Refer to the exhibit.
Which statement correctly explains the bgp graceful-restart command?
A . This command is used to enable NSR and is entered on the NSR-capable router, and also on any NSR-aware peer
B . This command is used to enable NSF and is entered on the NSF-capable router, and also on any NSF-aware peer
C . This command is only required on the NSF-capable routers to enable BGP graceful restart with the BGP peers
D . This command is only required on the NSF-aware routers to enable BGP graceful restart with the BGP peers
E . This command is only required on the NSR-capable routers to enable BGP graceful restart with the BGP peers
Answer: B
Explanation:
Graceful restart is supported in recent versions of Cisco IOS software (12.0S) and is supported in Cisco IOS XR software. Graceful restart is the mechanism by which BGP routing peers avoid changes to their forwarding paths following a switchover. If the BGP peer has received this capability, it is aware that the device sending the message is nonstop forwarding (NSF)-capable. Both the NSF-capable router and its BGP peers (NSFaware
peers) need to exchange the graceful restart capability in their OPEN messages, at the time of session
establishment. If both peers do not exchange the graceful restart capability, the session will not be graceful restart-capable.
If the BGP session is lost during a Route Processor (RP) switchover or BGP process restart, the
NSF-aware BGP peer marks all the routes associated with the NSF-capable router as stale; however, it continues to usethese routes to make forwarding decisions for a set period of time. This functionality means that no packets arelost while the newly active RP is waiting for convergence of the routing information with its BGP peers.
After a failover event occurs, the NSF-capable router reestablishes the session with the BGP peer. Inestablishing the new session, it sends a new graceful restart message that identifies the NSF- capable router ashaving restarted.At this point, the routing information is exchanged between the two BGP peers. Once this exchange iscomplete, the NSF-capable device uses the newly received routing information to update the RIB and theForwarding Information Base (FIB) with the new forwarding information. The NSF-aware device uses thenetwork information to remove stale routes from its BGP table. The BGP protocol is then fully converged.
If a BGP peer does not support the graceful restart capability, it will ignore the graceful restart capability in anOPEN message but will establish a BGP session with the NSF-capable device. This functionality will allowinteroperability with non-NSF-aware BGP peers (and without NSF functionality), but the BGP session with non-NSF-aware BGP peers will not be graceful restart- capable.