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Point-to-Point Protocol (PPP) and Serial Line Internet Protocol (SLIP) define methods of sending IP packets over standard EIA/TIA-232 asynchronous serial lines with minimum line speeds of 1200 baud. This chapter describes the commands used to configure your router to enable PPP and SLIP on asynchronous interfaces.
Using PPP or SLIP encapsulation over asynchronous lines is an inexpensive way of connecting PCs to a network. PPP and SLIP over asynchronous dial-up modems allow a home computer to be connected to a network without the cost of a leased line. Dial-up PPP and SLIP links can also be used for remote sites that need only occasional remote node or backup connectivity. Both public-domain and vendor-supported PPP and SLIP implementations are available for a variety of computer applications.
Use the commands in this chapter to configure PPP and SLIP on your router. For configuration information and examples, refer to the chapter "Configuring Asynchronous PPP and SLIP" in the Dial Solutions Configuration Guide.
To place a line into dedicated asynchronous mode using SLIP or PPP encapsulation, use the async mode dedicated interface configuration command. To return the line to interactive mode, use the no form of this command.
async mode dedicatedThis command has no arguments or keywords.
Asynchronous mode is disabled.
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
With dedicated asynchronous network mode, the interface will use either SLIP or PPP encapsulation, depending on which encapsulation method is configured for the interface. An EXEC prompt does not appear, and the router is not available for normal interactive use.
If you configure a line for dedicated mode, you will not be able to use the async dynamic address command, because there is no user prompt.
The following example assigns an IP address to an asynchronous line and places the line into network mode. Setting the stop bits to 1 enhances performance.
interface async 4 async default ip address 172.31.7.51 async mode dedicated encapsulation slip line 20 location Joe's computer stopbits 1 speed 115200
You can use the master indexes or search online to find documentation of related commands.
To return a line that has been placed into dedicated asynchronous network mode to interactive mode, thereby enabling the slip and ppp EXEC commands, use the async mode interactive interface configuration command. To prevent users from implementing SLIP and PPP at the EXEC level, use the no form of this command.
async mode interactiveThis command has no arguments or keywords.
Asynchronous mode is disabled.
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
Interactive mode enables the slip and ppp EXEC commands. In dedicated mode, there is no user EXEC level. The user does not enter any commands, and a connection is automatically established when the user logs in, according to the configuration.
The following example places async interface 6 into interactive asynchronous mode:
interface async 6 async default ip address 172.31.7.51 async mode interactive ip unnumbered ethernet 0
You can use the master indexes or search online to find documentation of related commands.
To configure a line to start an ARA, PPP, or SLIP session, use the autoselect line configuration command. Use the no form of this command to disable this function on a line.
autoselect {arap | ppp | slip | during-login}
arap | Configures the Cisco IOS software to allow an ARA session to start up automatically. |
ppp | Configures the Cisco IOS software to allow a PPP session to start up automatically. |
slip | Configures the Cisco IOS software to allow a SLIP session to start up automatically. |
during-login | The username and/or password prompt is displayed without pressing the Return key. After the user logs in, the autoselect function begins. |
ARA session
Line configuration
This command first appeared in Cisco IOS Release 10.3. The no autoselect command and the during-login command first appeared in Cisco IOS Release 11.0.
This command eliminates the need for users to enter an EXEC command to start an ARA, PPP, or SLIP session.
The autoselect command configures the Cisco IOS software to identify the type of connection being requested. For example, when a user on a Macintosh running ARA selects the Connect button, the Cisco IOS software automatically starts an ARAP session. If, on the other hand, the user is running SLIP or PPP and uses the autoselect ppp or autoselect slip command, the Cisco IOS software automatically starts a PPP or SLIP session, respectively. This command is used on lines making different types of connections.
A line that does not have autoselect configured views an attempt to open a connection as noise. The router does not respond and the user client times out.
The following example enables ARA on a line:
line 3 arap enable autoselect arap
The following example enables PPP on a line:
line 7 autoselect ppp
The following example enables ARA on a line and allows logins from users with a modified CCL script and an unmodified script to log in:
line 3 arap enable autoselect arap autoselect during-login arap noguest if-needed
You can use the master indexes or search online to find documentation of related commands.
arap use-tacacs
ppp authentication chap
ppp authentication pap
ppp use-tacacs
To configure SLIP or PPP encapsulation as the default on an asynchronous interface, use the encapsulation interface configuration command. To disable encapsulation, use the no form of this command.
encapsulation {slip | ppp}
slip | Specifies SLIP encapsulation for an interface configured for dedicated asynchronous mode or DDR. |
ppp | Specifies PPP encapsulation for an interface configured for dedicated asynchronous mode or dial-on-demand routing (DDR). |
SLIP encapsulation is enabled by default.
