Brocade Communications Systems ICX 7250 series, Руководство по настройке

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OSPFv2 graceful restart
The graceful restart (GR) feature provides a routing device with the capability to inform its neighbors when it is performing a restart.
Neighboring devices, known as GR helpers, are informed via protocol extensions that the device is undergoing a restart and assist in the
restart. For the duration of the graceful restart, the restarting device and its neighbors continue forwarding packets ensuring there is no
disruption to network performance or topology. Disruptions in forwarding are minimized and route flapping diminished. When the restart
is complete, the device is able to quickly resume full operation due to the assistance of the GR helpers. The adjacent devices then return
to normal operation.
There are two types of OSPFv2 graceful restart:
• Planned restart : the restarting routing device informs its neighbors before performing the restart. The GR helpers act as if the
routing device is still within the network topology, continuing to forward traffic to the restarting routing device. A defined interval,
known as a “grace period” is set to specify when the neighbors should consider the restart complete and the restarting routing
device as part of the network topology again.
• Unplanned restart : the routing device restarts without warning due to a software fault.
NOTE
In order for a graceful restart on a routing device to be successful, the OSPFv2 neighbors must have GR-helper mode enabled.
GR-helper mode is enabled by default.
OSPFv2 stub router advertisement
OSPFv2 stub router advertisement is an open standard based feature and it is specified in RFC 3137. This feature provides a user with
the ability to gracefully introduce and remove an OSPFv2 device from the network, by controlling when the data traffic can start and stop
flowing through the device in cases where there are other OSPFv2 devices present on the network providing alternative paths for the
traffic. This feature does not work if there is no alternative for the traffic through other OSPFv2 routers. The device can control the data
traffic flowing through it by changing the cost of the paths passing through the configured device. By setting the path cost high the traffic
will be redirected to other OSPFv2 devices 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 device is ready to forward the traffic, the links are
advertised with the real metric value instead of the maximum value.
OSPFv2 stub router advertisement is useful for avoiding a loss of traffic during short periods when adjacency failures are detected and
traffic is rerouted. Using this feature, traffic can be rerouted before an adjacency failure occurs due to common services interruptions such
as a router being shutdown for maintenance.
OSPFv2 stub router advertisement is also useful during startup because it gives the device enough time to build up its routing table
before forwarding traffic. This can be useful where BGP is enabled on the device because it takes time for the BGP routing table to
converge.
You can also configure and set a metric value for the following LSA types:
• Summary (type 3 and type 4)
• External (type 5 and type 7)
• Opaque (type 10, TE link)
OSPFv2 stub router advertisement
Brocade FastIron Layer 3 Routing Configuration Guide
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