Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

iBGP session's MSS misalignment

Dear all,

I have two Cisco 7609S (R1 runs 15.2(4)S1 and R2 runs 15.3(3)S1) running iBGP between them, apart from OSPF as IGP, and connected by a STM4 SONET link. After the IOS upgrade of R2 from 15.2(4)S1 to 15.3(3)S1, I saw that iBGP session's MSSs  were misaligned even though PMTUD discovery is enabled by default.

I get the following outputs:

R2#sh ip bgp neighbors [R1's ip address]

....

  BGP state = Established, up for 1w0d

...

  Connections established 1; dropped 0

  Last reset never

  Transport(tcp) path-mtu-discovery is enabled

  Graceful-Restart is enabled, restart-time 120 seconds, stalepath-time 360 seconds

...

SRTT: 1000 ms, RTTO: 1003 ms, RTV: 3 ms, KRTT: 0 ms

minRTT: 120 ms, maxRTT: 1000 ms, ACK hold: 200 ms

Status Flags: active open

Option Flags: nagle, path mtu capable

IP Precedence value : 6

Datagrams (max data segment is 4406 bytes):

...

R1#sh ip bgp neighbors [R2's ip address]

...

BGP state = Established, up for 1w0d

...

Connections established 2; dropped 1

  Last reset 1w0d, due to Active open failed

Transport(tcp) path-mtu-discovery is enabled

  Graceful-Restart is enabled, restart-time 120 seconds, stalepath-time 360 seconds

...

SRTT: 320 ms, RTTO: 323 ms, RTV: 3 ms, KRTT: 0 ms

minRTT: 120 ms, maxRTT: 976 ms, ACK hold: 200 ms

Status Flags: passive open, gen tcbs

Option Flags: nagle

Datagrams (max data segment is 556 bytes):

...

As you can see:

  • both routers have PMTUD enabled to find the biggest MSS previously to BGP negotiation
  • however, R2 shows an MSS of 4406 bytes and R1 only 556 bytes
  • additionaly, R1 doesn't show "path mtu capable" among the "Option Flags"

STM4 interfaces IP MTU is 4470 bytes in both sides of the link and there is no bottleneck in the link that blocks of fragment packets of that size.

From my point of view there has to be with the path mtu discovery, but I'm not sure.

I would really appreciate any comment to help me to understand why this misalignment takes place.

Thanks in advance

Regards

Octavio

156
Views
0
Helpful
0
Replies
CreatePlease login to create content