Adjust the TCP Maximum Segment Size

Note

Note

This procedure only applies to XA1400 Series.

Adjust the TCP maximum segment size (MSS) to improve the throughput for the TCP session over a Fabric Extend (FE) adjacency. By default, this functionality is enabled and auto-derived.

Tip

Tip

As a best practice, turn off this enhancement on the head-end side and keep this enabled only at the branch side.

About this task

The MSS adjustment functionality only activates when at least one FE tunnel with a maximum transmission unit (MTU) greater than or equal to 1500 is configured. The functionality is inactive if no FE tunnels with MTU greater than or equal to 1500 are configured. Deleting the last tunnel with MTU greater than or equal to 1500 results in the functionality becoming inactive.

The switch can auto-derive the MSS value based of the tunnel MTUs or you can manually configure the MSS value. The formula for the auto-derived value is min(Tunnels MTUs) - 250B (size for VXLAN + MIM + IPSEC + IP+TCP headers). If you configure multiple FE tunnels with MTU greater than or equal to 1500, then the switch uses the lowest of all tunnel MTUs to auto-derive the TCP MSS adjust value. The same value applies to all TCP syn packets that transmit NNI to UNI and UNI to NNI.

The switch does not support different TCP adjust MSS values if you configure different FE tunnel MTUs on different tunnels.

If you configure FE tunnels and regular NNIs on the same adjacency, then the TCP adjust MSS value applies to all TCP packets traversing across regular NNIs and FE tunnels.

Procedure

  1. Enter IS-IS Router Configuration mode:

    enable

    configure terminal

    router isis

  2. Use one of the following commands to configure the MSS adjustment functionality as required:
    1. Configure an explicit MSS adjust value:

      ip tcp adjust-mss mss <500-1250>

    2. Disable MSS adjustment explicitly:

      no ip tcp adjust-mss

    3. Disable a configured MSS adjustment value and return to the auto-derived value:

      no ip tcp adjust-mss mss

  3. Verify the configuration:

    show isis tcp adjust-mss

Example

Configure an MSS value of 1100 and verify the configuration.

Switch:1>enable
Switch:1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch:1(config)#router isis
Switch:1(config-isis)#ip tcp adjust-mss mss 1100
Switch:1>show isis tcp adjust-mss
================================================================================
                              ISIS TCP Adjust MSS
================================================================================
ENABLE            STATUS            TCP MSS                   TCP MSS
                                    TYPE                      VALUE
--------------------------------------------------------------------------------
TRUE              ACTIVE            MANUAL-CONFIG             1100
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