command, the activation character should be set to the default Return, and exec-character-bits to 7. If you change these defaults, the application cannot recognize the activation request.

Define a Command String for Automatic Execution

You can set up a command to execute automatically when the router connects to another host. The Cisco IOS can execute any appropriate EXEC command and any switch or host name that occurs with the EXEC command. To define a command, perform the following task in line configuration mode:

Task	Command
Define a command to be automatically executed.	autocommand command

Configure Rotary Groups

Connections can be made to the next free line in a group of lines, also called a rotary group or hunt group. A line can be in only one rotary group; a rotary group can consist of a single line or several contiguous lines. The console line (line 0) cannot be in a rotary group.

To configure a rotary group, perform the following task in line configuration mode:

Task	Command
Add a line to the specified rotary group.	rotary group

Specify Decimal TCP Port Numbers when Connecting to Lines

Connections to an individual line are most useful when a dial-out modem, parallel printer, or serial printer is attached to that line. To connect to an individual line, the remote host or terminal must specify a particular TCP port on the router.

If reverse XRemote is required, that port is 9000 (decimal) plus the decimal value of the line number.

If a raw TCP stream is required, the port is 4000 (decimal) plus the decimal line number. The raw TCP stream is usually the required mode for sending data to a printer.

If Telnet protocols are required, that port is 2000 (decimal) plus the decimal value of the line number. The Telnet protocol might require that Return characters be translated into Return and line-feed character pairs. You can turn off this translation by specifying the Telnet binary mode option. To specify this option, connect to port 6000 (decimal) plus the decimal line number.

For example, a laser printer is attached to line 10 of a Cisco 2511 router. Such a printer usually uses XON/XOFF software flow control. Because the Cisco IOS software cannot receive an incoming connection if the line already has a process, you must ensure that an EXEC session is not accidentally started. You must, therefore, configure it as follows:

line 10
 flowcontrol software
 no exec

A host that wants to send data to the printer would connect to the router on TCP port 4008, send the data, and then close the connection. (Remember that line number 10 octal equals 8 decimal.)

Optimize Available Bandwidth

Asynchronous lines have relatively low bandwidth and can easily be overloaded, resulting in slow traffic across these lines.

To optimize available bandwidth, perform any of the following tasks:

Configure Header Compression

One way to optimize available bandwidth is by using TCP header compression. Van Jacobson TCP header compression (defined by RFC 1144) can increase bandwidth availability between two and five times when compared to lines not using header compression. Theoretically, it can improve bandwidth availability by a ratio of seven to one.

To configure header compression, perform the following task in interface configuration mode:

Task	Command
Configure Van Jacobson TCP header compression on the asynchronous link.	ip tcp header-compression [on \| off \| passive]

Force Header Compression at the EXEC Level

On SLIP interfaces, you can force header compression at the EXEC prompt on a line on which header compression has been set to passive. This allows more efficient use of the available bandwidth and does not require entering privileged configuration mode.

To implement header compression, perform the following task in interface configuration mode:

Task	Command
Allow status of header compression to be assigned at the user level.	ip tcp header compression passive

For PPP interfaces, the passive option functions the same as the on option.

Asynchronous Configuration Examples

This section illustrates different communication requirements for the following asynchronous scenarios:

Restricting Access on the Asynchronous Interface Example

The following example assumes that users are restricted to certain servers designated as asynchronous servers, but that normal terminal users can access anything on the local network.

! access list for normal connections
access-list 1 permit 131.108.0.0 0.0.255.255
!
access-list 2 permit 131.108.42.55
access-list 2 permit 131.108.111.1
access-list 2 permit 131.108.55.99
!
line 1
 speed 19200
 flow hardware
 modem inout
interface async 1
 async mode interactive
 async dynamic address
 ip access-group 1 out
 ip access-group 2 in

Address Pooling on Asynchronous Interfaces Examples

The following sections provide examples of the use of DHCP and local pooling mechanisms.

