The router is able to process update messages. -->established
The router is waiting for the neighbor's open message. -->open sent
Routers have agreed on matching feature sets. -->open confirm
The session establishment has timed out. -->idle
This question requires matching BGP connection states from the BGP Finite State Machine (FSM) to descriptions of conditions that occur within or lead to those states.
Idle:This is the initial state where BGP awaits a start event or retries after a failure. It's also the state entered upon error detection or session closure, including timeouts during connection attempts.
Matches:"The session establishment has timed out." - A timeout during the connection process forces the BGP process back to the Idle state to potentially retry later.
OpenSent:After a TCP connection is established, the local router sends a BGP OPEN message with its parameters (AS number, capabilities, etc.) and transitions to the OpenSent state while waiting to receive an OPEN message from its BGP neighbor.
OpenConfirm:Once the router receives an OPEN message from its neighbor and validates the parameters (e.g., matching AS, compatible capabilities), it sends a KEEPALIVE message and moves to the OpenConfirm state. It waits for a KEEPALIVE from the neighbor to confirm the session. Basic parameter checks and capability negotiations are successfully completed in this phase.
Matches:"Routers have agreed on matching feature sets." - This agreement happens upon successful validation of the OPEN messages exchanged.
Established:This is the final, stable state where BGP peering is successful. Both routers have accepted each other's parameters via the OPEN messages and confirmed the session with KEEPALIVEs. In this state, the routers can exchange UPDATE messages containing routing information.
[References:RFC 4271 (BGP4 specification - Section 8, Finite State Machine), BGP configuration and troubleshooting guides for AOS-CX. This relates to the "Routing" (16%) and "Troubleshooting" (10%) objectives., , ]
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