User`s guide

9-6 Preliminary November 23, 1998 MultiVoice Gateway for the MAX— User’s Guide
Configuring OSPF Routing
Introduction to OSPF
example, for a single B-channel connection, the cost would be 24 times greater than for a T1
link.
Note: Be careful when assigning costs. Incorrect cost metrics can cause delays and
congestion on the network.
Hierarchical routing (areas)
If a network is large, the size of the database, time required for route computation, and related
network traffic become excessive. An administrator can partition an AS into areas to provide
hierarchical routing connected by a backbone.
The backbone area is special and always has the area number 0.0.0.0. Other areas are assigned
area numbers that are unique within the autonomous system.
Each areas acts like its own network: All area-specific routing information stays within the
area, and all routers within an area must have a synchronized topological database. To tie the
areas together, some routers belong to the backbone area and to another area. These routers are
area border routers (ABRs). In Figure 9-5, all of the routers are ABRs.If the ABRs and area
boundaries are set up correctly, link-state databases are unique to an area.
Figure 9-5. Dividing an AS into areas
Note: At this release, Ascend recommends that you do not configure the MultiVoice Gateway
as an ABR. The current recommendation is that you use the same area number for the Ethernet
interface of the MultiVoice Gateway and each of its WAN links. That number does not have to
be the backbone area number. The MultiVoice Gateway can reside in any OSPF area.
Stub areas
To reduce the cost of routing, OSPF supports stub areas, in which a default route summarizes
all external routes. For areas that are connected to the backbone by only one ABR (that is, the
area has one exit point), there is no need to maintain information about external routes. Stub
areas are similar to regular areas except that the routers do not enter external routes in the
area’s databases.
To prevent flooding of external routes throughout the AS, you can configure an area as a stub if
the area has a single exit point, or if the choice of exit point need not be made on a
per-external-destination basis. You might need to specify a stub area with no default cost
(StubNoDefault) if the area has more than one exit point.