3Com S7902E, Configuration Manual

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Session establishment and maintenance
In this phase, LSRs pass through two steps to establish sessions between them:
1) Establishing transport layer connections (that is, TCP connections) between them.
2) Initializing sessions and negotiating session parameters such as the LDP version, label distribution
mode, timers, and label spaces.
After establishing sessions between them, LSRs send Hello messages and Keepalive messages to
maintain those sessions.
LSP establishment and maintenance
Establishing an LSP is to bind FECs with labels and notify adjacent LSRs of the bindings. This is
implemented by LDP. The following gives the primary steps when LDP works in DU mode and ordered
mode:
1) When the network topology changes and an LER finds in its routing table a new destination
address that does not correspond to any existing FEC, the LER creates a new FEC for the
destination address.
2) If the LER has upstream LSRs and has at least one free label, it assigns a label to the FEC and
sends the label binding information to the upstream LSRs.
3) Upon receiving the label binding information, an upstream LSR records the binding. Then, if the
LSR which sent the binding information is the next hop of the FEC, it adds an entry in its LFIB,
assigns a label to the FEC, and sends the new label binding information to its own upstream LSRs.
4) When the ingress LER receives the label binding message, it adds an entry in its LFIB. Thus, an
LSP is established for the FEC, and packets of the FEC can be label switched along the LSP.
Session termination
LDP checks Hello messages to determine adjacency and checks Keepalive messages to determine the
integrity of sessions.
LDP uses different timers for adjacency and session maintenance:
z
Hello timer: 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 its peer in a Hello interval, it
removes the Hello adjacency.
z
Keepalive timer: LDP peers keep LDP sessions by periodically sending Keepalive messages over
LDP session connections. If an LSR does not receive any Keepalive message from its peer during
a Keepalive interval, it closes the connection and terminates the LDP session.
LDP Loop Detection
LSPs established in an MPLS domain may be looping. The LDP loop detection mechanism can detect
looping LSPs and prevent LDP messages from looping forever.
For the LDP loop detection mechanism to work, all LSRs must have the same LDP loop detection
configuration. However, establishing an LDP session does not require that the LDP loop detection
configuration on the LDP peers be the same.
LDP loop detection can be in either of the following two modes: