300-410 ENARSI: Implementing Cisco Enterprise Advanced Routing and ServicesDemo

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QuestionQ398

Layer 3 Technologies

How are LDP neighbors discovered?

  • A Broadcasts hellos are sent to the 255.255.255.255 broadcast address.
  • B Multicast hellos are sent to the 224.0.0.2 group address.
  • C Unicast hellos are sent to directly connected neighbors IP addresses.
  • D Multicast hellos are sent to the 224.0.0.5 group address.
Explanation

LDP uses periodic Hello messages sent as UDP packets to the all-routers multicast address 224.0.0.2 (not 255.255.255.255 broadcast or 224.0.0.5 used by OSPF) on UDP port 646. These multicast Hellos enable basic discovery of adjacent LDP-enabled routers on a segment. Once neighbors are discovered via multicast, LDP establishes a TCP session for label exchange.

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