What is an ABR

According to RFC 2328, the OSPF backbone area contains the Area Border Routers (ABRs) of all the areas. That is, an ABR must belong to the backbone area. If area 2 and area 3 are configured on a device but none of the interfaces of the device belongs to the backbone area, the device is not an ABR.

RFC 3509 defines what is an ABR.

In the switch implementation, an ABR can belong to two or more areas, and one of the areas must be a backbone area. For the configuration of these areas, each area does not need to contain at least one interface in the Up state.

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