HP 200 Series Services And Applications page 267

Routing with OSPF
Routing Improvements
Stub Areas. Stub areas are areas into which external routes are not
propagated. The term external route has a particular significance in
OSPF. Routing information provided to OSPF from any protocol other
than OSPF itself is considered external. Thus routes provided by RIP or
EGP as well as static routes are considered external. An OSPF router
that interfaces to an external router is called an "AS Boundary Router."
Consider an AS connected to the open internet through EGP. There are
potentially thousands of routes for which reachability information could
be obtained. To prevent this information from being propagated into an
area, an area can be configured as a stub area. In figure 1, area A is a
stub area. Therefore, external routing information received in area D
will not be propagated into area A.
Virtual Links. In figure 1, area C is not directly attached to the Back-
bone. Instead, it is attached to area B through a "backbone virtual link".
Virtual links allow areas to be configured where it would otherwise be
inconvenient to do so due to the distance or cost to attach to the Back-
bone.
Metrics
The OSPF routing algorithm calculates the shortest path to each destination
network. A path is composed of a series of network links from a router to a
particular destination network. Each link is assigned a metric. The metric
for a path is simply the sum of all the link metrics. The shortest path to a
network is thus the path with the lowest metric. As networks become more
complex, the result of alternate routes and varying link speeds, metrics
become more important. OSPF provides a 16-bit (0 to 65535) dimensionless
metric for the assignment of link costs and allows 24 bits for inter-area paths.
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