Costs are defined as metric values associated with a link. The
cost of a path is determined by adding up the cost of each link in a
path. Forwarding decisions are based on the cost of each path.
Calculation of the cost for summary and external routes is as
follows: The cost of a summary route is the smallest cost of a given
inter-area route that appears in the summary plus the cost of the
ABR link to the backbone. So if the ABR link to the backbone was 50,
and the summary router had two inter-area routes, one at cost 49 and
the other at cost 50, the total cost associated with the summary
route would be 99. This calculation is done automatically for each
summary route.
Calculating the cost for external routes
The cost of an external route differs depending on the external
type configured on the ASBR. You configure the router to generate
one of the following external packet types:
- Type-1 (E1) --- If a packet is an E1, then the metric is
calculated by adding the external cost to the internal cost of
each link the packet crosses. Use this packet type when you have
multiple ASBRs advertising a route to the same autonomous
system.
- Type-2 (E2) --- If a packet is an E2, then the packet will
always have the external cost assigned, no matter where in the
area it crosses (this is the default). Use this packet type if
only one router is advertising a route to the autonomous system.
Type-2 routes are preferred over Type-1 routes unless two equal
cost routes exist to the destination.