Interface configuration
This command first appeared in Cisco IOS Release 10.3.
On lines configured for interactive use, encapsulation is selected by the user when they establish a connection with the slip or ppp EXEC command.
IP Control Protocol (IPCP) is the part of PPP that brings up and configures IP links. After devices at both ends of a connection communicate and bring up PPP, they bring up the control protocol for each network protocol that they intend to run over the PPP link such as IP or IPX. If you have problems passing IP packets and the show interface command shows that line is up, use the negotiations command to see if and where the negotiations are failing. You might have different versions of software running, or different versions of PPP, in which case you might need to upgrade your software or turn off PPP option negotiations. All IPCP options as listed in RFC 1332 are supported on asynchronous lines. Only Option 2, TCP/IP header compression, is supported on synchronous interfaces.
PPP echo requests are used as keepalive packets to detect line failure. The no keepalive command can be used to disable echo requests. For more information about the no keepalive command, refer to the chapter "IP Services Commands" in the Networking Protocols Command Reference, Part 1 and the chapter "Configuring IP Services" in the Networking Protocols Configuration Guide, Part 1.
In order to use SLIP or PPP, the Cisco IOS software must be configured with an IP routing protocol or with the ip host-routing command. This configuration is done automatically if you are using old-style slip address commands. However, you must configure it manually if you configure SLIP or PPP via the interface async command.
In the following example, async interface 1 is configured for PPP encapsulation.
router# config Configuring from terminal, memory, or network [terminal]? Enter configuration commands, one per line. End with CNTL/Z. router(config)# interface async1 router(config-if)# encapsulation ppp
You can use the master indexes or search online to find documentation of related commands.
keepalive
To configure an access list to be used for packets transmitted to and from the asynchronous host, use the ip access-group interface configuration command. To disable control over packets transmitted to or from an asynchronous host, use the no form of this command.
ip access-group access-list-number {in | out}
access-list-number | Assigned IP access list number. |
in | Defines access control on packets transmitted from the asynchronous host. |
out | Defines access control on packets being sent to the asynchronous host. |
Disabled
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
With this command in effect, the IP destination address of each packet is run through the access list for acceptability then dropped or passed.
The following example assumes that users are restricted to certain servers designated as SLIP or PPP servers, but that normal terminal users can access anything on the local network:
! access list for normal connections access-list 1 permit 172.16.0.0 0.0.255.255 ! ! access list for SLIP packets. access-list 2 permit 172.16.42.55 access-list 2 permit 172.16.111.1 access-list 2 permit 172.16.55.99 ! ! Specify the access list interface async 6 async dynamic address ip access-group 1 out ip access-group 2 in
To set IP addresses for an interface, use the ip address interface configuration command. To remove the specified addresses, use the no form of this command.
ip address address mask [secondary]
address | IP address. |
mask | Network mask for the associated IP network. |
secondary | (Optional) Specifies additional IP addresses. |
No IP addresses are specified.
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
The subnet mask must be the same for all interfaces connected to subnets of the same network. Hosts can determine subnet masks using the Internet Control Message Protocol (ICMP) Mask Request message. The Cisco IOS software responds to this request with an ICMP Mask Reply message.
You can disable IP processing on a particular interface by removing its IP address with the no ip address interface configuration command. If the router detects another host using one of its IP addresses, it will print an error message on the console.
In the example that follows, 172.16.1.27 is the primary address and 192.168.7.17 and 192.168.8.17 are secondary addresses for async interface 1:
interface async 1 ip address 172.16.1.27 255.255.255.0 ip address 192.168.7.17 255.255.255.0 secondary ip address 192.168.8.17 255.255.255.0 secondary
To enable asychronous listening, which in turn allows TCP connections to TCP port 57, enter the ip tcp async-mobility server global configuration command. Enter the no form of this command to turn listening off.
ip tcp async-mobility serverThis command has no keywords or arguments.
Disabled. Asynchronous listening is turned off.
Global Configuration
This command first appeared in Cisco IOS Release 11.2.
After asynchronous listening is turned on by the ip tcp async-mobility server command, enter the tunnel host command to establish a network layer connection to a remote host. Both commands must be used to enable asynchronous mobility.
