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What are the difference and relationship between Hello messages and Keepalive messages of LDP sessions?
In an LDP session, UDP is used to discover peers by sending Hello messages, while TCP is used to maintain peer relationships by sending Keepalive messages. Both UDP and TCP use port 646. LDP Hello messages are sent to multicast address 184.108.40.206 for a directly-connected peer and to a specific unicast address for an indirectly-connected peer. After the peer relationship is established, Hello messages and Keepalive messages are periodically sent. By default, the NE40E/NE80E/NE5000E sends the messages at the following intervals:
The timeout period of the Link-Hello hold timer is 15s. The timeout period of the Targeted-Hello hold timer is 45s.
The timeout period of the Keepalive hold timer is 45s for both directly-connected LDP sessions and targeted LDP sessions.
TCP Keepalive messages are sent to maintain the TCP connection between peers, regardless of which interface sends or receives Keepalive messages.
Whereas, LDP Hello messages are sent to determine which interface receives messages from a peer (that is, the interface that learns peer information).
When two routers are connected by multiple links, LDP Hello messages are sent on each link to locate the interface where a peer resides, whereas in per-destination load balancing, Keepalive messages are not sent on each link. That is, sending Hello messages keeps each link alive, and sending Keepalive messages keeps each peer alive.