The reason why 2 peers wouldn’t be able to create a BGP sessions are :
If you can work thru this checklist it would be far easier to find the issues with.
- Peer Establishment
- Missing Routes
- Inconsistent Route Selection
- Loops and Convergence Issues
- Internet Reachability Problems
Routers establish a TCP session
Port 179—Permit in ACLs
IP connectivity (route from IGP)
OPEN messages are exchanged
Peering addresses must match the TCP session
Local AS configuration parameters
Problems for Peer Establishment include:
Sessions are not established
No IP reachability
Peers are flapping
Layer 2 problems
Things to check:
Is the Local AS configured correctly?
Is the remote-as assigned correctly?
Verify with your diagram or other documentation!
Remember to allow TCP/179 through filters
Common eBGP implementation error include not allowing TCP sessions.
• Need to be careful with ebgp-multihop
Peer between loopback interfaces
Needed to loadshare
Remember update-source loopback 0
Using passwords on iBGP and eBGP sessions
- Link won’t come up
- Been through all the previous troubleshooting steps.
• Extended ping/traceroute allow you to verify
Loopback to loopback IP connectivity TTL issues
• “show ip bgp summary”
Displays the state of all peers
• “show ip bgp neighbor”
Gives a lot of information regarding the peer
• “debug ip bgp”
Should give you a good hint as to why a peer will not establish
• “debug ip bgp events”
Displays state transitions for peers
• “show ip bgp neighbor | include Last reset”
Will show you the last reset reason for all peers