LDP GR

LDP Graceful Restart (GR) ensures uninterrupted traffic transmission during a protocol restart or a primary/backup switchover because the forwarding plane is separated from the control plane.

Background

On an MPLS network, when the GR Restarter restarts a protocol or performs a primary/backup switchover, label forwarding entries on the forwarding plane are deleted, interrupting data forwarding.

LDP GR addresses this issue and therefore improves network reliability. During a protocol restart or primary/backup switchover, LDP GR retains label forwarding entries because the forwarding plane is separated from the control plane. The switch still forwards packets based on the label forwarding entries, ensuring data transmission. After the protocol restart or primary/backup switchover is complete, the GR Restarter can restore to the original state with the help of the GR Helper.

Related Concepts

LDP GR is a high-reliability technology based on non-stop forwarding (NSF). The GR process involves GR Restarter and GR Helper devices:

GR Restarter has GR capability.
GR Helper is a GR-capable neighbor of the GR Restarter.

LDP GR uses the following timers:

Forwarding State Holding timer: specifies the duration of the LDP GR process.
Reconnect timer: controls the time the GR Helper waits for LDP session reestablishment. After a protocol restart or primary/backup switchover occurs on the GR Restarter, the GR Helper detects the LDP session as Down. The GR Helper then starts this timer to wait for the LDP session to be reestablished.
Recovery timer: controls the time the GR Helper waits for LSP recovery. After the LDP session is reestablished, the GR Helper starts this timer to wait for the LSP to recover.

Implementation

Figure 1 shows LDP GR implementation.

Figure 1 LDP GR implementation

The implementation of LDP GR is as follows:

An LDP session is set up between the GR Restarter and GR Helper. The GR Restarter and GR Helper negotiate GR capabilities during LDP session setup.
When restarting a protocol or performing a primary/backup switchover, the GR Restarter starts the Forwarding State Holding timer, retains label forwarding entries, and sends an LDP Initialization message to the GR Helper. When the GR Helper detects that the LDP session with the GR Restarter is Down, it retains label forwarding entries of the GR Restarter and starts the Reconnect timer.
After the protocol restart or primary/backup switchover, the GR Restarter reestablishes an LDP session with the GR Helper. If an LDP session is not reestablished before the Reconnect timer expires, the GR Helper deletes label forwarding entries of the GR Restarter.
After the GR Restarter reestablishes an LDP session with the GR Helper, the GR Helper starts the Recovery timer. Before the Recovery timer expires, the GR Restarter and GR Helper exchange Label Mapping messages over the LDP session. The GR Restarter and GR Helper then restore forwarding entries with each other's help. After the Recovery timer expires, the GR Helper deletes all forwarding entries that have not been restored.
After the Forwarding State Holding timer expires, the GR Restarter deletes label forwarding entries and completes the implementation process.