2. Compare the networks that have the same network address, to determine which network is more specic. The more specic
network is the one that has more contiguous one bits in its network mask. For example, network 10.0.0.0 255.255.0.0 is more
specic than network 10.0.0.0 255.0.0.0, because the rst network has 16 ones bits (255.255.0.0) whereas the second
network has only 8 ones bits (255.0.0.0).
• – For the less specic network, use the networks address as the ID.
– For the more specic network, use the network’s broadcast address as the ID. The broadcast address is the network
address, with all ones bits in the host portion of the address. For example, the broadcast address for network 10.0.0.0
255.255.0.0 is 10.0.255.255.
If this comparison results in a change to the ID of an LSA that has already been generated, the router generates a new LSA to
replace the previous one. For example, if the router has already generated an LSA for network with ID 10.0.0.0 for network
10.0.0.0 255.255.255.0, the router must generate a new LSA for the network, if the router needs to generate an LSA for
network 10.0.0.0 255.255.0.0 or 10.0.0.0 255.0.0.0.
OSPF graceful restart
The OSPF Graceful Restart feature provides support for high-availability routing. With this feature enabled, disruptions in forwarding are
minimized and route
apping diminished to provide continuous service during times when a router experiences a restart.
With OSPF graceful restart enabled, a restarting router sends special LSAs to its neighbors called grace LSAs. These LSAs are sent to
neighbors either before a planned OSPF restart or immediately after an unplanned restart. The grace LSA species a grace period for
the neighbors of the restarting router to continue using the existing routes to and through the router after a restart. The restarting router
comes up, it continues to use its existing OSPF routes as if nothing has occurred. In the background, the router re-acquires its neighbors
prior to the restart and recalculates its OSPF routes and replaces them with new routes as necessary. Once the grace period has passed,
the adjacent routers return to normal operation.
NOTE
By default, graceful restart is enabled.
NOTE
If a Brocade ICX 6650 device is congured for OSPF graceful restart and is intended to be used in switch over, the OSPF
dead-interval should be changed to 60 seconds on OSPF interfaces to ensure that the graceful restart process succeeds
without a timeout.
OSPF stub router advertisement
OSPFv2 stub router advertisement is an open standard based feature and it is specied in RFC 3137. This feature provides a user with
the ability to gracefully introduce and remove an OSPFv2 router from the network by controlling when the data trac can start and stop
owing through the router in case where there are other OSPFv2 routers present on the network providing alternative paths for the trac.
This feature does not work if there is no alternative for the trac through other OSPFv2 routers. The router can control the data trac
owing through it by changing the cost of the paths passing through the congured router. By setting the path cost high the trac will be
redirected to other OSPFv2 routers providing a lower cost path. This change in path cost is accomplished by setting the metric of the
links advertised in the Router LSA to a maximum value. When the OSPFv2 router is ready to forward the trac the links are advertised
with the real metric value instead of the maximum value.
The feature is useful for avoiding a loss of trac during short periods when adjacency failures are detected and trac is rerouted. Using
this feature, trac can be rerouted before an adjacency failure occurs due to common services interruptions such as a router being
shutdown for maintenance.
OSPF graceful restart
FastIron Ethernet Switch Layer 3 Routing
230 53-1003627-04
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