Open Shortest Path First - Path Preference

Path Preference

OSPF uses path cost as its basic routing metric, which was defined by the standard not to equate to any standard value such as speed, so the network designer could pick a metric important to the design. In practice, it is determined by the speed (bandwidth) of the interface addressing the given route, although that tends to need network-specific scaling factors now that links faster than 100 Mbit/s are common. Cisco uses a metric like 10^8/bandwidth (the base value, 10^8 by default, can be adjusted). So, a 100Mbit/s link will have a cost of 1, a 10Mbit/s a cost of 10 and so on. But for links faster than 100Mbit/s, the cost would be <1.

Metrics, however, are only directly comparable when of the same type. Four types of metrics are recognized. An intra-area route is always preferred to an External route regardless of metric. In decreasing preference, these types are:

1. Intra-area
2. Inter-area
3. External Type 1, which includes both the external path cost and the sum of internal path costs to the ASBR that advertises the route,
4. External Type 2, the value of which is solely that of the external path cost