Extreme Networks Summit Summit24 Installation And User Manual page 188

Interior Gateway Routing Protocols
• Area Border Router (ABR)
An ABR has interfaces in multiple areas. It is responsible for exchanging summary advertisements
with other ABRs. You can create a maximum of 7 non-zero areas.
• 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.
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:
config ospf vlan <name> area <areaid>
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 <areaid>
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.
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.
• External routes originating from the NSSA can be propagated to other areas, including the backbone
area.
The CLI command to control the NSSA function is similar to the command used for configuring a stub
area, as follows:
config ospf area <area_id> nssa {summary | nosummary} stub-default-cost <cost>
{translate}
The option determines whether type 7 LSAs are translated into type 5 LSAs. When
translate
configuring an OSPF area as an NSSA, the
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should only be used on NSSA border routers,
translate
Summit24e3 Switch Installation and User Guide