Multi-Area OSPF. Multi-area OSPF networks can be difficult to design, and typically demand more administrative attention than any other popular interior gateway protocol. OSPF Advantages.
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Multi-area OSPF networks can be difficult to design, and typically demand more administrative attention than any other popular interior gateway protocol.
The hierarchical topology possibilities of OSPF yield several important advantages:
There are four different types of OSPF Routers:
A router can be more than one router type. If a router interconnects to Area 0 and Area 1, as well as to a non-OSPF network, it would be both an ABR and an ASBR.
An area qualifies as stub or totally stubby when it meets the following criteria:
All these criteria are important because a stub/totally stubby area is configured primarily to exclude external routes.
To configure a stub or totally stubby area, use the following on all router interfaces in that area:
An optional no-summary keyword is added only on ABRs. This configures the ABR to block interarea routes (Type 3 and Type 4 LSAs). The no-summary creates a totally stubby area.
The area stub command is configured on each router in the stub location and is essential for the routers to become neighbors and exchange routing information.
On ABRs only, there is an option of defining a cost of the default route to be automatically injected in the stub/totally stubby area:
To configure an area as a NSSA, you must configure all OSPF router interfaces that belong to the area using the following command syntax:
Theno-summarykeyword is used on the ABR and typically makes the NSSA totally stubby.
To control the summarization or filtering during the translation using the following syntax:
Router(config)#summary-address prefix mask [not-advertise] [tag tag]
Thenot-advertisekeyword is used to suppress routes that match the prefix/mask pair. This keyword applies to OSPF only. The tag value can also be assigned but is not required.
The ABR does not generate default routes in an NSSA. To force the ABR to generate the default route, use this command only on the ABR for the NSSA:
router ospf pid
area id nssa default-information originate
To verify that NSSA is defined on a given router, you can use the show ip ospf command
If a new area is added after the OSPF internetwork has been designed, and it is not possible to provide that new area with direct access to the backbone, a virtual link can be defined to provide the needed connectivity to the backbone area. Because all areas must be connected to Area 0, the virtual link provides the disconnected area a logical path to the backbone.
The virtual link has the two requirements:
On each router that will use the virtual link, create the "virtual link" configuration. The routers that make the links are the ABR that connects the remote area to the transit area and the ABR that connects the transit area to the backbone area: