C7XXX router of C company connects to NE20 through FE directly for IBGP neighbor. Execute disp bgp peer command at NE20 to check the neighbor status, always in active or connect, failing to set up neighbor.
1. The route from NE20 to loopback address of C7XXX is reachable, and no packet is lost in ping to each other. so it is not the route and link that fails BGP TCP connection.
2. Execute debug bgp at C equipment.
2w2d: BGP: 18.104.22.168 bad OPEN, wrong router identifier 22.214.171.124
2w2d: BGP: 126.96.36.199 went from OpenSent to Closing
2w2d: BGP: 188.8.131.52 sending NOTIFICATION 2/3 (BGP identifier wrong) 4 bytes 870A233E
2w2d: BGP: 184.108.40.206 local error close, erroneous BGP update received
2w2d: BGP: 220.127.116.11 reset due to BGP Notification sent
2w2d: BGP: 18.104.22.168 went from Closing to Idle
2w2d: BGP: 22.214.171.124 closing
According to the debug information above, C7XXX transmits notification packet to disconnect the TCP connection with NE20 because it thinks that open packet of NE20 carries false router identifier, failing the setup of neighbor.
3. Check the cofigurations of NE20 BGP, and they are correct; 126.96.36.199 is the address of loopback interface of NE20.
4. Check C7XXX BGP:
TR7206router#show ip bgp summary
BGP router identifier 188.8.131.52, local AS number 64746
It is obvious that C7XXX is problematic, and its default router identifier is the address of loopback interface of NE20.
5. At C7XXX, configure bgp router-id 184.108.40.206 command to set the identifier to the address of its loopback interface; NE20 and C7XXX set up BGP neighbor.
1. The route to loopback interface of the peer is not reachable or the link connection is problematic.
2. Neighbor negotiation fails.
3. Causes of other products.
BGP neighbor cannot set up normally. If both the route and links are normal, analyze the open packets in interaction through debug, and find the detailed reasons why negotiation fails.