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) > [email protected] > > _______________________________________________ > cisco-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/cisco-nsp > archive at http://puck.nether.net/pipermail/cisco-nsp/
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