You can adjust the TCP maximum segment size (MSS) to improve the throughput for the TCP session over a Fabric Extend (FE) adjacency.
When a client initiates a connection with a server, it uses TCP SYN packets to negotiate the MSS to avoid fragmentation. The client and server use the outgoing maximum transmission unit (MTU) to advertise the MSS.
If a tunnel exists between the client and server, the encapsulation consumes more room in the outer IP header. As a result, the router that performs the tunnel encapsulation fragments the packet to fit over the tunnel. Adjust the MSS to modify the value in the TCP SYN packet so the client and server negotiate a lower number and leave headroom for tunnel encapsulation.
This adjustment functionality applies to IPv4 only.
Important
If you enable this functionality and port mirroring simultaneously, the switch does not mirror CP-generated packets.
TCP MSS adjustment applies unidirectionally when a packet is forwarded from a UNI interface to any other interface. To use this functionality, you must enable TCP MSS adjustment on both sides of the FE tunnel.
TCP MSS adjustment applies unidirectionally when a packet is forwarded into an FE tunnel. To use this functionality, you must enable TCP MSS adjustment on both sides of the FE tunnel.
The MSS adjustment functionality only activates when at least one Fabric Extend (FE) tunnel with a MTU less than or equal to 1500 is configured. The functionality is inactive if no FE tunnels with MTU less than or equal to 1500 are configured. Deleting the last tunnel with MTU less than or equal to 1500 results in the functionality becoming inactive.
min(Tunnels MTUs) - 200B (size for VXLAN + MIM + IPSEC + IP+TCP headers)
If you configure multiple FE tunnels with MTU less 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 bidirectionally, to all TCP syn packets that transmit NNI to UNI and UNI to NNI.
Tip
As a best practice, disable this enhancement on the head-end side and enable only at the branch side.
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.