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3
Configuring Virtual Links
All OSPF areas must connect to the
backbone. If an area does not have a
direct physical connection to the
backbone, you can configure a
virtual link that provides a logical
path to the backbone. To connect an
isolated area to the backbone, the
logical path can cross a single
non-backbone area (i.e., transit area)
to reach the backbone. To define this
path, you must configure an ABR
that serves as an endpoint
connecting the isolated area to the
common transit area, and specify a
neighboring ABR as the other
endpoint connecting the common transit area to the backbone itself. (Note that you
cannot configure a virtual link that runs through a stub or NSSA area.)
Virtual links can also be used to create a redundant link between any area and the
backbone to help prevent partitioning, or to connect two existing backbone areas
into a common backbone.
Command Attributes
• Area ID – Identifies the transit area for the virtual link.
(The area ID must be in the form of an IP address.)
• Neighbor Router ID – Neighbor router at other end of the virtual link. This must be
an Area Border Router (ABR) that is adjacent to both the backbone and the transit
area for the virtual link.
• Events – The number of state changes or error events on this virtual link.
The other items are described under “Configuring OSPF Interfaces,” page 3-243.
Note:
This router supports up 64 virtual links.
backbone
normal
area
isolated
area
ABR
ABR
virtual
link