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Discovery Messages
Discovery messages announce and maintain the presence
of a router in a network. Routers 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 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.
- Extended discovery—LDP sessions between routers
not directly connected are supported by LDP extended discovery. A
router 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
decides whether to respond to or ignore the targeted hello message.
A targeted router that chooses to respond does so by periodically
sending targeted hello messages to the initiating router.
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