H3C Technologies H3C SR8800 - Manual (Page 20)

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Every LSR that wants to establish LDP sessions sends Hello messages periodically to notify
neighboring LSRs of its presence. In this way, LSRs can automatically discover their LDP peers. LDP
provides the following discovery mechanisms:
Basic discovery mechanism—Discovers local LDP peers, or, 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, or, all routers on the subnet, so that all LSRs directly connected at the link
layer can discover this LSR.
Extended discovery mechanism—Discovers remote LDP peers, or, 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.
2. Session establishment and maintenance
After an LSR finds an LDP peer, the LSRs go through the following steps to establish a session:
a. Establish a TCP connection between them.
b. Initialize negotiation of session parameters such as the LDP version, label advertisement mode,
and Keepalive interval.
After a session is established between them, the two LDP peers send Hello messages and
Keepalive messages to maintain the session.
3. LSP establishment and maintenance
LDP sends label requests and label binding messages, so as to advertise label bindings between
LDP peers and thereby establish LSPs.
For the LSP establishment process, see “
LSP establishment and label distribution .”
4. Session termination
An LSR terminates its LDP session with an LDP peer in the following cases:
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 Hello timer
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.
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.
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:
• RFC 3031, Multiprotocol Label Switching Architecture
• RFC 3032, MPLS Label Stack Encoding