[c-nsp] BGP MSS=576 bytes

[Oliver Boehmer (oboehmer)]

Antonio Soares <> wrote on Thursday, February 19, 2009 18:38:

> This is what i got with 12.2(18)SXF15a and "ip tcp
> path-mtu-discovery" enabled: 
> 
> 6500#sh ip bgp neighbors 1.1.1.1 | inc (path-mtu|max data)
> Datagrams (max data segment is 536 bytes):
> 6500#
> 6500#
> 6500#sh ip bgp neighbors 10.10.10.1 | inc (path-mtu|max data)
> Datagrams (max data segment is 1460 bytes):
> 6500#
> 
> MSS=1460 for directly connected peerings and MSS=536 for non-directly
> connected peerings. Got the same behavior for iBGP and eBGP. 

did you enable it on both ends and did you reset the session?

> So basically it didn't work as i was expecting. It seems SXI puts
> some order here. 
 
As far as I know, SXI allows to enable/disable PMTUD per BGP neighbor,
and defaults to PMTUD being enabled..

	oli


> 
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Phil Mayers 
> Sent: domingo, 15 de Fevereiro de 2009 11:53
> To: Antonio M. Soares
> Cc: cisco-nsp at puck.nether.net
> Subject: Re: [c-nsp] BGP MSS=576 bytes
> 
> Antonio M. Soares wrote:
>> Hello group,
>> 
>> I have a 6500 running 122-18.SXF7 with lots of BGP peers and all of
>> the BGP sessions have negotiated a MSS of 536 bytes. Here's an
> 
> I think you need a newer IOS. Certainly under SXI I see:
> 
> ac-core#sh ip bgp neighbors  | inc path-mtu|max data seg
>    Transport(tcp) path-mtu-discovery is enabled Datagrams (max data
>    segment is 536 bytes): Transport(tcp) path-mtu-discovery is
>    enabled Datagrams (max data segment is 1460 bytes): Transport(tcp)
>    path-mtu-discovery is enabled Datagrams (max data segment is 9060
> bytes): Transport(tcp) path-mtu-discovery is enabled Datagrams (max
> data segment is 536 bytes):  
> 
> ...depending on whether the neighbor is similarly equipped to be
> jumbo-framed and PMTU. _______________________________________________
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