Cisco NCS 5500 Series Configuration Manual page 24

Details of Label Distribution Protocol Graceful Restart
Recovery with Graceful Restart
Figure 4: Recovering with Graceful Restart
1 The R4 LSR control plane restarts.
2 LIB is lost when the control plane restarts.
3 The forwarding states installed by the R4 LDP control plane are immediately deleted.
4 Any in-transit packets flowing from R3 to R4 (still labeled with L4) arrive at R4.
5 The MPLS forwarding plane at R4 performs a lookup on local label L4 which fails. Because of this failure,
the packet is dropped and NSF is not met.
6 The R3 LDP peer detects the failure of the control plane channel and deletes its label bindings from R4.
7 The R3 control plane stops using outgoing labels from R4 and deletes the corresponding forwarding state
(rewrites), which in turn causes forwarding disruption.
8 The established LSPs connected to R4 are terminated at R3, resulting in broken end-to-end LSPs from R1
to R4.
9 The established LSPs connected to R4 are terminated at R3, resulting in broken LSPs end-to-end from R2
to R4.
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
MPLS Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
16
Implementing MPLS Label Distribution Protocol