LDP GR

LDP supports graceful restart (GR) that enables a Restarter, together with a Helper, to perform a master/backup switchover or protocol restart, without interrupting traffic.

Figure 1 LDP GR

After a device without the GR capability performs a master/backup switchover, an LDP session between the device and its neighbor node goes Down. As a result, the neighbor node deletes the LSP established over the LDP session, and services are interrupted for a short period of time. LDP GR can be configured to prevent interruptions because LDP GR remains capable of forwarding entries after a master/backup device switchover or protocol restart is performed. LDP GR helps implement uninterrupted MPLS forwarding. Figure 1 illustrates the process of LDP GR:

Before a master/slave switchover is performed, LDP neighbors negotiate the GR capability when establishing an LDP session.
When the GR Helper is aware that the Restarter has performed a master/slave switchover or LDP is restarted, the Helper starts a Reconnect timer, and reserves the forwarding entries of the Restarter before the timer expires to prevent forwarding interruptions.
If an LDP session between the Restarter and Helper is reestablished before the Reconnect timer expires, the Helper deletes the Reconnect timer and starts a Recovery timer.
The Helper and the Restarter help each other restore the forwarding entries before the Recovery timer expires. After the timer expires, the Helper deletes all Restarter-related forwarding entries that were not restored.
After the Restarter performs the master/backup switchover or protocol restart, the Restarter starts a Forwarding State Holding timer. The Restarter preserves the forwarding entries before a restart and restores the forwarding entries before the timer expires with the help of the Helper. After the Forwarding State Holding timer expires, the Restarter deletes all forwarding entries that were not restored.

The NetEngine 8000 F can function as a Helper to help the Restarter implement uninterrupted forwarding during a master/backup switchover or protocol restart.