[c-nsp] 6500, SUP720, 12.2(33)SXI3, BGP path-mtu flap

[Tim Durack]

6500, SUP720, 12.2(33)SXI3.

Couple of iBGP neighbors keep flapping on one box. Looks like a
path-mtu issue, yet the discovered path looks correct:

sh bgp ipv4 unicast summary | i Neighbor|10.116.0.1
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ
Up/Down  State/PfxRcd
10.116.0.1      4       65116  165937  690347   268786    0   61
00:00:50        0

path-mtu discovers 9116:

  Address tracking is enabled, the RIB does have a route to 10.116.0.1
  Connections established 3937; dropped 3936
  Last reset 00:02:22, due to BGP Notification sent, hold time expired
  External BGP neighbor may be up to 255 hops away.
  Transport(tcp) path-mtu-discovery is enabled
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Mininum incoming TTL 0, Outgoing TTL 255
Local host: 10.1.0.1, Local port: 46013
Foreign host: 10.116.0.1, Foreign port: 179

Enqueued packets for retransmit: 2, input: 0  mis-ordered: 0 (0 bytes)

Event Timers (current time is 0x146C10A6C):
Timer          Starts    Wakeups            Next
Retrans             9          5     0x146C121A4
TimeWait            0          0             0x0
AckHold             1          0             0x0
SendWnd             0          0             0x0
KeepAlive           0          0             0x0
GiveUp              0          0             0x0
PmtuAger            1          0             0x0
DeadWait            0          0             0x0

iss: 3884779463  snduna: 3884779560  sndnxt: 3884788712     sndwnd:  16288
irs: 1703625207  rcvnxt: 1703625304  rcvwnd:      16288  delrcvwnd:     96

SRTT: 99 ms, RTTO: 1539 ms, RTV: 1440 ms, KRTT: 49248 ms
minRTT: 8 ms, maxRTT: 300 ms, ACK hold: 200 ms
Flags: higher precedence, nagle, path mtu capable, path mtu discovery

Datagrams (max data segment is 9116 bytes):
Rcvd: 4 (out of order: 0), with data: 2, total data bytes: 96
Sent: 5 (retransmit: 5), with data: 3, total data bytes: 9248

Pinging between loopbacks with df-bit and mtu 9156 is successful.

Disabling path-mtu on the neighbor session "fixes" the problem. The
problem only started occurring after some outages caused the sessions
to flap.

Anyone seen odd path-mtu related issues?
-- 
Tim:>
Sent from New York, NY, United States