DHCP Pooling Examples

The following global configuration example enables DHCP proxy-client status on all asynchronous interfaces on the access server:

ip address-pool dhcp-proxy-client

The following global configuration example illustrates how to specify which DHCP servers are used on your network. You can specify up to four servers using IP addresses or names. If you do not specify servers, the default is to use the IP limited broadcast address of 255.255.255.255 for transactions with any and all discovered DHCP servers.

ip dhcp-server jones smith wesson

The following interface configuration example illustrates how to disable DHCP proxy-client functionality on asynchronous interface 1:

async interface
interface 1
 no peer default ip address

Local Pooling Example

The following example shows how to select the IP pooling mechanism and how to create a pool of local IP addresses that are used when a client dials in on an asynchronous line. The default address pool comprises IP addresses 172.30.0.1 through 172.30.0.28.

! this command tells the access server to use a local pool
ip address-pool local
! this command defines the ip address pool. 
! The address pool is named group1 and comprised of addresses 
! 10.1.2.1through 10.1.2.5 inclusive
ip local-pool group1 10.1.2.1 10.1.2.5

Configure Specific IP Addresses for an Interface Example

This example shows how to configure the access server so that it will use the default address pool on all interfaces except interface 7, on which it will use an address pool called lass:

ip address-pool local
ip local-pool lass 172.30.0.1
 async interface
 interface 7
 peer default ip address lass

Group and Member Asynchronous Interfaces Examples

The following example shows how to create an asynchronous group interface 0 with group interface members 2 through 7, starting in global configuration mode:

interface group-async 0
 group-range 2 7

The following example shows how you need to configure asynchronous interfaces 1, 2, and 3 separately if you do not have a group interface configured:

interface Async1
 ip unnumbered Ethernet0
 encapsulation ppp
 async default ip address 172.30.1.1
 async mode interactive
 async dynamic routing
!
interface Async2
 ip unnumbered Ethernet0
 encapsulation ppp
 async default ip address 172.30.1.2
 async mode interactive
 async dynamic routing
!
interface Async3
 ip unnumbered Ethernet0
!
 encapsulation ppp
 async default ip address 172.30.1.3
 async mode interactive
 async dynamic routing

The following example configures the same interfaces, but from a single group asynchronous interface:

!
interface Group-Async 0
 ip unnumbered Ethernet0
 encapsulation ppp
 async mode interactive
 async dynamic routing
 group-range 1 3
 member 1 async default ip address 172.30.1.1
 member 2 async default ip address 172.30.1.2
 member 3 async default ip address 172.30.1.3

Dedicated Asynchronous Interface Configuration Example

The following example assigns an IP address to an asynchronous interface and places the line in dedicated network mode. Setting the stop bit to 1 is a performance enhancement.

line 20
 location Department PC Lab
 stopbits 1
 speed 19200
!
interface async 20
 async default ip address 182.32.7.51
 async mode dedicated

IP-SLIP--Asynchronous Interface Example

The following example configures IP-SLIP on asynchronous interface 6. The IP address for the interface is assigned to Ethernet 0, interactive mode has been enabled, and the IP address of the client PC running SLIP has been specified.

IP and the appropriate IP routing protocols have already been enabled on the access server or router.

interface async 6 
 ip unnumbered ethernet 0 
 encapsulation slip 
 async mode interactive 
 async default ip address 172.18.1.128

AppleTalk-PPP Example

The following example configures asynchronous interface 4 on the router so that users can access AppleTalk zones by dialing into the router via PPP to this interface. Users accessing the network can run AppleTalk and IP natively on a remote Macintosh, access any available AppleTalk zones from Chooser, use networked peripherals, and share files with other Macintosh users. Routing is not supported on the asynchronous interface 6.

interface async 6 
 encapsulation ppp
 appletalk virtual-net 12345 saivite
 appletalk client-mode

IP-PPP Example

The following example configures IP-PPP on asynchronous interface 6. The IP address for the interface is assigned to Ethernet 0, interactive mode has been enabled, and the IP address of the client PC running PPP has been specified.

