Recovery With Graceful-Restart - Cisco CRS Configuration Manual

LDP Graceful Restart

Recovery with Graceful-Restart

This figure illustrates the process of failure recovery using graceful restart.
Figure 5: Recovering with Graceful Restart
1 The router R4 LSR control plane restarts.
2 With the control plane restart, LIB is gone but forwarding states installed by R4's LDP control plane are
not immediately deleted but are marked as stale.
3 Any in-transit packets from R3 to R4 (still labeled with L4) arrive at R4.
4 The MPLS forwarding plane at R4 performs a successful lookup for the local label L4 as forwarding is
still intact. The packet is forwarded accordingly.
5 The router R3 LDP peer detects the failure of the control plane and channel and deletes the label bindings
from R4. The peer, however, does not delete the corresponding forwarding states but marks them as stale.
6 At this point there are no forwarding disruptions.
7 The peer also starts the neighbor reconnect timer using the reconnect time value.
8 The established LSPs going toward the router R4 are still intact, and there are no broken LSPs.
When the LDP control plane recovers, the restarting LSR starts its forwarding state hold timer and restores
its forwarding state from the checkpointed data. This action reinstates the forwarding state and entries and
marks them as old.
The restarting LSR reconnects to its peer, indicated in the FT Session TLV, that it either was or was not able
to restore its state successfully. If it was able to restore the state, the bindings are resynchronized.
The peer LSR stops the neighbor reconnect timer (started by the restarting LSR), when the restarting peer
connects and starts the neighbor recovery timer. The peer LSR checks the FT Session TLV if the restarting
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
14
Implementing MPLS Label Distribution Protocol