ip tcp adjust-mss

[UthayakumarNetp]

Hi All,

Please suggest to me. Right now i'm observing bgp hold timer expired on my router.

Current config:

ip mtu 1476

ip tcp adjust-mss 1460

If i changes the Mtu mss size it will works or not (any suggestion) 

myRouter(config-if)# ip mtu 1476

myRouter(config-if)# ip tcp adjust-mss 1436 ------------>what is the exact value (For MTU 1467)

Mail@ uthaya.network7@gmail.com

[Abac]

TCP MSS is the interface MTU minus the IP header and minus the TCP header. So assuming no IP options (a reasonable assumption) then in your case it would be 1476 - 20 -20 = 1436.

[Joseph W. Doherty]

Abac correctly provides the usual less 40 bytes for setting adjust-mss, but if the device itself is the end host and it has a smaller MTU adjust-mss shouldn't be needed (as the TCP session should auto adjust its MSS during TCP setup - i.e. adjust-mss is used for transit traffic - where both end hosts have a larger MTU than some transit device).

[Richard Burts]

Joseph makes a good point that adjust-mss is useful if the router where it is configured is in the transit path from some source to some destination. But not very useful to apply it if the router is the source.

I believe that there is a more fundamental question to address. The original post identifies the particular issue as this

bgp hold timer expired

I have trouble thinking of a scenario where bgp hold timer issue would be related to mss. We do not have enough information to be able to give good advice about the bgp hold timer. As a start can we learn what model of device is this? What version of code is it running? Can we see at least the configuration of the interfaces used and of the BGP configuration? Are we sure that there is good IP connectivity between the BGP peering addresses?

HTH

Rick