CCNP Routing and Switching ROUTE 300-101 Official Cert Guide

Redistributing into OSPF as E1 Routes

OSPF’s external metric type feature gives engineers a design tool for influencing the choice of best route. E2 routes work well when the design needs to choose the best route based on the external metric—in other words, the metric as perceived outside the OSPF domain. E2 routes ignore the internal OSPF cost (except when breaking ties for best route). Therefore, when OSPF compares two E2 routes for the same subnet, that first choice to pick the lowest-metric route is based on the external metric only.

OSPF routers calculate the metrics of E1 routes by adding the internal cost to reach the ASBR to the external cost defined on the redistributing ASBR. As a result, an engineer can influence the choice of routes ...