LDP GR

LDP Graceful Restart (GR) preserves label forwarding information when the signaling protocol or control plane fails, so that LSRs can still forward packets according to forwarding entries.

As shown in Figure 12, GR defines the following roles:

• GR restarter—An LSR that performs GR. It must be GR-capable.

• GR helper—A neighbor LSR that helps the GR restarter to complete GR.

The device can act as a GR restarter or a GR helper.

As shown in Figure 13, LDP GR operates as follows:

1. LSRs establish an LDP session. The L flag of the Fault Tolerance TLV in their Initialization messages is set to 1 to indicate that they support LDP GR.

2. When LDP restarts, the GR restarter starts the MPLS Forwarding State Holding timer, and marks the MPLS forwarding entries as stale. When the GR helper detects that the LDP session to the GR restarter goes down, it performs the following operations:

   1. Marks the FEC-label mappings learned from the session as stale.

   2. Starts the Reconnect timer received from the GR restarter.

3. After LDP completes restart, the GR restarter re-establishes an LDP session to the GR helper.

   • If the LDP session is not set up before the Reconnect timer expires, the GR helper deletes the stale FEC-label mappings and the corresponding MPLS forwarding entries.

   • If the LDP session is successfully set up before the Reconnect timer expires, the GR restarter sends the remaining time of the MPLS Forwarding State Holding timer to the GR helper.

     The remaining time is sent as the LDP Recovery time.

4. After the LDP session is re-established, the GR helper starts the LDP Recovery timer.

5. The GR restarter and the GR helper exchange label mappings and update their MPLS forwarding tables.

   The GR restarter compares each received label mapping against stale MPLS forwarding entries. If a match is found, the restarter deletes the stale mark for the matching entry. Otherwise, it adds a new entry for the label mapping.

   The GR helper compares each received label mapping against stale FEC-label mappings. If a match is found, the helper deletes the stale mark for the matching mapping. Otherwise, it adds the received FEC-label mapping and a new MPLS forwarding entry for the mapping.

6. When the MPLS Forwarding State Holding timer expires, the GR restarter deletes all stale MPLS forwarding entries.

7. When the LDP Recovery timer expires, the GR helper deletes all stale FEC-label mappings.