BGP Troubleshooting Scenario: Link is Up but neighbor is Down.

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 Scenario :


A city Bank network administrator has taken connectivity between 2 ISP : Vodafone and Jio. /24 pool has been occupied for the same at both ends. An enterprise customer allows prefix /28 at Vodafone ISP end but suddenly customer end BGP session is stuck in idle state even though link is up.

 

Kindly check possible solutions for it.



LOG :





 Troubleshooting: 


1.       BGP is a path vector protocol and used port no 179 for TCP three-way handshaking process.

2.       First of all we should check physical media, port status and H/w details:

n   Port is flapped or not à show port status

n Link status: by IGP protocol configuration & adjacency status, we can confirm it.

3.       When we try to ping far end IP it should result without RTO (Ping drop) for multiple no. of packets echo response at periodic interval -à it indicates link is up and stable.

4.       When we try to check BGP neighbor status in table if it is indicated by idle status (Pfxct : Prefix limit crossed threshold) so, BGP state stuck in Active state even though reset it multiple times. 

solution:

Pfxct indicates that particular length prefix block is already allowed at both ends in BGP .
 
 If no. of subscribers increases than allowed prefix block length need to do 
supernating of block.
 
here /28 means total number of subscribers allowed = 14 (2^4 -2). so, when more than 14 subscribers at customer end is trying to access internet from Vodafone ISP end then BGP peer state stuck into Idle state and not come up. Possible resolution is needed to allow prefix-length block more than /28 for that issue. 









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