The following example shows how to configure asynchronous mobility. The tunnel host command is used to establish a network layer connection with an IBM host called mktg.
5300# configure terminal Enter configuration commands, one per line. End with CNTL/Z. 5300(config)# ip tcp async-mobility server 5300(config)# exit 5300# %SYS-5-CONFIG_I: Configured from console by console 5300# tunnel ? WORD Address or hostname of a remote system 5300# tunnel mktg
You can use the master indexes or search online to find documentation of related commands.
tunnel
To configure TCP header compression on the asynchronous link, use the ip tcp header-compression interface configuration command. To disable header compression, use the no form of this command.
ip tcp header-compression [on | off | passive]
on | (Optional) Turns header compression on. |
off | (Optional) Turns header compression off. |
passive | (Optional) On SLIP lines, prevents transmission of compressed packets until a compressed packet arrives from the asynchronous link, unless a user specifies SLIP on the command line. For PPP, this option functions the same as the on option. |
Header compression is on.
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
Header compression data areas are initialized to handle up to 16 simultaneous TCP connections. Currently, you cannot change this number. You can only turn header compression on or off or use the passive keyword.
On lines configured for PPP encapsulation, the keywords passive and on cause the same behavior because, before attempting header compression, PPP automatically negotiates whether compression is available at each end of the connection.
There are two ways to implement header compression when the line is configured for ip tcp header-compression passive:
If a line is configured for passive header compression and you use the slip or ppp EXEC command to enter asynchronous mode, you will see that the interface is set to match the compression status used by the host at the other end of the asynchronous line.
router> slip 10.0.0.1
Password:
Entering SLIP mode.
Interface IP address is 10.0.0.1, MTU is 1500 bytes
Header compression will match your system.
The message "Header compression will match your system" indicates that the interface is set to match the compression status used by the host at the other end of the asynchronous line. If the line was configured to have header compression on, this line would read "Header compression is On."
The following example enables Van Jacobson TCP header compression. The passive keyword prevents transmission of compressed packets until a compressed packet arrives from the IP link. Notice that asynchronous routing and dynamic addressing are also enabled.
interface async 6
async dynamic routing
async dynamic address
ip tcp header-compression passive
You can use the master indexes or search online to find documentation of related commands.
ppp
slip
slip default
slip /compressed
To conserve network resources, use the ip unnumbered interface configuration command. To disable unnumbered interfaces, use the no form of this command.
ip unnumbered type number
type | Interface type. |
number | Interface number. |
Disabled
Interface configuration
This command first appeared in Cisco IOS Release 10.0.
You must use either the ip address or ip unnumbered command to provide the local address for an interface.
Unnumbered interfaces do not have an address. Network resources are conserved because fewer network numbers are used and routing tables are smaller.
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. Restrictions include the following:
The following example configures async interface 6 as unnumbered:
interface async 6ip unnumbered ethernet 0
You can use the master indexes or search online to find documentation of related commands.
To enable compression of IPX packet headers in a PPP session, use the ipx compression cipx interface configuration command. To disable compression of IPX packet headers in a PPP session, use the no form of this command.
ipx compression cipx number-of-slots
number-of-slots | Number of stored IPX headers allowed. The range is from 10 to 256. The default is 16. A slot is similar to a table entry for a complete IPX header. When a packet is received, the receiver stores the complete IPX header in a slot and tells the destination which slot it used. As subsequent CIPX packets are sent, the receiver uses the slot number field to determine which complete IPX header to associate with the CIPX packet before passing the packet up to IPX. |
No compression of IPX packets during a PPP session.
Interface configuration
This command first appeared in Cisco IOS Release 11.1.
This interface configuration command enables IPX header compression on PPP links.
The following example enables IPX header compression for PPP:
encapsulation ppp ipx compression cipx 128
You can use the master indexes or search online to find documentation of related commands.
show ipx compression
To enable a non-routing IPX client to connect to an asynchronous interface, the interface must be associated with a loopback interface configured to run IPX. To permit such connections, use the ipx ppp-client interface configuration command. To disable a non-routing IPX client, use the no form of this command.
ipx ppp-client loopback number
loopback | Loopback interface configured with a unique IPX network number. |
number | Number of the loopback interface. |
IPX client connections are not permitted over PPP.
Interface configuration
This command first appeared in Cisco IOS Release 11.1.
