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
As a best practice, turn off this enhancement on the head-end side and keep this enabled only at the branch side.
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.
enable
configure terminal
router isis
ip tcp adjust-mss mss <500-1250>
no ip tcp adjust-mss
no ip tcp adjust-mss mss
show isis tcp adjust-mss
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