[c-nsp] BGP MSS=576 bytes

[Gergely Antal]

is "ip mtu 1500" set on the interfaces?

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
> example:
> 
> ++++++++++++++++++++++++++
> 6500>sh ip bgp neighbors x.x.x.x
> 
> ...
> 
> Datagrams (max data segment is 536 bytes):
> 
> Rcvd: 439340 (out of order: 252), with data: 406672, total data bytes: 94316052
> 
> Sent: 296303 (retransmit: 727), with data: 35046, total data bytes: 994215
> 
> 6500>
> ++++++++++++++++++++++++++
> 
> The documentation says that PMTUD is enabled by default so this should not be happening:
> 
> ++++++++++++++++++++++++++
> BGP Neighbor Session TCP PMTUD
> 
> TCP path MTU discovery is enabled by default for all BGP neighbor sessions, but there are situations when you may want to disable
> TCP path MTU discovery for one or all BGP neighbor sessions. While PMTUD works well for larger transmission links (for example,
> Packet over Sonet links), a badly configured TCP implementation or a firewall may slow or stop the TCP connections from forwarding
> any packets. In this type of situation, you may need to disable TCP path MTU discovery. In Cisco IOS Release 12.2(33)SRA,
> 12.2(31)SB, 12.2(33)SXH, 12.4(20)T, Cisco IOS XE Release 2.1, and later releases, configuration options were introduced to permit
> TCP path MTU discovery to be disabled, or subsequently reenabled, either for a single BGP neighbor session or for all BGP sessions.
> To disable the TCP path MTU discovery globally for all BGP neighbors, use the no bgp transport path-mtu-discovery command under
> router configuration mode. To disable the TCP path MTU discovery for a single neighbor, use the no neighbor transport
> path-mtu-discovery command under router or address family configuration modes. 
> ++++++++++++++++++++++++++
> 
> I have for example a direct eBGP peering over TenGiga interfaces where i see the same problem:
> 
> ++++++++++++++++++++++++++
> 6500>sh int tenGigabitEthernet x/x | inc MTU
>   MTU 1500 bytes, BW 10000000 Kbit, DLY 10 usec, 
> 6500>
> 6500>
> 6500>sh ip int tenGigabitEthernet x/x | inc MTU
>   MTU is 1500 bytes
> 6500>
> ++++++++++++++++++++++++++
> 
> 
> 
> Any explanation to this strange behavior ?
> 
> 
> Thanks.
> 
> Regards,
> 
> Antonio Soares, CCIE #18473 (R&S)
> amsoares at netcabo.pt
> 
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