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This chapter presents information and configuration tasks required to prepare a serial interface for such uses as ISDN PRI signalling, for example, or to configure dial-on-demand routing (DDR). It includes the following main sections:
For a complete description of the synchronous interface commands in this chapter, see the Cisco IOS Dial Services Command Reference publication. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Synchronous serial ports can be used for leased-line or dialup communications. In addition, high-speed lines (E1 or T1) can be configured to support multiple serial interfaces that can themselves be configured as if they were attached to dialup lines. Dialers can be configured on synchronous serial lines to support DDR.
See the section "Relationship Between Lines and Interfaces" in the chapter "Interfaces, Controllers, and Lines Used for Dial Access Overview" for more information about Cisco serial interfaces.
See the chapter "Configuring ISDN PRI and Other Signalling on E1 and T1 Lines" for information about configuring ISDN PRI lines.
For information about dialup uses of the serial interfaces, see the chapters in the "Dial-on-Demand Routing" part of this manual. For protocol-specific routing configuration tasks, see the relevant routing protocol chapters in other volumes of the Cisco IOS software configuration guides.
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Note As of Cisco IOS Release 11.3, all commands supported on the Cisco 7500 series are also supported on the Cisco 7000 series. |
To configure a synchronous serial interface, perform the tasks in the following sections. The first task is required; the remaining tasks are optional.
For examples of synchronous serial interface configuration, see the section "Synchronous Serial Interface Configuration Examples" at the end of this chapter.
To specify a synchronous serial interface and enter interface configuration mode, use one of the following commands in global configuration mode:
| Command | Purpose |
|---|---|
interface serial number | Specifies an interface and enter interface configuration mode. |
See the section "Basic Serial Interface Configuration" later in this chapter for a configuration example.
Encapsulation methods are set according to the type of protocol or application you configure in the CiscoIOS software. By default, synchronous serial lines use the High-Level Data Link Control (HDLC) serial encapsulation method, which provides the synchronous framing and error detection functions of HDLC without windowing or retransmission. Synchronous serial interfaces support the following serial encapsulation methods for dial solutions:
In addition, synchronous serial interfaces support the following encapsulation methods that are discussed in other books in the Cisco IOS software documentation set:
These encapsulation methods are defined in their respective books and chapters describing the protocols or applications. Serial encapsulation methods are also discussed in the Cisco IOS Configuration Interface Command Reference publication under the encapsulation command.
To define the encapsulation method for dial solutions, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
encapsulation {hdlc | ppp}
| Configures synchronous serial encapsulation for dial solutions. |
The default synchronous serial encapsulation is HDLC.
To configure PPP (including PPP compression), see the chapters in the "PPP Configuration" part of this publication.
You can configure point-to-point software compression on serial interfaces that use HDLC encapsulation. Compression reduces the size of an HDLC frame via lossless data compression. The compression algorithm used is a Stacker (LZS) algorithm.
Compression is performed in software and might significantly affect system performance. We recommend that you disable compression if central processor unit (CPU) load exceeds 65 percent. To display the CPU load, use the show process cpu EXEC command.
If the majority of traffic is already compressed files, do not use compression.
To configure compression over HDLC, use the following commands in interface configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | encapsulation hdlc | Enables encapsulation of a single protocol on the serial line. |
Step2 | compress stac | Enables compression. |
| Command | Purpose |
|---|---|
crc size | Sets the length of the CRC. |
All FSIP interface types on the Cisco 7000 series with RSP7000 support nonreturn to zero (NRZ) and nonreturn to zero inverted (NRZI) format. This is a line-coding format that is required for serial connections in some environments. NRZ encoding is most common. NRZI encoding is used primarily with EIA/TIA-232 connections in IBM environments.
The default configuration for all serial interfaces is NRZ format. The default is no nrzi-encoding. To enable NRZI format, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
nrzi-encoding | Enables NRZI encoding format. |
To enable the internally generated clock on a serial interface when a DTE device does not return a transmit clock, use the following interface configuration command on the Cisco 7000 series with RSP7000:
| Command | Purpose |
|---|---|
transmit-clock-internal | Enables the internally generated clock on a serial interface. |
Delays between the serial clock transmit external (SCTE) clock and data transmission indicate that the transmit clock signal might not be appropriate for the interface rate and length of cable being used. Different ends of the wire may have variances that differ slightly. To invert the clock signal to compensate for these factors, use the following command in interface configuration mode on a Cisco7000 series router with RSP7000, and Cisco7200 and Cisco 7500 series routers:
| Command | Purpose |
|---|---|
invert-txclock | Inverts the clock signal on an interface. |
It is possible to send back-to-back data packets over serial interfaces faster than some hosts can receive them. You can specify a minimum dead time after sending a packet to alleviate this condition. This setting is available for serial interfaces on the MCI and SCI interface cards and for the High-Speed Serial Interface (HSSI) or MultiChannel Interface Processor (MIP). Use one of the following interface configuration commands, as appropriate, for your system to set the transmit delay:
| Command | Purpose |
|---|---|
transmitter-delay microseconds | Sets the transmit delay on the MCI and SCI synchronous serial interfaces. |
transmitter-delay hdlc-flags | Sets the transmit delay on the HSSI or MIP. |
| Command | Purpose |
|---|---|
pulse-time seconds | Configures DTR signal pulsing. |
This task applies to Quad Serial network interface modules on the Cisco 4000 series routers and Hitachi-based serial interfaces on the Cisco2500 series and Cisco 3000 series routers.
