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LDP Messages

When you configure LDP on an LSR, the router begins sending LDP discovery messages out all LDP-enabled interfaces. When an adjacent LSR receives LDP discovery messages, it establishes an underlying TCP session. An LDP session is then created on top of the TCP session. The TCP three-way handshake ensures that the LDP session has bidirectional connectivity. After they establish the LDP session, the LDP neighbors maintain, and terminate, the session by exchanging messages.

LDP advertisement messages allow LSRs to exchange label information to determine the next hops within a particular LSP.

Any topology changes, such as a router failure, generate LDP notifications that can terminate the LDP session or generate additional LDP advertisements to propagate an LSP change.

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