[c-nsp] Which is the actual TCP MSS for this iBGP connection on ASR9K running IOS-XR?

[Everton Marques]

On Thu, Aug 25, 2016 at 7:57 AM, James Bensley <jwbensley at gmail.com> wrote:

> On 19 August 2016 at 13:38, Everton Marques <everton.marques at gmail.com>
> wrote:
> > Hi all,
> >
> > I have this iBGP session established between a Cisco ASR9K and a Cisco
> 7600
> > router.
> >
> > On the ASR9K device, 'show tcp detail pcb all' displays 'MSS 1240, peer
> MSS
> > 9152, min MSS 1240, max MSS 1240' for this specific iBGP connection.
>
> What is the MTU on the egress interface on the 7600 towards the ASR9K,
> and vice versa what is the egress interface MTU on the ASR9K facing
> the 7600?
>

ASR9K#sh ip int TenGigE0/1/0/2 | i MTU
  MTU is 9216 (9202 is available to IP)

7600#sh ip int Vlan6 | i MTU
  MTU is 9216 bytes

Those boxes are not directly attached to each other, as there is an
intermediate router between them.


> I assume you have a lower MTU on the ASR9K based on the above, they
> should both agress on the lower MSS of the two device so the MSS here
> should be 1240.
>

It seems the 1240 size is related to PMTUD being disabled:
https://supportforums.cisco.com/discussion/12029911/jumbo-frame-support-bgp-mss-size

What I am exactly trying to find out is which value each device is actually
using to limit its own outgoing TCP segments.
My understanding is that both routers should be limiting their TCP segments
to 1240.
However, is the ASR9K limiting its own outgoing TCP segments to 1240 or to
9152?

I have found this past thread over the same issue that did answer the
question:
http://www.gossamer-threads.com/lists/cisco/nsp/186300

Thanks,
Everton