Configuring Serial Interfaces

A serial interface uses a serial line protocol—such as EIA-530, X.21, RS-449/422, RS-232, or V.35—to control the transmission of signals across the interface. You do not need to explicitly configure the serial line protocol, because it is automatically detected by the Juniper Networks device based on the cable plugged into the serial interface.

To configure properties on a serial interface:

  1. From the configuration page, as shown in Figure 1, select the serial interface you want to configure.

    The properties you can configure on a serial interface are displayed, as shown in Figure 9.

    Figure 9: Serial Interfaces Configuration Page

    Serial Interfaces Configuration Page
  2. Enter information into the configuration page, as described in Table 12.
  3. Click one of the following buttons:
    • To apply the configuration and stay on the Configure Interfaces page, click Apply.
    • To apply the configuration and return to the configuration page that lists all interfaces, click OK.
    • To cancel your entries and return to the Configure Interfaces page, click Cancel.

Table 12: Serial Configuration Summary



Your Action

Logical Interfaces

Add logical interfaces

Defines one or more logical units that you connect to this physical serial interface. You must define at least one logical unit for a serial interface. You can define multiple units if the encapsulation type is Frame Relay.

Click Add.

Logical Interface Description

(Optional) Describes the logical interface.

Type a text description of the logical interface to more clearly identify it in monitoring displays.

IPv4 Addresses and Prefixes

Specifies one or more IPv4 addresses for the interface.

  1. Click Add.
  2. Type one or more IPv4 addresses and prefixes. For example:

  3. Click OK.

Physical Interface Description

(Optional) Adds supplementary information about the physical serial interface.

Type a text description of the serial interface to more clearly identify it in monitoring displays.

MTU (bytes)

Specifies the maximum transmission unit (MTU) size for a serial interface.

Type a value between 256 and 9192 bytes. The default MTU for serial interfaces is 1504.

Per Unit Scheduler

Enables scheduling on logical interfaces.

Allows you to configure multiple output queues on a logical interface and associate an output scheduler and shaping rate with the queues.

  • To enable scheduling, select the check box.
  • To disable scheduling, clear the check box.


Specifies the encapsulation type for traffic on the interface.

From the list, select the encapsulation for this serial interface:

  • PPP
  • Frame Relay
  • Cisco HDLC

Enable CHAP

Enables or disables CHAP authentication on a serial interface with PPP encapsulation only.

  • To enable CHAP, select the check box.
  • To disable CHAP, clear the check box.
CHAP Local Identity (available if CHAP is enabled)

Use System Host Name

Specifies that the serial interface use the device's system hostname in CHAP challenge and response packets.

  • To enable, select the check box (the default).
  • To disable, clear the check box.

Local Name

If Use System Host Name is disabled, specifies the local name for CHAP to use.

Type a local name for this serial interface.

CHAP Peer Identity

Identifies the client or peer with which the device communicates on this serial interface.

Type the CHAP client name.

CHAP Secret

Specifies the secret password for CHAP authentication, known to both sides of the connection.

Type a password that is known to the other side of the connection. Use a combination of letters and numbers that is difficult for others to guess.

Serial Options

Clocking Mode

Specifies the clock source to determine the timing on serial interfaces.

If you use an externally timed clocking mode—dce or loop—long cables might introduce a phase shift of DTE-transmitted clock and data. At high speeds, this phase shift might cause errors.

Inverting the transmit clock corrects the phase shift, thereby reducing error rates. By default, the transmit clock is not inverted. To invert the transmit clock, do either of the following:

  • In the J-Web configuration editor, set the Transmit clock value to invert on the Interfaces>interface-name> Serial options page.
  • In the CLI configuration editor, include the transmit-clock invert statement at the [edit interfaces se-pim/0/port serial-options] hierarchy level.

From the list, select one of the following timing sources:

  • dce—Uses a transmit clock generated by the data circuit-terminating equipment (DCE) for the device's DTE.
  • internal—Uses the device's internal clock.
  • loop—Uses the DCE's or DTE's receive clock (the default).

For X.21 serial interfaces, you must use the loop clocking mode.

When the device is functioning as DTE, you must use the dce clocking mode for all interfaces except X.21 serial interfaces.

When the device is functioning as DCE, we recommend using the internal clocking mode for all interfaces.

Clock Rate

Note: RS-232 serial interfaces cannot function error-free with a clock rate greater than 200 KHz.

Specifies the line speed, in kilohertz or megahertz, for serial interfaces that use the DTE clocking mode.

From the list, select one of the following clock rates:

  • 1.2 KHz
  • 2.4 KHz
  • 9.6 KHz
  • 19.2 KHz
  • 38.4 KHz
  • 56.0 KHz
  • 64.0 KHz
  • 72.0 KHz
  • 125.0 KHz
  • 148.0 KHz
  • 250.0 KHz
  • 500.0 KHz
  • 800.0 KHz
  • 1.0 MHz
  • 1.3 MHz
  • 2.0 MHz
  • 4.0 MHz
  • 8.0 MHz