The OSPF RFC 3623 Graceful Restart feature allows you to configure IETF NSF in ultivendor
networks. When using OSPF Graceful Restart, which mechanism is used to continue forwarding
packets during a switchover?
A.
Reverse Path Forwarding
B.
Hardware-based forwarding
C.
UDP forwarding
D.
Layer 2 Forwarding
Explanation:
Graceful Restart Router Operation
Graceful Restart Initiation
The restarting router becomes aware that it should start the graceful restart process when the
network administrator issues the appropriate command or when an RP reloads and forces and
Redundancy Facility (RF) switchover. The length of the grace period can be set by the network
administrator or calculated by the OSPF software of the restarting router. In order to prevent the
LSAs from the restarting router from aging out, the grace period should no t exceed an LSA
refresh time of 1800 seconds. In preparation for graceful restart, the restarting router must perform
the following action before its software can be reloaded: The restarting router must ensure that its
forwarding table is updated and will remain in place during the restart. No OSPF shutdown
procedures are performed since neighbor routers must act as if the restarting router is still in
service. The OSPF software is reloaded on the router (it undergoes graceful restart).
OSPF Processes during Graceful Restart After the router has reloaded; it must modify its OSPF
processes until it reestablishes full adjacencies with all former fully adjacent OSPF neighbors.
During graceful restart, the restarting router modifies its OSPF processes in the following ways:
The restarting router does not originate LSAs with LS types 1, 5, or 7 so that the other routers in
the OSPF domain will use the LSAs that the restarting router had originated prior to reloading. The
router does not modify or flush any self-originated LSAs.
The restarting router runs its OSPF routing calculations in order to return any OSPF virtual links to
operation.However, the restarting router does not install OSPF routes into the system’s forwarding table, and
the router relies on the forwarding entries that it had installed prior to undergoing the graceful
restart process. If the restarting router determines that is was the Designated Router on a given
segment prior to the graceful restart, it will reelect itself.
Graceful Restart Process Exit
The restarting router exits the graceful restart process when one of the following events occur:
The router has reestablished all adjacencies. The graceful restart was successful. The router
receives an LSA that is inconsistent with an LSA from the same router prior to the graceful restart.
The inconsistency can be mean either that the router does not support the graceful restart feature
or that the router has terminated its helper mode for some reason. The graceful restart was
unsuccessful. The grace period has expired. The graceful restart was not successful. Once the
restarting router has completed the graceful restart process, it returns to normal OSPF operation,
reoriginating LSAs based on the current state of the router and updating its forwarding tables
based on current link-state database contents. At this time, it flushes the grace-lsa’s that it had
originated during the initiation of the graceful restart process.