IP and the appropriate IP routing protocols have already been enabled on the access server or router.

interface async 6
 ip unnumbered ethernet 0
 encapsulation ppp
 async mode interactive
 peer default ip address 172.18.1.128

IPX-PPP--Loopback Interface Example

The following example shows how to configure IPX to run over PPP on an asynchronous interface. The asynchronous interface is associated with a loopback interface configured to run IPX. This example enables a non-routing IPX client to connect to the router.

ipx routing 0000.0c07.b509
interface loopback0
 no ip address
 ipx network 544
 ipx sap-interval 2000
interface ethernet0
 ip address 172.21.14.64
 ipx network AC150E00
 ipx encapsulation SAP
interface async 3
 ip unnumbered ethernet0
 encapsulation ppp
 async mode interactive
 async default ip address 172.18.1.128
 ipx ppp-client loopback0
 ipx sap-interval 0

In this example, IPX client connections are permitted to asynchronous interface 3, which is associated with loopback interface 0. Loopback interface 0 is configured to run IPX. Routing updates have been filtered on asynchronous interface 3. Routing updates take up a great deal of bandwidth, and asynchronous interfaces have low bandwidth.

IPX-PPP--Using Dedicated IPX Network Numbers for Each Interface Example

The following example shows how to configure IPX to run over PPP on an asynchronous interface. A dedicated IPX network number has been specified for each interface, which can require a substantial number of network numbers for a large number of interfaces. This example permits an IPX client with routing enabled to connect with the router.

ipx routing 0000.0c07.b509
interface async 6
 ip unnumbered ethernet0
 encapsulation ppp
 async mode interactive
 ipx network AC150E00
 ipx sap-interval 0

In this example, IPX client connections are permitted to asynchronous interface 6, which has a unique IPX network number. Routing updates have been filtered on asynchronous interface 6. Routing updates take up a great deal of bandwidth, and asynchronous interfaces have low bandwidth.

IPX-PPP over X.25 to an IPX Network on VTY Lines Example

The following example shows how to enable IPX-PPP on VTY lines. First, you enable PPP to run on VTY lines, then you associate the VTY line with a loopback interface configured to run IPX. This example enables a non-routing IPX client to connect to the router.

ipx routing 0000.0c07.b509
interface loopback0
 no ip address
 ipx network 544
vty-async ipx ppp-client loopback0

In this example, IPX client connections are permitted to VTY lines, which have been associated with loopback interface 0. Loopback interface 0 is configured with an IPX network number that is used by the VTY lines.

Remote Node NetBEUI Example

In the following example, asynchronous interface 7 and ethernet interface 0 are configured to enable NetBEUI connectivity between the corporate telecommuter's client and the remote access (NetBEUI) server. The PC client is running a legacy application--Chat--in Windows NT, to connect with the remote server. Refer to Figure 43.

Figure 43: Connecting a Remote NetBEUI Client to a Server through a Router

The configuration for the router is as follows:

interface async 7
 netbios nbf
 encapsulation ppp

You would also need to configure security, such as TACACS+, RADIUS, or another form of login authentication on the router.

Asynchronous Routing and Dynamic Addressing Configuration Example

The following example shows a simple configuration that allows routing and dynamic addressing. With this configuration, if the user specifies /routing in the EXEC slip or ppp command, routing protocols will be sent and received.

interface async 6
 async dynamic routing
 async dynamic address

TCP Header Compression Configuration Example

The following example configures async interface 7 with a default IP address, allowing header compression if it is specified in the slip or ppp connection command entered by the user or if the connecting system sends compressed packets.

interface async 7
 ip address 172.31.79.1
 async default ip address 172.31.79.2
 ip tcp header-compression passive

