Troubleshooting BGP peer flapping issue

Enable “bgp log-neighbor-changes” so you get a log message when a peer flaps

%BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down BGP Notification sent
%BGP-3-NOTIFICATION: sent to neighbor 1.1.1.1 4/0 (hold time expired) 0 bytes
R2#show ip bgp neighbor 1.1.1.1 | include Last reset
Last reset 00:01:02, due to BGP Notification sent, hold time expired

• We are not receiving keepalives from the other side!

Let’s take a look at our peer!

• Hellos are stuck in OutQ behind update packets!
• Notice that the MsgSent counter has not moved

Possible causes
Remote router is not healthy (OutQ)
Lower layer problems (IP)

Things to check
MTU values
Traffic shaping
Rate-limiting parameters

• Looks like a Layer 2 problem
• At this point we have verified that BGP is not at fault
• Next step is to troubleshoot layer 2…

Ref: http://meetings.ripe.net/ripe-44/presentations/ripe44-eof-bgp.pdf
http://docstore.mik.ua/cisco/pdf/routing/Cisco%20Networkers%20-%20Troubleshooting%20Bgp%20In%20Large%20Ip%20Networks.pdf