Open Shortest Path First (OSPF) - Router Alley

OSPF v1.31 – Aaron Balchunas
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The OSPF Metric
OSPF determines the best (or shortest) path to a destination network using a
cost metric, which is based on the bandwidth of interfaces. The total cost of
a route is the sum of all outgoing interface costs. Lowest cost is preferred.
Cisco applies default costs to specific interface types:
Type
Cost
Serial (56K) 1785
Serial (64K) 1562
T1 (1.544Mbps) 64
Token Ring (4Mbps) 25
Ethernet (10 Mbps) 10
Token Ring (16 Mbps) 6
Fast Ethernet 1
On Serial interfaces, OSPF will use the configured bandwidth (measured in
Kbps) to determine the cost:
Router(config)# interface s0
Router(config-if)# bandwidth 64
The default cost of an interface can be superseded:
Router(config)# interface e0
Router(config-if)# ip ospf cost 5
Changing the cost of an interface can alter which path OSPF deems the
“shortest,” and thus should be used with great care.
To alter how OSPF calculates its default metrics for interfaces:
Router(config)# router ospf 1
Router(config-router)# ospf auto-cost reference-bandwidth 100
The above ospf auto-cost command has a value of 100 configured, which is
actually the default. This indicates that a 100Mbps link will have a cost of 1
(because 100/100 is 1). All other costs are based off of this. For example, the
cost of 4 Mbps Token Ring is 25 because 100/4 = 25.
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