because it preserves its MPLS forwarding state during the restart. When RSVP-TE comes
back up, the restarted router sends the first hello message to its neighbors with a new
source instance value to indicate that it had a control plane failure. The destination
instance value in the hello message is set to zero. The recovery time included in this hello
message is set to zero only if the router was unable to preserve the MPLS forwarding
state or to support control state recovery.
When a neighboring router that has been configured as a graceful restart helper
determines that the number of continuous missing hellos has reached the configured
hello miss limit, it declares the router to be down. The helper router then waits for a period
equal to the restart time that it received from the router and stored before the failure.
During this period, the helper router preserves the restarting router's RSVP-TE state and
MPLS forwarding state for the established LSPs and keeps forwarding MPLS traffic.
However, the helper router suspends the refreshing of path and resv state to the restarting
router. The helper router keeps sending hello messages to the restarting router with an
unchanged source instance value and a destination instance value set to zero. The helper
router removes the RSVP-TE state for any LSP that was in the process of being established
when the neighbor was declared to be down.
If the helper router does not receive a hello message from the restarting router during
the restart period, the helper router immediately exits the recovery procedure and cleans
up the states associated with the restarting router. The helper router determines that
the failed neighbor has restarted when it finds a new source instance in the neighbor's
hello message. When a nonzero recovery time is received in that hello message, the
helper router determines that the restarted neighbor supports state recovery. The helper
router then starts the recovery procedures. However, if the recovery time specified in the
hello message is zero, then the helper router exits the recovery procedure and cleans up
the states associated with the restarting router.
Recovery Behavior
In the recovery period, neighboring helper routers and the restarting router resynchronize
the RSVP-TE state and MPLS forwarding state. During this period, MPLS traffic continues
to be forwarded.
The helper router starts the recovery procedure by marking as stale the RSVP-TE state
associated with the restarting router. The upstream helper router then refreshes all the
path messages shared with the downstream restarting router. The upstream helper
router includes the recovery_label object in the path message to the downstream
restarting router for the label binding information that the restarting router specified
before the restart. The downstream helper router does not refresh the reservation state
control block (RSB) shared with the restarting router until a corresponding path message
is received from the restarting router.
During the recovery period, the restarting router checks for the state associated with an
incoming path message. If the RSVP-TE state already exists, the restarting router handles
the path message as usual. Otherwise, the restarting router examines the path message
for the recovery_label object. If the recovery_label object is not found, the restarting router
treats the path message as a setup request for a new LSP and handles the path message
as usual.
Copyright © 2010, Juniper Networks, Inc.
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