if stateful SRP switchover is not configured on the router, the graceful restart capability
will not function.
You can configure an LSR to restart itself gracefully and to support graceful restart in its
neighbors (helper mode), or helper mode alone. In either case, the LSR includes the fault
tolerant (FT) session TLV in the LDP initialization messages it sends at session startup.
The TLV includes values for the reconnect timeout and the recovery time. When both
graceful restart and the helper mode are disabled, the LSR does not include the TLV in
its LDP initialization messages.
The configurable reconnect time specifies how long you want the LSR’s neighbors to
wait for the LSR to resume exchanging LDP messages with the neighbors after the
connection failure. The reconnect timeout value is nonzero when graceful restart is
enabled.
When you disable graceful restart but enable helper mode, the reconnect timeout is set
to zero to announce to the neighbors that the LSR does not preserve MPLS forwarding
state across the restart. The presence of the TLV indicates to the neighbors that the LSR
supports them if they gracefully restart. That is, the LSR in this case waits for a gracefully
restarting neighbor to resume sending messages.
Table 54 on page 257 summarizes the states possible for LDP graceful restart.
Table 54: Summary of LDP Graceful Restart States
Reconnect timeout value
sent in TLV
FT TLV sent to
neighbor?
Helper mode
enabled?
Graceful restart
enabled?
NonzeroYesYesYes
ZeroYesYesNo
–NoNoNo
The recovery time specifies how long the LSR retains its MPLS forwarding state across
the restart. When the LSR restarts, it marks the forwarding state entries as stale. The
forwarding state holding timer begins counting down from the configured recovery time
value. If the timer expires before the restart completes, the LSR deletes all stale entries.
When the LSR sends new LDP initialization messages to its neighbors, the messages
contain the current value of the timer.
When the LSR restarts, if a neighbor of the LSR has previously received the FT session
TLV from the LSR with a nonzero reconnect timeout value, the neighbor retains the label
mapping information that it has previously received from the LSR and marks that
information as stale. Alternatively, if the neighbor received an FT session TLV with a
timeout value of zero (indicating that only helper mode is enabled) or no TLV at all
(indicating that both graceful restart and helper mode are disabled), it deletes the label
mapping information.
Also when the LSR restarts, the neighbor sets its neighbor liveness timer to the lesser of
the two values, the reconnect timeout value and its own configurable neighbor liveness
257Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview
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