HP 3500YL - Manual (Page 262)

IP Routing Features
Configuring OSPF
13. Configuring an ABR To Use a Virtual Link to the
Backbone
All ABRs (area border routers) must have either a direct, physical or indirect,
virtual link to the OSPF backbone area (0.0.0.0 or 0). If an ABR does not have
a physical link to the area backbone, the ABR can use a virtual link to provide
a logical connection to another ABR having a direct physical connection to
the area backbone. Both ABRs must belong to the same area, and this area
becomes a transit area for traffic to and from the indirectly connected ABR.
N o t e A backbone area can be purely virtual with no physical backbone links. Also
note that virtual links can be “daisy chained”. If so, it may not have one end
physically connected to the backbone.
Because both ABRs in a virtual link connection are in the same OSPF area,
they use the same transit area ID. This setting is automatically determined
by the ABRs and should match the area ID value configured on both ABRs in
the virtual link.
The ABRs in a virtual link connection also identify each other with a neighbor
router
setting:
■ On the ABR having the direct connection to the backbone area, the
neighbor router is the IP address of the router interface needing a logical
connection to the backbone.
■ On the opposite ABR (the one needing a logical connection to the back­
bone), the neighbor router is the IP address of the ABR that is directly
connected to the backbone.
N o t e s By default, the router ID is the lowest numbered IP address or (user-config­
ured) loopback interface configured on the device. For more information or
to change the router ID, see “Changing the Router ID” on page 5-15.
When you establish an area virtual link, you must configure it on both of the
ABRs (both ends of the virtual link).
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