Extreme Networks ExtremeWare XOS 11.3, Manual

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OSPF
ExtremeWare XOS 11.3 Concepts Guide
546
Planned and Unplanned Restarts
Two types of graceful restarts are defined: planned and unplanned. A planned restart would occur if the
software module for OSPF 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 reboot or a crash of OSPF, or an MSM failover occurs. As OSPF restarts, it
informs its neighbors that it is in the midst 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.
Configuring Graceful OSPF Restart
To configure a router to perform graceful OSPF restart, use the following command:
configure ospf restart [none | planned | unplanned | both]
Since a router can act as a restart helper router to multiple neighbors, you will specify which neighbors
to help. To configure a router to act as a graceful OSPF restart helper, use the following command:
configure ospf [vlan [all | <vlan-name>] | area <area-identifier> | virtual-link
<router-identifier> <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 ospf restart grace-period <seconds>
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. You can disable this behavior by using the following command:
disable ospf [vlan [all | <vlan-name>] | area <area-identifier> | virtual-link
<router-identifier> <area-identifier>] restart-helper-lsa-check
Areas
OSPF allows parts of a network to be grouped together into areas . The topology within an area is
hidden from the rest of the AS. Hiding this information enables a significant reduction in LSA traffic
and reduces the computations needed to maintain the LSDB. Routing within the area is determined
only by the topology of the area.
The three types of routers defined by OSPF are as follows:
●
Internal router (IR)—An internal router has all of its interfaces within the same area.
●
Area border router (ABR)—An ABR has interfaces in multiple areas. It is responsible for exchanging
summary advertisements with other ABRs.
●
Autonomous system border router (ASBR)—An ASBR acts as a gateway between OSPF and other
routing protocols, or other autonomous systems.