Graceful Restart (GR) is part of the LDP handshake process (that is, the LDP peering session initialization) and needs to be supported by both peers. GR provides a mechanism that allows the peers to cope with a service interruption because of a CSM switchover, which is a period of time when the standby CSM is not capable of synchronizing the states of the LDP sessions and labels being advertised and received.
Graceful Restart Helper (GR-Helper) decouples the data plane from the control plane so that if the control plane is not responding (that is, there is no LDP message exchange between peers), then the data plane can still forward frames based on the last known (advertised) labels.
Because the 7705 SAR supports non-stop services / high-availability for LDP (and MPLS), the full implementation of GR is not needed. However, GR-Helper is implemented on the 7705 SAR to support non-high-availability devices. With GR-Helper, if an LDP peer of the 7705 SAR requests GR during the LDP handshake, the 7705 SAR agrees to it but does not request GR. For the duration of the LDP session, if the 7705 SAR LDP peer fails, the 7705 SAR continues to forward MPLS packets based on the last advertised labels and does not declare the peer dead until the GR timer expires.