Extreme Networks ExtremeWare XOS 10.1, Manual

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190 ExtremeWare XOS 10.1 Concepts Guide
Interior Gateway Protocols
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.
Backbone Area (Area 0.0.0.0)
Any OSPF network that contains more than one area is required to have an area configured as area
0.0.0.0, also called the backbone. All areas in an autonomous system must be connected to the backbone.
When designing networks, you should start with area 0.0.0.0, and then expand into other areas.
NOTE
Area 0.0.0.0 exists by default and cannot be deleted or changed.
The backbone allows summary information to be exchanged between ABRs. Every ABR hears the area
summaries from all other ABRs. The ABR then forms a picture of the distance to all networks outside of
its area by examining the collected advertisements, and adding in the backbone distance to each
advertising router.
When a VLAN is configured to run OSPF, you must configure the area for the VLAN. If you want to
configure the VLAN to be part of a different OSPF area, use the following command:
configure ospf vlan <vlan-name> area <area-identifier>
If this is the first instance of the OSPF area being used, you must create the area first using the
following command:
create ospf area <area-identifier>
Stub Areas
OSPF allows certain areas to be configured as stub areas. A stub area is connected to only one other area.
The area that connects to a stub area can be the backbone area. External route information is not
distributed into stub areas. Stub areas are used to reduce memory consumption and computation
requirements on OSPF routers. Use the following command to configure an OSPF area as a stub area:
configure ospf area <area-identifier> stub [summary | nosummary] stub-default-cost
<cost>
Not-So-Stubby-Areas (NSSA)
NSSAs are similar to the existing OSPF stub area configuration option, but have the following two
additional capabilities:
• External routes originating from an ASBR connected to the NSSA can be advertised within the
NSSA.