Symptom
+ 3-way handshake failing from a peer (verifiable via ethanalyzer) - SYN packets are ignored:
F241.06.09-9364C-1(TAH-elam-insel6)# ethanalyzer local interface inband display-filter tcp.port==179 limit-c 0
Capturing on inband
2023-10-03 06:17:32.558431 192.168.1.24 -> 192.168.1.2 TCP 37886 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24892736 TSER=0 WS=2
2023-10-03 06:18:52.662236 192.168.1.24 -> 192.168.1.2 TCP 16168 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24916767 TSER=0 WS=2
2023-10-03 06:18:53.665114 192.168.1.24 -> 192.168.1.2 TCP 16168 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24917068 TSER=0 WS=2
2023-10-03 06:18:55.671773 192.168.1.24 -> 192.168.1.2 TCP 16168 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24917670 TSER=0 WS=2
2023-10-03 06:18:59.678443 192.168.1.24 -> 192.168.1.2 TCP 16168 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24918872 TSER=0 WS=2
2023-10-03 06:19:07.705108 192.168.1.24 -> 192.168.1.2 TCP 16168 > bgp [SYN] Seq=0 Win=27528 Len=0 MSS=9176 TSV=24921280 TSER=0 WS=2
Note: Switch running the capture owns the 192.168.1.2 IP, it's not responding to these TCP packets.
+ BGP update packets ignored from peer - resulting in prefixes missing from the BGP RIB.
Conditions
Overlapping /31 IP configuration which was previously active on the switch with a BGP prefix peer configuration with neighbors using the same IP's as seen in the /31 range.
Workaround
+ Removal of the IP from the previously active interface results in proper handling of the initial TCP 3-way handshake, resulting in BGP being able to exchange OPEN messages. (Thus proceeding with an attempt to establish BGP neighbors)
Further Problem Description
