When troubleshooting a BGP session that remains in the Idle or Active state, one of the most common issues is an incorrect configuration of the remote AS (Autonomous System) on one of the routers. In BGP, both peers must have matching AS numbers in their respective remote-as statements for the session to be successfully established.
Symptom:
The BGP session does not progress beyond the Idle or Active state.
Root Cause:
A misconfigured remote AS value. This typically happens when one router’s remote-as statement does not match the actual AS number of its neighbor.
Solution:
Verify and correct the remote-as value on each router so that each router’s configuration accurately reflects the neighbor's AS number.
Suppose we have two routers, Router0 and Router1, that are supposed to form a BGP session.
Router0(config)# router bgp 5110 Router0(config-router)# neighbor 192.168.158.51 remote-as 2640 <-- Correct
Router1(config)# router bgp 2640
Router1(config-router)# neighbor 192.168.158.49 remote-as 5111 <-- Incorrect AS!
In this case, Router1 is incorrectly configured with a remote AS of 5111 instead of the correct AS, which should be 5110. To resolve the issue, update Router1’s configuration as follows:
Router1(config)# router bgp 2640
Router1(config-router)# neighbor 192.168.158.49 remote-as 5110 <-- Corrected
Why It Matters:
The remote-as command tells the router which AS number to expect from its neighbor. If these values do not match, BGP will not form a session, as the protocol uses AS numbers to validate the legitimacy of the connection.
Best Practice:
Always double-check that the remote-as values on both ends of the BGP session are correctly configured and correspond to the intended neighbor’s AS number. This ensures a successful and stable BGP peering relationship.
Understanding this configuration aspect is essential for maintaining robust and efficient BGP sessions. By ensuring accurate remote-as values, you help prevent connectivity issues that could affect network routing and overall performance.