BGP Troubleshooting in advanced

BGP Troubleshooting.

Problem: BGP peer is not getting establish between routers.
Cause :
1) Peer ip address is not reachable .
2) port 179 is blocked by firewall or access-list.
3) BGP configuration is not correct like wrong peer address local-as or remote-as, wrong AS number, wrong authentication/MD5 password or wrong update-source loopback.
4) static route is missing for loopback address end to end.
5) TTL=1 for ebgp neigbor. ebgp-multihop is not configured.
6) MD5 authentication has "space" in password.
7)Duplicate router-id between BGP neighbors. 

Debugging command :
debug ip bgp
debug ip tcp transaction
debug ip bgp events 

Solution:
a) Ensure BGP local and remote AS configuration is correct.
b) MD5 authentication password is correct on both sides without spaces.
c)Verify update-source loopbak and ebgp-multihop.
d) port 179 is allowed in the path boh end via access-list. Ensure firwall is not blocking.




Problem : BGP neighbor ship is flapping and getting reset.

Cause:
1. Keep alive mismatch
2. MTU mismatch.
3. Hellos are stuck in OutQ behind update packets.
4. Remote router rebooting continually (typical with a 3-5 minute BGP peering cycle time)
5. Remote router BGP process unstable, restarting
6.Traffic Shaping & Rate Limiting parameters
7.MTU incorrectly set on links, PMTU discovery disabled on router.
8. Output drops on the interface or congestion on the queue.
9. High CPU on the router or CPU spike on router. 

Solution :
a) Make sure you  have same keep alive on both routes.
b) Make sure you  have same MTU  and can ping MTU 1500 with df bit set.
c) If MTU mismatch can not be fixed, can use path-mtu discovery to overcome.  

Tip: BGP update packets are packed to the size of the MTU – keepalives and BGP OPEN packets are not packed to the size of the MTU ⇒ Path MTU problem.

Problem : Router is not installing routes in RIB and ignoring.

Cause :
1. Paths that are marked as not synchronized in the show ip bgp longer-prefixes output
2.Paths for which the NEXT_HOP is inaccessible
3.Paths from an external BGP (eBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH
4.If you enabled bgp enforce-first-as and the UPDATE does not contain the AS of the neighbor as the first AS number in the AS_SEQUENCE 
5.Paths that are marked as (received-only) in the show ip bgp longer-prefixes output 


Solution:
a) Be sure that there is an Interior Gateway Protocol (IGP) route to the NEXT_HOP that is associated with the path.

Problem : Router is not installing routes in BGP Table.
Cause:
1. No router in global routing table for the network command configured in BGP.
2.  No matching route in BGP table for the aggregate-command configured in BGP.
3. Same router-ID in routes coming from Router-reflector in Originator-ID attributes.
4. Same router-IS in routes coming from router-reflector from same cluster-ID.  



Problem : Router is not advertising route to another BGP peer.
Cause :
1. No matching route in routing table.
2. Filter is  discarding route.