MPLS LDP Graceful Restart (GR)

The LDP devices must negotiate GR capability and fault tolerance (FT) during the formation of an MPLS LDP session. The formed session can only be FT/GR capable if both devices enable GR. A GR device must backup the FECs and label data in order to implement GR.

Fig 1.1- MPLS topology

When an LDP session is capable of GR

• When the GR restarter resumes, the GR helper will notice that the associated LDP session is down and will continue to be its neighbor while keeping track of the session's details until the reconnect timeout expires.
• Before the reconnect timeout expires, if the GR helper gets a session request from the GR restarter, it saves the session's LSP and label data and restores the session using the GR restarter. In every other case, it removes all of the session's LSP and label data.
• After the session has recovered, the GR restarter and GR helper turn on their neighbor liveness and recovery timers, restore all the LSP data pertinent to this session, and send label mapping and label request messages to one another.
• After the neighbor liveness timer and recovery timer expire, the GR restarter and GR helper erase the LSP stale flag and any associated session LSP data upon receiving mapping messages from one another.

Conclusion
To conclude, MPLS packets can continue to be transported without interruption during a graceful restart since the forwarding plane's LSP information is kept.

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