This command enables IPX clients to log into the router from a device running a virtual terminal protocol, then issue the PPP command at the EXEC prompt to connect to a remote device.
You must first configure a loopback interface with a unique IPX network number. The loopback interface is then assigned to an asynchronous interface, which permits IPX clients to connect to the asynchronous interface.
The following example configures IPX to run over PPP on asynchronous interface 3:
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
You can use the master indexes or search online to find documentation of related commands.
interface loopback
ipx network
Use the peer default ip address interface configuration command to specify an IP address, an address from a specific IP address pool, or an address from the DHCP mechanism to be returned to a remote peer connecting to this interface. This command sets the address used on the remote (PC) side. Use the no form of this command to disable a prior peer IP address pooling configuration on an interface.
To remove the default address from your configuration, use the no form of this command also.
peer default ip address {ip-address | dhcp | pool [poolname]}
ip-address | Specific IP address to be assigned to a remote peer dialing in to this interface. To prevent the assignment of duplicate IP addresses on two or more interfaces, this form of the command cannot be applied to a dialer rotary group nor to an ISDN interface. |
dhcp | Retrieve an IP address from the DHCP server. |
pool | Use the Global Default Mechanism as defined by the ip address-pool command unless the optional poolname is supplied. |
poolname | (Optional) Name of a local address pool created using the ip local pool command. The router retrieves an address from this pool regardless of the Global Default Mechanism setting. |
pool
Interface configuration
This command first appeared in Cisco IOS Release 11.0.
This command applies to point-to-point interfaces that support the PPP or SLIP encapsulation.
This command allows an administrator to configure all possible address pooling mechanisms on a interface-by-interface basis. The peer default ip address command can be used to override the Global Default Mechanism defined by the ip address-pool command on an interface-by-interface basis.
For all interfaces not configured with a peer default IP address mechanism (equivalent to selecting the peer default ip address pool command), the router uses the Global Default Mechanism that is defined by the ip address-pool command.
If you select the peer default ip address pool poolname command, then the router uses the locally configured pool on this interface and does not follow the Global Default Mechanism.
If you select the peer default ip address ip-address form of this command, the specified IP address is assigned to any peer connecting to this interface and any Global Default Mechanism is overridden for this interface.
If you select the peer default ip address dhcp form of this command, the DHCP proxy-client mechanism is used by default on this interface and any Global Default Mechanism is overridden for this interface.
The following example specifies address 192.31.7.51 for async interface 6:
line 20 speed 115200 interface async 6 peer default ip address 192.31.7.51
You can use the master indexes or search online to find documentation of related commands.
async dynamic address
To start an asynchronous connection using PPP, use the ppp EXEC command.
ppp {/default | {remote-ip-address | remote-name} [@tacacs-server]} [/routing]
/default | (Optional) Makes a PPP connection when a default address has been configured. |
remote-ip-address | IP address of the client workstation or PC. This parameter can only be specified if the line is set for dynamic addresses using the async address dynamic line configuration command. |
remote-name | Name of the client workstation or PC. This parameter can be specified if the line is set for dynamic addresses using the async address dynamic line configuration command. |
@tacacs-server | (Optional) IP address or IP host name of the TACACS server to which the user's TACACS authentication request is sent. |
/routing | (Optional) Indicates that the remote system is a router and that routing messages should be exchanged over the link. The line must be configured for asynchronous routing using PPP encapsulation. |
EXEC
This command first appeared in a release prior to Cisco IOS Release 10.0.
When you connect from a remote node computer to an EXEC session on the access server and want to connect from the access server to a device on the network, issue the ppp command.
If you specify an address for the TACACS server (either /default or tacacs-server), the address must be the first parameter in the command after you type ppp. If you do not specify an address or enter default, you are prompted for an IP address or host name. You can enter default at this point.
To terminate a session, disconnect from the device on the network using the command specific to that device. Then, exit from the EXEC by using the exit command.
The following example shows a line that is in asynchronous mode using PPP encapsulation (see Figure 4). The PC's name is ntpc---assuming that the name ntpc is in the Domain Naming System (DNS) so that it can be resolved to a real IP address). The PC must be running a terminal emulator program.
router> ppp ntpc@server1
To provide backward compatibility for client software scripts expecting SLIP and PPP dialogs to be formatted with software release 9.1 or earlier, use the service old-slip-prompts global configuration command. Use the no form of this command to disable this function.
service old-slip-promptsThis command has no arguments or keywords.
