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488 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
A stub area can have multiple ABRs and multiple exit points. However, all
of the exit points and routers must contain the same external routing
data so that the choice of an exit point does not need to be made for
each external destination.
Important
Considerations
Consider the following guidelines when you define stub default metrics:
■
By default, area border routers advertise a stub default metric of 1.
■
Stub default metrics are relevant only for area border routers (ABRs)
that are attached to stub networks.
■
If your network does not have external routes, you do not need to
configure the stub default metric; and you do not need a stub area.
■
If you remove the stub default metric, the ABR does not advertise a
default route into the stub area.
Virtual Links
The backbone area (0.0.0.0) must link to all areas. If any areas are
disconnected from the backbone, some areas of the autonomous system
(AS) become unreachable. In the rare case that it is impossible to
physically connect an area to the backbone, you can use a virtual link. The
virtual link provides a logical path to the backbone for the disconnected
area.
Virtual links are used to ensure that the OSPF backbone is contiguous.
You can use virtual links to:
■
Introduce new areas that do not have physical access to the backbone.
■
Add redundancy to the backbone to help prevent partitioning.
■
Patch the backbone when discontinuity occurs.
■
Connect area backbones. For example, you can merge two existing
OSPF networks into one network sharing a common backbone.
A virtual link must be established between two ABRs that share a
common nonbackbone area, with one of those ABRs directly connected
to the backbone. The nonbackbone area through which the virtual link
runs is called a
transit area .