R3303-HP HSR6800 Routers MPLS Configuration Guide

Configuring LDP GR

MPLS has two separate planes: the forwarding plane and the control plane. Using this feature, LDP

Graceful Restart (GR) preserves the LFIB information when the signaling protocol or control plane fails, so

that LSRs can still forward packets according to LFIB, ensuring continuous data transmission.

A device that participates in a GR process can be a GR restarter or a GR helper.

• GR restarter—Router that gracefully restarts due to a manually configured command or a fault. It

must be GR-capable.

• GR helper—Neighbor of the GR restarter. A GR helper maintains the neighbor relationship with the

GR restarter and helps the GR restarter restore its LFIB information. A GR helper must be

GR-capable.

Figure 12 LDP GR

Two LDP peers perform GR negotiation when establishing an LDP session. The LDP session established is

GR capable only when both peers support LDP GR.

LDP GR works in the following procedure:

1. Whenever restarting, the GR restarter preserves all MPLS forwarding entries, marks them as stale,

and starts the MPLS forwarding state holding timer for them.

2. After a GR helper detects that the LDP session with the GR restarter is down, it marks the FEC-label

bindings learned from the session as stale and keeps these FEC-label bindings for a period of time

defined by the fault tolerant (FT) reconnect time argument. The FT reconnect time is the smaller one

between the reconnect time advertised from the peer GR restarter and the neighbor liveness time

configured locally.

3. During the FT reconnect time, if the LDP session fails to be re-established, the GR helper deletes the

FEC-label bindings marked stale.

4. If the session is re-established successfully, during the LDP recovery time, the GR helper and the GR

restarter uses the new LDP session to exchange the label mapping information, update the LFIB,

and delete the stale marks of the corresponding forwarding entries. The LDP recovery time is the

smaller one between the recovery time configured locally and that configured on the peer GR

restarter.

GR helper

GR helper GR helper

GR restarter

LDP session with GR capability