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Discovery Messages
Discovery messages announce and maintain the presence of a router or switch in a network. They indicate their presence in a network by sending hello messages periodically. Hello messages are transmitted as UDP packets to the LDP port at the group multicast address for all routers on the subnet.
LDP uses the following discovery procedures:
- Basic discovery—A router or switch periodically sends LDP link hello messages through an interface. LDP link hello messages are sent as UDP packets addressed to the LDP discovery port. Receipt of an LDP link hello message on an interface identifies an adjacency with the LDP peer router or switch.
- Extended discovery—LDP sessions between routers or switches not directly connected are supported by LDP extended discovery. A router or switch periodically sends LDP targeted hello messages to a specific address. Targeted hello messages are sent as UDP packets addressed to the LDP discovery port at the specific address. The targeted router or switch decides whether to respond to or ignore the targeted hello message. A targeted router or switch that chooses to respond does so by periodically sending targeted hello messages to the initiating router or switch.
