H3C Technologies H3C S7500E Series Switches - Manual (Page 52)

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Basic discovery mechanism: Used to discover local LDP peers, that is, LSRs directly connected
at the link layer. In this mechanism, an LSR periodically sends LDP link Hello messages to
multicast address 224.0.0.2, that is, all routers on the subnet, so that all LSRs directly connected
at the link layer can discover this LSR.
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Extended discovery mechanism: Used to discover remote LDP peers, that is, LSRs not directly
connected at the link layer. In this mechanism, an LSR periodically sends LDP targeted Hello
messages to a given IP address so that the LSR with the IP address can discover the LDP peer.
Session establishment and maintenance
After an LSR finds a LDP peer, they start to establish a session. The LSRs goes through two steps to
establish a session:
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Establishing a transport layer connection, that is, a TCP connection between them.
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Initializing negotiation of session parameters such as the LDP version, label advertisement mode,
and Keepalive interval.
After establishing a session between them, the two LDP peers send Hello messages and Keepalive
messages to maintain the session.
LSP establishment and maintenance
LDP binds a FEC with a label on each LSR involved and notify adjacent LSRs of the binding, so as to
establish an LSP.
For the LSP establishment process, see LSP Establishment and Label Distribution .
Session termination
An LSR terminates its LDP session with an LDP peer in the following cases:
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All Hello adjacencies are deleted between the two peers
LDP peers periodically send Hello messages to indicate that they intend to keep the Hello adjacency.
If an LSR does not receive any Hello message from a peer before the hold timer for the Hello
adjacency expires, it deletes the Hello adjacency with this peer. An LDP session has one or more
Hello adjacencies. When the last Hello adjacency for the session is deleted, the LSR will send a
Notification message to terminate the LDP session.
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Loss of session connectivity
An LSR determines the integrity of an LDP session according to the LDP PDU (which carries one or
more LDP messages) transmitted on the session. Before the Keepalive timer times out, if two LDP
peers have no information to exchange, they can send Keepalive messages to each other to maintain
the LDP session. If an LSR does not receive any LDP PDU from its peer during a Keepalive interval, it
closes the TCP connection and terminates the LDP session.
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Receiving a shutdown message from the peer
An LSR can also send a Shutdown message to its LDP peer to terminate the LDP session. Therefore,
when receiving the Shutdown message from an LDP peer, an LSR will terminate the session with the
LDP peer.
Protocols
MPLS related protocols include:
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RFC 3031: Multiprotocol Label Switching Architecture
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RFC 3032: MPLS Label Stack Encoding