By default, when the serial interface is operating in DTE mode, it monitors the Data Carrier Detect (DCD) signal as the line up/down indicator. By default, the attached DCE device sends the DCD signal. When the DTE interface detects the DCD signal, it changes the state of the interface to up.
In some configurations, such as an SDLC multidrop environment, the DCE device sends the data set ready (DSR) signal instead of the DCD signal, which prevents the interface from coming up. To tell the interface to monitor the DSR signal instead of the DCD signal as the line up/down indicator, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
ignore-dcd | Configures the serial interface to monitor the DSR signal as the line up/down indicator. |
To configure the clock rate for the connector hardware of the serial interface to an acceptable bit rate, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
clock rate bps | Configures the clock rate on serial interfaces. |
See the section "Serial Interface in DCE Mode" for an example of how to configure DCE mode.
On Cisco 4000 series routers, you can specify the serial NPM timing signal configuration. When the board is operating as a DCE and the DTE provides terminal timing (SCTE or TT), you can configure the DCE to use SCTE from the DTE. When running the line at high speeds and long distances, this strategy prevents phase shifting of the data with respect to the clock.
To configure the DCE to use SCTE from the DTE, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
dce-terminal-timing enable | Configures the DCE to use SCTE from the DTE. |
When the board is operating as a DTE device, you can invert the transmit clock (TXC) signal that it receives from the DCE, which the DTE uses to send data. Invert the clock signal if the DCE cannot receive SCTE from the DTE, the data is running at high speeds, and the transmission line is long. Again, this prevents phase shifting of the data with respect to the clock.
To configure the interface so that the router inverts the TXC clock signal, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
dte-invert-txc | Specifies timing configuration to invert TXC clock signal. |
This section describes the optional tasks for configuring a G.703 serial interface:
Interfaces that meet the G.703 electrical and mechanical specifications operate at E1 data rates (2.048Mbps).
See the section "G.703 Serial Interface Configuration" for a configuration example.
| Command | Purpose |
|---|---|
timeslot start-slot - stop-slot | Enables framed mode, and sets the range of time slot start and stop times. |
To restore the default, use the no form of this command or set the starting time slot to 0.
By default, the G.703 CRC4 (cyclic redundancy check) is not generated. To enable generation of the G.703 CRC4, which is useful for checking data integrity while operating in framed mode, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
crc4 | Enables CRC4 generation. |
By default, time slot 16 is used for signalling. It can also be used for data. To control the use of time slot 16 for data, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
ts16 | Specifies that time slot 16 is used for data. |
A G.703 interface can clock its sent data from either its internal clock or from a clock recovered from the receive data stream of the line. By default, the clock source is the receive data stream of the line. To control which clock is used, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
clock source {line | internal} | Specifies the clock used for sent data. |
This section contains the following examples:
For more information about synchronous serial interfaces configured on channelized E1/T1 and used for dialing, see the chapter "Configuring ISDN PRI and Other Signalling on E1 and T1 Lines" later in this publication.
The following example illustrates how to begin interface configuration on a serial interface. It assigns PPP encapsulation to serial interface 0.
interface serial 0 encapsulation ppp
The same example on a Cisco 7500 requires the following commands:
interface serial 1/0 encapsulation ppp
The following example configures a serial interface for DCE mode on a Cisco 7500 series. Because the DTE does not return the SCTE signal, the transmit-clock-internal is needed.
interface serial 0/0 ip address 170.1.8.2 255.255.255.0 clockrate 72000 transmit-clock-internal
The following example configures a serial interface for DCE mode on a Cisco 4000 series router. Because the DTE does not return the SCTE signal, the dce-terminal-timing-enable command is needed. In this example, the default NRZ encoding and 16-bit cyclic redundancy check (CRC) are accepted.
interface serial 1 clockrate 72000 dce-terminal-timing-enable nrz-encoding
The following example shows a configuration for serial interface 1/0/0 on a PA-2JT2 serial port adapter in a Cisco 7500 series router. In this example, the interface is configured to clock data using an internal clock source rather than the default line-derived clock source and to allow the frame alignment search criteria to use CRC5.
interface serial 1/0/0 ip address 1.1.1.10 255.255.255.0 clock source internal crc bits 5 no shutdown
The following example inverts data on serial interface 3/1/0:
interface serial 3/1/0 invert data
The following example inverts the clock signal on serial interface 3/0:
interface serial 3/0 invert txclock
The following example specifies NRZI mark encoding for serial interface 4/0/2:
interface serial 4/0/2 nrzi-encoding mark
The following example shows a basic configuration for serial interface 9/1/3 on an E1-G.703/G.704 serial port adapter in a Cisco 7500 series router. In this example, the interface is configured for framed (G.704) operation, and time slot 16 is used for data.
interface serial 9/1/3 ip address 1.1.1.10 255.255.255.0 no keepalive no fair-queue timeslot 1-31 crc4 ts16
The following example shuts down a T1 circuit number 23 running on a Cisco 7500 series router:
interface serial 4/0:23 shutdown
The following example shuts down the entire T1 line physically connected to a Cisco 7500 series router:
controller t1 4/0 shutdown
Posted: Tue Jul 18 13:28:56 PDT 2000
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