Conserving Network Addresses Using the IP Unnumbered Feature Example

The following example shows how to configure your router for routing using unnumbered interfaces. The source (local) address is shared between Ethernet 0 and async 6 (172.18.1.1). The default remote address is 172.18.1.2.

interface ethernet 0
 ip address 172.18.1.1 255.255.255.0
!
interface async 6
 ip unnumbered ethernet 0
 async dynamic routing
! default address is on the local subnet
 async dynamic address
 async default ip address 172.18.1.2
 ip tcp header-compression passive

The following example shows how the IP unnumbered configuration works. Although the user assigned an address, the system response shows the interface as unnumbered, and the address entered by the user will be used only in response to BOOTP requests.

router> slip /compressed 10.11.11.254
Password:
Entering async mode.
Interface IP address is unnumbered, MTU is 1500 bytes.
Header compression is On.

Configuring Routing on a Dedicated Dial-In Router Example

In the following example, the router is set up as a dedicated dial-in router. Interfaces are configured as IP unnumbered to conserve network resources, primarily IP addresses.

ip routing
interface ether 0
 ip address 10.129.128.2 255.255.255.0
!
interface async 1
 ip unnumbered ethernet 0
 async dynamic routing
! The addresses assigned with SLIP or PPP EXEC commands are not used except 
! to reply to BOOTP requests.
! Normally, the routers dialing in will have their own address
! and not use BOOTP at all.
 async default ip address 10.11.11.254 
!
interface async 2
 ip unnumbered ethernet 0
 async default ip address 10.11.12.16
 ip tcp header-compression passive
 async mode dedicated
!
! run RIP on the asynchronous lines, because few implementations of SLIP 
! understand IGRP. Run IGRP on the ethernet (and in the local network).
!
router igrp 110
 network 10.11.12.0
! send routes from the asynchronous lines on the production network.
 redistribute RIP
! don't send IGRP updates on the async interfaces
 passive-interface async 1
!
 router RIP
 network 10.11.12.0
 redistribute igrp
 passive-interface ethernet 0
! consider filtering everything except a default route from the routing 
! updates sent on the (slow) asynchronous lines 
 distribute-list 1 out
 ip unnumbered async 2
 async dynamic routing

Configuring an Asynchronous Interface as the Only Network Interface Example

In the following example, one of the asynchronous lines is used as the only network interface. The router is used primarily as a terminal server, but is at a remote location and dials into the central site for its only network connection.

ip default-gateway 10.11.12.2
interface ethernet 0
 shutdown
interface async 1
 async dynamic routing
 ip tcp header-compression on
 async default ip address 10.11.16.12
 async mode dedicated
 ip address 10.11.12.32 255.255.255.0

Configuring IGRP Example

In the following example, only the IGRP TCP/IP routing protocol is running; it is assumed that the systems that are dialing in to use routing will either support IGRP or have some other method (for example, a static default route) of determining that the router is the best place to send most of its packets.

router igrp 111
 network 10.11.12.0
interface ethernet 0
 ip address 10.11.12.92 255.255.255.0
!
interface async 1
 async default ip address 10.11.12.96
 async dynamic routing
 ip tcp header-compression passive
 ip unnumbered ethernet 0
line 1
 modem ri-is-cd

Configuring an Interface Example

The following configuration shows interface and line configuration. The interface is configured with access lists, passive header compression and a default address. The line is configured for TACACS authentication.

interface async 1
 ip access-group 1 in
 ip access-group 1 out
 ip tcp header-compression passive
 async default ip address 172.31.176.201
line 1
 login tacacs
 location 457-5xxx
 exec-timeout 20 0
 password XXXXXXXX
 session-timeout 20 
 stopbits 1

Remote Network Access Using PPP--A Basic Configuration Example

Figure 44 illustrates a simple network configuration comprised of remote PCs with modems connected via modem to a router. The cloud is a public switched telephone network (PSTN). The modems are connected via asynchronous lines, and the access server is connected to a local network.

