Configuring ip tcp adjust-mss on ASR-920-12CZ-A

[dcrozier7]

I have a need to adjust the tcp mss value to support GRE tunnels on ASR-920-12CZ-A running 16.12.05. I've configured 

ip tcp adjust-mss 

1436 on the egress interface, but I can see in a packet capture that the value is still 1460. I've verified that internet bound traffic does traverse this interface, and I also run this through a CSR1000v in CML and it worked as it should. Are there any device limitations that I'm running into? I'm going crazy trying to get this working. I've looked through many articles and forum posts, and haven't seen anything relevant aside from how the command/function is supposed to work. 

Any help would be greatly appreciated. 

[Joseph W. Doherty]

Ah, didn't understand there was a third party device that ingress traffic was being filtered through!

Unclear, to me, why there's a GRE tunnel from the device to customer router.  Perhaps to verify Internet traffic has gone through the Cloudflare device?

In any case, what you describe you're now doing, using

adjust-mss

on both the physical interface and virtual interfaces, would seem to be the correct approach.  (My recommendation for just setting

adjust-mss

on the virtual interface, assumed, all traffic flows, using the tunnel, used it bidirectionally.)

The bug you referenced, does seem to correspond to what you also noted in your OP.

Assuming it is a bug, w/o a useful work around, the question then remains, how soon for Cisco to correct?  If their timeline is longer than your need, your quickest "fix" would be to use another IOS version or router w/o this bug, or another physical device "inline" with your ingress/egress traffic, in such a manner, it can effectively apply

adjust-mss.

[dcrozier7 · Accepted solution]

Thanks for all of the input. I finally got confirmation from Cisco TAC and their Business Unit, this feature/command is not supported by the ASR-920 and is not on the roadmap to be implemented. So we will be eventually replacing these with another router at some point, meanwhile we've applied the MSS change to the endpoints which is less than ideal but it works for now. 

[Joseph W. Doherty]

Thanks for the update.  I.e. it's not a bug, it's a feature (or the lack of one).