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Figure 8-3. OSPF costs for different types of links
The MAX has a default cost of one for a connected route (Ethernet) and ten for a WAN link. If
you have two paths to the same destination, the MAX selects the one with the lower cost. You
might want to account for the bandwidth of a connection when assigning costs. For 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 can 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 AS.
Each area 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 8-4, all of the routers are ABRs. If you set up the ABRs and
area boundaries correctly, link-state databases are unique to an area.
MAX 6000/3000 Network Configuration Guide
Cost = 10
Router-2
T1
A
Router-2
Router-1
Cost = 10
Router-3
T1
56Kbps
Router-3
Cost = 240
Configuring OSPF Routing
OSPF overview
B
8-5
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