LDP Graceful Restart

LDP Graceful Restart (GR) helps minimize MPLS traffic loss when an LDP component is restarting in a router that is capable of preserving its MPLS forwarding states across restart. LDP GR is based on RFC 3478 (Graceful Restart mechanism for Label Distribution Protocol).

LDP GR works between a router and its neighbor and its capability must be advertised when sending an LDP Initialization message. An LDP restart triggered by an MP failover due to a fault of the active MP or a command-initiated switchover is the only scenario where the MPLS forwarding state is preserved.

The router can also support LDP GR in helper-only mode. In this mode, a router does not preserve its forwarding entries on a LDP GR restart. It indicates to its peers that forwarding state is not preserved by sending an initialization message with the Reconnect Time and the Recovery Time set to zero (0) in FT session TLV. However, it can help a neighboring router recover its forwarding entries when the neighbor is going through restart.

A LDP GR enabled router goes into helper-only mode (GR helper) when any of the following events occur on the router‘s neighbors.

• MP failover occurs
• HLOS upgrade occurs
• Remove and re-add of the MPLS configuration
• TCP communication broken (such as, session keepalive timer expires)
• UDP communication broken (for example, an adjacency goes down)
• Restarting LDP component by disabling and enabling the loopback
• Restarting a LDP session by issuing the clear mpls ldp neighbor command

In helper-only mode, the LDP GR procedure works at the session level. Any of the previous events causes the helper to detect session down and start the GR procedure. The operation of the GR helper is the same independent of what has happened on the restarting LSR that triggers the GR procedure.

When LDP GR is enabled on a router, the configuration does not apply to the current sessions. The LDP GR configuration is applied for the new sessions brought up after the configuration is added.