What is the grace period in a graceful restart situation?

A.
how long the supervisor waits for NSF replies

B.
how often graceful restart messages are sent after a switchover

C.
how long NSF-aware neighbors should wait after a graceful restart has started before tearing
down adjacencies

D.
how long the NSF-capable switches should wait after detecting that a graceful restart has started,
before verifying that adjacencies are still valid

Explanation:
Graceful restart (GR) refers to the capability of the control plane to delay advertising the absence of
a peer (going through control-plane switchover) for a “grace period,” and thus help minimize
disruption during that time (assuming the standby control plane comes up). GR is based on
extensions per routing protocol, which are interoperable across vendors. The downside of the grace
period is huge when the peer completely fails and never comes up, because that slows down the
overall network convergence, which brings us to the final concept: nonstop routing (NSR).
NSR is an internal (vendor-specific) mechanism to extend the awareness of routing to the standby
routing plane so that in case of failover, the newly active routing plane can take charge of the
already established sessions.

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