Graceful Restart Helper; Graceful Handling Of Resource Exhaustion - Nokia 7705 SAR-W Series Manual

Label Distribution Protocol
When unlabeled packets arrive at the ingress LER, the forwarding plane consults the
LTN entry and uses a hashing algorithm to map the packet to one of the NHLFEs
(PUSH label) and forward the packet to the corresponding next-hop peer. For a
labeled packet arriving at a transit or egress LSR, the forwarding plane consults the
ILM entry and either uses a hashing algorithm to map it to one of the NHLFEs if they
exist (SWAP label) or routes the packet if there are no NHLFEs (POP label).

5.1.10 Graceful Restart Helper

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 due to 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 will not declare the peer dead until the GR
timer expires.

5.1.11 Graceful Handling of Resource Exhaustion

Graceful handling of resource exhaustion enhances the behavior of LDP when a data
path or a CSM resource required for the resolution of a FEC is exhausted. In prior
releases, the entire LDP protocol was shut down, causing all LDP peering sessions
to be torn down and therefore impacting all peers. The user was required to fix the
issue that caused the FEC scaling to be exceeded, and to restart the LDP session
by executing the no shutdown CLI command. With graceful handling of resource
exhaustion, only the responsible session or sessions are shut down, which impacts
only the appropriate peer or peers.
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MPLS Guide
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