External Routes

External routing information can be gained by OSPF through a number of means. The most common means is through redistribution from another routing protocol. This external route information must then be made available throughout the OSPF AS in order for it to be of use. The AS boundary routers (ASBRs) do not summarize the external routing information, but ASBRs flood the external route information throughout the AS. Every router receives this information, with the exception of stub areas.

NOTE Summarization at ASBR happens only when configured with an outbound distribute list or an OSPF Summary statement, but this not the default. By default, Cisco routers let all external routes in without summarization.

The types of external routes used in OSPF are as follows:

• E1 routes-E1 routes' costs are the sum of internal and external (remote AS) OSPF metrics. For example, if a packet is destined for another AS, an E1 route takes the remote AS metric and adds all internal OSPF costs. They are identified by the E1 designation within the OSPF routing table.
• E2 routes-E2 routes are the default external routes for OSPF. They do not add the internal OSPF metrics; they use the remote AS only, regardless of where they are in the AS. For example, if a packet is destined for another AS, E2 routes add only the metrics from the destination AS associated with reaching the destination.

TIP Multiple routes to the same destination use the following order of preference: intra-area, inter-area, E1, and E2.