BGP Neighbor Stabilization

D.

The answer is D. Established.

Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol that is used to exchange routing information between different autonomous systems (ASes) on the Internet. BGP establishes a TCP session between two BGP routers, and before BGP update messages can be exchanged, the TCP session must be established and the BGP routers must go through a number of different neighbor states.

The different BGP neighbor states are:

1. Idle: This is the initial state of a BGP neighbor. In this state, the BGP router waits for a TCP connection request from the remote BGP peer.

2. Connect: Once the TCP connection request is received, the BGP router moves to the Connect state. In this state, the BGP router sends a TCP SYN packet to the remote peer to establish the TCP connection.

3. Active: If the remote BGP peer does not respond to the TCP SYN packet, the BGP router moves to the Active state. In this state, the BGP router sends a TCP connection request to the remote peer again and waits for a response.

4. OpenSent: Once the TCP connection is established, the BGP router moves to the OpenSent state. In this state, the BGP router sends an Open message to the remote peer to exchange the BGP capabilities.

5. OpenConfirm: Once the Open message is acknowledged by the remote peer, the BGP router moves to the OpenConfirm state. In this state, the BGP router waits for the remote peer to send an acknowledgment for the Open message.

6. Established: Once the BGP routers have exchanged the Open message and acknowledged it, the BGP router moves to the Established state. In this state, the BGP router can exchange BGP update messages with the remote peer.

Therefore, before BGP update messages can be sent, a neighbor must stabilize into the Established neighbor state.