Which high-availability routing feature requires the neighbor router to support the graceful restart
capability?

A.
BFD

B.
NSR

C.
NSF

D.
MTR

Explanation:

On Cisco IOS XR software, NSF minimizes the amount of time a network is unavailable to its
users following a route processor (RP) failover. The main objective of NSF is to continue
forwarding IP packets and perform a graceful restart following an RP failover.
When a router restarts, all routing peers of that device usually detect that the device went down
and then came back up. This transition results in what is called a routing flap, which could spread
across multiple routing domains. Routing flaps caused by routing restarts create routing
instabilities, which are detrimental to the overall network performance. NSF helps to suppress
routing flaps in NSF-aware devices, thus reducing network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing
protocol information is being restored following an RP failover. When the NSF feature is
configured, peer networking devices do not experience routing flaps. Data traffic is forwarded
through intelligent line cards while the standby RP assumes control from the failed active RP
during a failover. The ability of line cards to remain up through a failover and to be kept current
with the Forwarding Information Base (FIB) on the active RP is key to NSF operation.
When the Cisco IOS XR router running IS-IS routing performs an RP failover, the router must
perform two tasks to resynchronize its link-state database with its IS-IS neighbors. First, it must
relearn the available IS-IS neighbors on the network without causing a reset of the neighbor
relationship. Second, it must reacquire the contents of the link-state database for the network.
The IS-IS NSF feature offers two options when configuring NSF:
•IETF NSF
•Cisco NSF
If neighbor routers on a network segment are NSF aware, meaning that neighbor routers are
running a software version that supports the IETF Internet draft for router restartability, they assist

an IETF NSF router that is restarting. With IETF NSF, neighbor routers provide adjacency and
link-state information to help rebuild the routing information following a failover.
In Cisco IOS XR software, Cisco NSF checkpoints (stores persistently) all the state necessary to
recover from a restart without requiring any special cooperation from neighboring routers. The
state is recovered from the neighboring routers, but only using the standard features of the IS-IS
routing protocol. This capability makes Cisco NSF suitable for use in networks in which other
routers have not used the IETF standard implementation of NSF