The prompts and information transmitted by SLIP and PPP are formatted with the current release of Cisco IOS software.
Global configuration
This command first appeared in Cisco IOS Release 11.1.
The following example shows the output of a SLIP command after service old-slip-prompts is enabled:
router# configure terminal Enter configuration commands, one per line. End with CNTL/Z. router(config)# service old-slip-prompts router(config)# exit router# slip IP address or hostname: 2.2.2.2 Entering SLIP mode. Your IP address is 2.2.2.2. MTU is 1500 bytes
To show the current status and statistics of IPX header compression during PPP sessions, use the show ipx compression EXEC command.
show ipx compression detail int-spec
detail | Shows detailed link-state database information for NLSP. |
int-spec | Interface type, as listed in Table 114. |
| Keyword | Description |
|---|---|
Async | Asynchronous interface. |
Ethernet | Ethernet IEEE 802.3 interface. |
Null | Null interface. |
Serial | WAN serial interface. |
EXEC
This command first appeared in Cisco IOS Release 11.1.
You can use the master indexes or search online to find documentation of related commands.
ipx compression cipx
show ipx interface
To start a serial connection to a remote host by using SLIP, use the slip EXEC command.
slip [/default] {remote-ip-address | remote-name} [@tacacs-server] [/routing]} [/compressed]
/default | (Optional) Makes a SLIP connection when a default address has been configured. |
remote-ip-address | IP address of the client workstation or PC. |
remote-name | Name of the client workstation or PC. |
@tacacs-server | (Optional) IP address or IP host name of the TACACS server to which your TACACS authentication request is sent. |
/routing | (Optional) Indicates that the remote system is a router. Line must be configured for asynchronous routing using SLIP encapsulation. |
/compressed | (Optional) Indicates that IP header compression should be negotiated. |
EXEC
This command first appeared in a release prior to Cisco IOS Release 10.0.
When you connect from a remote node computer to the EXEC facility on a router and want to connect from the router to a device on the network, issue the slip command.
If you specify an address for the TACACS server by using /default or tacacs-server, the address must be the first parameter in the command after you enter slip. If you do not specify an address or enter /default, you are prompted for an IP address or host name. You can enter /default at this point.
If you do not use the tacacs-server argument to specify a TACACS server for SLIP address authentication, the TACACS server specified at login (if any) is used for the SLIP address query.
To optimize bandwidth on a line, SLIP enables compression of the SLIP packets using Van Jacobson TCP header compression as defined in RFC 1144.
Your system administrator must configure the system with the ip tcp header-compression passive command for the /compressed command option to be valid in EXEC mode. The ip tcp header-compression command forces header compression on or off. The default is to not compress the packets. The configuration file must have header compression on and the slip /compressed EXEC command must be entered for header compression to occur.
To terminate a session initiated with the slip command, disconnect from the device on the network using the command specific to that device. Then, exit from the EXEC by using the exit command.
The following example makes a connection when a default IP address is assigned. Once a correct password is entered, you are placed in SLIP mode, and the IP address is displayed.
router> slip
Password: Entering SLIP mode. Your IP address is 192.31.7.28, MTU is 1524 bytes
The following example illustrates the prompts displayed and the response required when you use dynamic addressing to assign the SLIP address:
router> slip
IP address or hostname? 192.31.6.15 Password: Entering SLIP mode Your IP address is 192.31.6.15, MTU is 1524 bytes
In the preceding example, the address 192.31.6.15 has been assigned as the default. Password verification is still required before SLIP mode can be enabled.
router> slip /default
Password: Entering SLIP mode Your IP address is 192.31.6.15, MTU is 1524 bytes
The following example illustrates the implementation of header compression on the interface with the IP address 128.66.2.1:
router> slip 128.66.2.1 /compressed Password: Entering SLIP mode. Interface IP address is 128.66.2.1, MTU is 1500 bytes. Header compression will match your system.
In the preceding example, the interface is configured for ip tcp header-compression passive, which permits the user to enter the /compressed keyword at the EXEC mode prompt. The message "Header compression will match your system" indicates that the user specified compression. If the line was configured for ip tcp header-compression on, this line would read "Header compression is On."
The following example specifies a TACACS server named server1 for address authentication:
router> slip 1.0.0.1@server1 Password: Entering SLIP mode. Interface IP address is 1.0.0.1, MTU is 1500 bytes Header compression will match your system.
Posted: Mon Apr 26 16:18:23 PDT 1999
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