Symptom
BGP L2VPN session can flap continuously on N9K.
The flap are occurring every interval.
Conditions
- Observed on N9K running NXOS 9.2(x).
- Observed on eBGP sessions only
- Observed for L2VPN sessions only
Workaround
Workarounds:
1) Enable path MTU discovery.
2) Avoid fragmentation in between the BGP speakers.
Further Problem Description
The sequence of events leading to the issue are the following:
1) A and B are BGP speakers and try to form a BGP L2VPN peering
2) TCP handshake goes fine and BGP session comes up
3) The first UPDATE sent by A is huge and gets fragmented in between A and B
4) B receives the fragmented update (all fragments are received), but never ACK those packets.
5) A stays idle, as no ACK are received for the initial UPDATE
6) B reach holdtimer and shut down the BGP session.
NB: The fragmented packets are received up to the CPU of B, but BGP process never receives the re-assembled UPDATE packet.
The issue is fixed starting with NXOS 9.3(1) and 10.1(1).
