[c-nsp] BGP MSS=576 bytes

[Paul Cosgrove]

TCP sessions normally use 536 if they are established between IPs which 
are not directly connected.  You may see the same on MSDP peerings.  
Enabling Path MTU Discovery allows the actual end to end MSS to be 
determined, provided the ICMP type 3 code 4 messages are not blocked 
along the way.

Paul.

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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