HP 3500YL - Manual (Page 225)

IP Routing Features
Configuring OSPF
All areas in an AS must connect with the backbone through one or more area
border routers (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.
ABR
ASBR
NSSA
ABR
ABR
ABR
Virtual Link
Backbone Area
Normal
Area
Stub
Area
Normal
Area
OSPF Domain
External
(IGP)
Domain
ASBR
External
(IGP)
Domain
Figure 5-21. Example of an Autonomous System (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.
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