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Example: Impose a Minimum MTU Setting

The maximum transmission unit (MTU) is the greatest amount of data or packet size (in bytes) that can be transferred in one physical frame on a network. In this example, a commit script tests the MTU of SONET/SDH interfaces. If the MTU is less than a specified minimum value, the commit script reports the error and causes the commit operation to fail.


This example uses a device running Junos OS with SONET/SDH interfaces.

Overview and Commit Script

This example tests the MTU of SONET/SDH interfaces, reports when the MTU is less than the value of the min-mtu parameter, here set to 2048, and causes the commit operation to fail. The for loop selects all SONET/SDH interfaces that start with so- and that have an MTU statement that is defined and less than the value of min-mtu. For the selected interfaces, the script generates an error, which includes the location of the interface in the configuration hierarchy and the MTU configured for that interface.

The example script is shown in both XSLT and SLAX syntax:

XSLT Syntax

SLAX Syntax



Step-by-Step Procedure

To download, enable, and test the script:

  1. Copy the script into a text file, name the file so-mtu.xsl or so-mtu.slax as appropriate, and copy it to the /var/db/scripts/commit/ directory on the device.

  2. Select the following test configuration stanzas, and press Ctrl+c to copy them to the clipboard.

    If you are using the SLAX version of the script, change the filename at the [edit system scripts commit file] hierarchy level to so-mtu.slax.

  3. In configuration mode, issue the load merge terminal command to merge the stanzas into your device configuration.

    1. At the prompt, paste the contents of the clipboard by using the mouse and the paste icon.

    2. Press Enter.

    3. Press Ctrl+d.

  4. Commit the configuration.


Verifying the Commit Script Output


Verify that the script behaves as expected.


Review the output of the commit command. The sample configuration stanzas configure two SONET/SDH interfaces so-1/2/2 and so-1/2/3. The so-1/2/3 interface is configured with an MTU of 576, so the script generates an error message, and the commit operation fails. The following output appears after issuing a commit command: