What may cause BGP to flap between idle and active?

• A. The neighbor IP is unreachable.
• B. The BGP process is hung.
• C. The AS numbers may be misconfigured.
• D. TTL expiry on BGP packets.

Answer: (C)

BGP sessions flip between idle and active when the peers repeatedly fail to establish and maintain the TCP connection because what each side expects to receive from the other doesn’t match. The most common trigger is a misconfigured AS number: the remote AS on one side doesn’t match the actual AS on the other side, so when the OPEN message is processed (or the TCP connection is attempted), the peer rejects the session. That rejection tears down the TCP connection, sending the state back to Idle, and the router again tries to connect, producing a rapid idle-active cycle. In eBGP, peers must be in different ASes, so an incorrect remote-as value directly leads to this churn as the session is repeatedly reset.

Other scenarios can cause problems, but they don’t explain the steady idle-active flaps as cleanly. If the neighbor IP were truly unreachable, the session would stall in Idle or Connect rather than repeatedly cycling to Active. If the BGP process were hung, you wouldn’t see a true flap. TTL expiry could drop a session after it starts, but the repeated cycling is most characteristic of an ASN mismatch that causes the remote side to reject the session.