Sequence of Hello Message Exchange

When a peer receives a hello message with a hello request object, the receiver generates a hello message with a hello ack object. If the receiver has never received a hello from the sender and the source instance is nonzero, then the receiver updates the destination instance that it sends in response with this new value. When the original sender first receives a hello ack from the peer in response to the hello request, the sender updates the destination instance that it sends in the subsequent hello request with the nonzero source instance it receives in the hello ack.

Consider the following example. An LSP has been established between peers A and B. These adjacent peers have not yet exchanged hello messages.

1. Peer A sends a hello request to Peer B. The request object contains the following:
   • Source instance = 5 (generated by Peer A for this adjacency)
   • Destination instance = 0 (because it has never exchanged messages with Peer B)
2. Peer B receives the hello request and sends a hello ack to Peer A. The ack object contains the following:
   • Source instance = 8 (generated by Peer B for this adjacency)
   • Destination instance = 5 (because that is what Peer B detected in the source instance from peer A)
3. Peer A receives the hello ack and sends another hello request to peer B. The request object contains the following:
   • Source instance = 5 (generated by Peer A for this adjacency)
   • Destination instance = 8 (the source instance generated by Peer B for this adjacency)

The two peers continue exchanging hello messages until the LSP is torn down. The following is true for these message exchanges unless a peer resets:

• Peer A always sends source instance= 5 and destination instance= 8 to Peer B.
• Peer B always sends instance= 8 and destination instance= 5 to Peer A.

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