HP 10500 SERIES Configuration Manual page 67

LDP session, advertisement, and notification messages use TCP for reliability. Discovery messages use
UDP for efficiency.
LDP operation
LDP goes through the following phases in operation:
Discovery
1.
Each LSR 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 directly connected LSRs and establishes link hello
adjacencies with them. 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 and establishes
targeted hello adjacencies. 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.
If two LSRs each have the same transport address (the source IP address used to establish a TCP
connection to the peer) for the basic and extended discovery mechanisms, the LSRs can establish
both a link hello adjacency and a targeted hello adjacency with each other, and the two
adjacencies are associated with the same session. This method is suited for two LDP peers that
have both a direct link (only one hop) and an indirect link (more than one hop) in between. It can
protect the LDP session between the two peers and avoid LDP convergence upon failure of one link.
When the direct link fails, the link hello adjacency is deleted. In this case, if the indirect link
operates normally, the targeted hello adjacency remains. Therefore, the LDP session is not deleted,
and the FEC-label bindings based on this session are not deleted either. When the direct link
recovers, the LDP peers do not need to re-establish the LDP session or re-learn the FEC-label
bindings.
If two LSRs each have different transport addresses for the basic and extended discovery
mechanisms, only one type of hello adjacency (either link or targeted) can exist between them.
Session establishment and maintenance
2.
After an LSR finds a LDP peer, the LSRs go through the following steps to establish a session:
Establish a TCP connection between them.
a.
Initialize negotiation of session parameters such as the LDP version, label advertisement mode,
b.
and Keepalive interval.
After establishing a session between them, the two LDP peers send Hello and Keepalive messages
to maintain the session.
LSP establishment and maintenance
3.
LDP sends label requests and label binding messages between LDP peers to establish LSPs.
For the LSP establishment process, see
Session termination
4.
An LSR terminates its LDP session with an LDP peer when:
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 sends a
Notification message to terminate the LDP session.
"LSP establishment and label
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distribution."