[c-nsp] RES: BGP MSS=576 bytes

[Tony Varriale]

I always enable this command (as well as a couple others) explicitly when 
doing BGP.

I'm looking at a 6500 with SHX1 right now and the command is in there...as 
in not default.

I also show all peers with a MSS of 1440 (2 internal, 2 external/ISP).

tv
----- Original Message ----- 
From: "Leonardo Gama Souza" <leonardo.souza at nec.com.br>
To: "cisco-nsp" <cisco-nsp at puck.nether.net>
Sent: Wednesday, February 11, 2009 10:09 AM
Subject: [c-nsp] RES: BGP MSS=576 bytes


> Wasn't it supposed to be enabled by default for all BGP neighbors?
> I think that's the point...
>
> -----Mensagem original-----
> De: cisco-nsp-bounces at puck.nether.net
> [mailto:cisco-nsp-bounces at puck.nether.net] Em nome de Jared Mauch
> Enviada em: quarta-feira, 11 de fevereiro de 2009 13:49
> Para: Gergely Antal
> Cc: Antonio M. Soares; cisco-nsp at puck.nether.net
> Assunto: Re: [c-nsp] BGP MSS=576 bytes
>
> You want to enable
>
> 'ip tcp path-mtu-discovery'
>
> globally.  This will allow it to scale outside the default 536,
> and
> if you are using jumbo mtu, may cause significantly reduced convergence
> times since it takes fewer packets to send those bgp updates.
>
> - Jared
>
>
> On Wed, Feb 11, 2009 at 04:46:17PM +0100, Gergely Antal wrote:
>> 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
>> >
>> > _______________________________________________
>> > cisco-nsp mailing list  cisco-nsp at puck.nether.net
>> > https://puck.nether.net/mailman/listinfo/cisco-nsp
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>>
>
>
>
>> _______________________________________________
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>
> -- 
> Jared Mauch  | pgp key available via finger from jared at puck.nether.net
> clue++;      | http://puck.nether.net/~jared/  My statements are only
> mine.
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