In this configuration you need to configure the following:

An asynchronous line on the access server configured to use PPP encapsulation
An interface on the access server for the modem connection; this interface also needs to be configured to accept incoming modem calls
A default IP address for each incoming line

Figure 44: Remote Network Access Using PPP

This default address indicates the address of the remote PC to the server, unless the user explicitly specifies another when starting the PPP session.

The server is configured for interactive mode with autoselect enabled, which allows the user to automatically begin a PPP session upon detection of a PPP packet from the remote PC; or, the remote PC can explicitly begin a PPP session by typing PPP at the prompt.

The configuration is as follows:

ip routing
!
interface ethernet 0
 ip address 192.168.32.12 255.255.255.0
!
interface async 1
 encapsulation ppp
 async mode interactive
 async default ip address 192.168.32.51
 async dynamic address
 ip unnumbered ethernet 0
line 1
 autoselect ppp
 modem callin
 speed 19200

Remote Network Access Using PPP--Routing IP Example

Figure 45 illustrates a network configuration that provides routing functionality, allowing routing updates to be passed across the asynchronous lines.

This network is comprised of remote and local PCs connected via modem and network connections to an access server. This access server is connected to a second access server via an asynchronous line running TCP/IP. The second access server is connected to a local network via modem.

For this scenario, you will need to configure the following:

An asynchronous line on both access servers configured to use PPP encapsulation
An interface on both access servers for the modem connection and for this interface to be configured to accept incoming modem calls
A default IP address for each incoming line
IP routing on all configured interfaces

Figure 45: Routing on an Asynchronous Line Using PPP

The configuration is as follows:

interface async 1
 encapsulation ppp
 async mode interactive
 async default ip address 192.168.32.10
 async dynamic address
 ip unnumbered ethernet 0
 async dynamic routing

If you want to pass IP routing updates across the asynchronous link, issue the following commands:

line 1
 autoselect ppp
 modem callin
 speed 19200

Next, enter these commands to configure the asynchronous lines between the access servers, starting in global configuration mode:

interface async 2
 async default ip address 192.168.32.55
 ip tcp header compression passive

Finally, configure routing as described in the Network Protocols Configuration Guide, Part 1 using one of the following methods. The server can route packets three different ways:

Use ARP, which is the default behavior.
Use a default-gateway by issuing the command ip default-gateway x.x.x.x, where x.x.x.x is the IP address of a locally attached router.
Run an IP routing protocol (RIP, IGRP, EIGRP, or OSPF).

Remote Network Access--Leased Line with Dial-Backup Using PPP Example

Figure 46 illustrates a scenario where two networks are connected via access servers on a leased line. Redundancy is provided by a dial-backup line over the public switched telephone network so that if the primary leased line goes down, the dial-backup line will be automatically brought up to restore the connection. This configuration would be useful for using an auxiliary port as the backup port for a synchronous port.

In this scenario, you will need to configure the following:

Two asynchronous interfaces on each access server
Two modem interfaces
A default IP address for each interface
Dial-backup on one modem interface per access server
An interface connecting to the access server's related network

Figure 46: Asynchronous Leased Line with Backup

The configuration is as follows:

hostname routerA
!
username routerB password cisco
chat-script backup "" "AT" TIMEOUT 30 OK atdt\T TIMEOUT 30 CONNECT \c !
!
interface Serial0
 backup interface Async1
 ip address 192.168.222.12 255.255.255.0
!
interface Async1
 ip address 172.16.199.1 255.255.255.0
 encapsulation ppp
 async default ip address 172.16.199.2
 async dynamic address
 async dynamic routing
 async mode dedicated
 dialer in-band
 dialer map IP 172.16.199.2 name routerB modem-script backup broadcast 3241129  dialer-group 1
 ppp authentication chap
!
 dialer-list 1 protocol ip permit
!
line aux 0
 modem InOut
 rxspeed 38400
 txspeed 38400

Table of Contents

Configuring Modem Support and Asynchronous Devices