OSPFv3 Graceful restart
To protect data plane during the
events on Control plane like failover, process restart etc., Graceful restart
can be used. In EXOS OSPFv3 Graceful restart supported from 16.2
Without Graceful restart
Without graceful restart, adjacent
routers will assume that information previously received from the restarting
router is stale and will not be used to forward traffic to that router.
With Graceful restart
However, in many cases, two conditions
exist that allow the router restarting OSPF to continue to forward traffic
correctly. The first condition is that forwarding can continue while the
control function is restarted. Most modern router system designs separate the
forwarding function from the control function so that traffic can still be
forwarded independent of the state of the OSPF function. Routes learned through
OSPF remain in the routing table and packets continue to be forwarded. The
second condition required for graceful restart is that the network remain
stable during the restart period. If the network topology is not changing, the
current routing table remains correct. Often, networks can remain stable during
the time for restarting OSPF.
Restarting and helper mode
Routers involved with graceful restart
fill one of two roles: the restarting router or the helper router.
With graceful restart, the router that
is restarting sends out Grace-LSAs informing its neighbors that it is in
graceful restart mode, how long the helper router should assist with the
restart (the grace period), and why the restart occurred. If the neighboring
routers are configured to help with the graceful restart (helper-mode), they
will continue to advertise the restarting router as if it was fully adjacent.
Traffic continues to be routed as though the restarting router is fully
functional.
If the network topology changes, the helper routers will stop
advertising the restarting router. The helper router will continue in helper
mode until the restarting router indicates successful termination of graceful
restart, the Grace-LSAs expire, or the network topology changes. A router can
be configured for graceful restart, and for helper-mode separately. A router
can be a helper when its neighbor restarts, and can in turn be
helped by a neighbor if it restarts.
Planned and Unplanned Restarts
Two types of graceful restarts are
defined: planned and unplanned.
A planned restart would occur if the
software module for OSPFv3 was upgraded, or if the router operator decided to
restart the OSPF control function for some reason. The router has advance
warning, and is able to inform its neighbors in advance that OSPF is
restarting.
An unplanned restart would occur if
there was some kind of system failure that caused a remote rebootor a crash of
OSPFv3, or a failover occurs. As OSPFv3 restarts, it informs its neighbors that
it is in themidst of an unplanned restart.
You can decide to configure a router to
enter graceful restart for only planned restarts, for only unplanned restarts,
or for both. Also, you can separately decide to configure a router to be a
helper for only planned, only unplanned, or for both kinds of restarts.
OSPFv3 Graceful restart Configuration
• Configure a router to perform graceful
OSPFv3 restart:
configure ospfv3 restart [none | planned | unplanned | both]
• Configure a router interface to act as a
graceful OSPFv3 restart helper:
configure ospfv3 [[vlan | tunnel] all | {vlan} vlan-name | {tunnel}
tunnelname | area area-identifier] restart-helper [none | planned | unplanned
|both]
• The graceful restart period sent out to
helper routers can be configured with the following command:
configure ospfv3 restart grace-period seconds
• Disable or enable helper router LSA check:
disable ospfv3 [[vlan | tunnel] all | vlanvlan-name | {tunnel}
tunnel-name |area area-identifier] restart-helper-lsa-check
enable ospfv3 [[vlan | tunnel] all | {vlan} vlan-name | {tunnel}
tunnel-name |area area-identifier] restart-helper-lsa-check
By default, a helper router will
terminate graceful restart if received LSAs would affect the restarting router.
This will occur when the restart-helper receives an LSA that will be flooded to
the restarting router or when there is a changed LSA on the restarting router's
retransmission list when graceful restart is initiated.
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