Question

An enterprise runs OSPF with three ASBRs (R1, R2, R3) redistributing the same external network 172.20.0.0/16 into area 0. Users report that traffic to a host in 172.20.5.0/24 always egresses through R3, even though R1 offers a much shorter internal path. All three ASBRs redistribute with default settings and no route maps. You run 'show ip ospf border-routers' and 'show ip route 172.20.5.0' on a core router and see the route points to R3 as a Type 5 external (E2) with metric 20. What is the MOST likely reason the router selected R3?

Accepted Answer

For E2 routes with equal external metrics, OSPF selects the path with the lowest cost to the ASBR (forwarding metric); R3 has the lowest internal cost to reach it from this router.

Answer

E2 routes always prefer the ASBR with the highest router ID, and R3 has the highest router ID of the three ASBRs.

Answer

R3 is advertising the external route as E1, which is always preferred over the E2 routes from R1 and R2.

Answer

The external metric of 20 on R3 is lower than the default of 110 used by R1 and R2, so R3 wins.