H3C Technologies H3C S10500 Series Switches - Manual (Page 66)

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• Basic discovery mechanism—Discovers directly connected LSRs. An LSR periodically sends LDP link
Hello messages to multicast address 224.0.0.2 that identifies all routers on the subnet to advertise
its presence.
• Extended discovery mechanism—Discovers indirectly connected LDP peers. An LSR periodically
sends LDP 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 a LDP peer, they start to establish a session through the following steps:
• Establish a TCP connection between them.
• Initialize 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.
3. LSP establishment and maintenance
LDP sends label requests and label binding messages between LDP peers to 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 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 terminates the session with the LDP peer.
Protocols
MPLS-related protocols are as follows:
• RFC 3031, Multiprotocol Label Switching Architecture
• RFC 3032, MPLS Label Stack Encoding
• RFC 5036, LDP Specification