All areas in an AS must connect with the backbone through one or more ABRs. If a normal area is not directly
connected to the backbone area, it must be configured with a virtual link to an ABR that is directly connected to
the backbone. The remaining area types do not allow virtual link connections to the backbone area.
Figure 42: Example of an AS with multiple areas and external routes
Backbone area
Every AS must have one (and only one) backbone area (identified as area 0 or 0.0.0.0.) The ABRs of all other
areas in the same AS connect to the backbone area, either physically through an ABR or through a configured,
virtual link. The backbone is a transit area that carries the type-3 summary LSAs, type-5 AS external link LSAs
and routed traffic between non-backbone areas, as well as the type-1 and type-2 LSAs and routed traffic internal
to the area. ASBRs are allowed in backbone areas.
Normal area
This area connects to the AS backbone area through one or more ABRs (physically or through a virtual link) and
supports type-3 summary LSAs and type-5 external link LSAs to and from the backbone area. ASBRs are allowed
in normal areas.
Not-so-stubby-area (NSSA)
This area is available and connects to the backbone area through one or more ABRs. NSSAs are intended for
use where an ASBR exists in an area where you want to control the following:
• Advertising the ASBR's external route paths to the backbone area
• Advertising the NSSA's summary routes to the backbone area
• Allowing LSAs from the backbone area to advertise in the NSSA:
◦ Summary routes (type-3 LSAs) from other areas
◦ External routes (type-5 LSAs) from other areas as a default external route (type-7 LSAs)
In the above operation, the ASBR in the NSSA injects external routes as type 7 LSAs. (Type 5 LSAs are not
allowed in an NSSA.) The ABR connecting the NSSA to the backbone converts the type 7 LSAs to type 5 LSAs
and injects them into the backbone area for propagation to networks in the backbone and to any normal areas
configured in the AS. The ABR also injects type-3 summary LSAs:
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
