A router's routing table contains two routes learned via OSPF for the same destination network 10.20.0.0/16: one as an intra-area route (O) with metric 20, and another as an external Type 2 route (O E2) with metric 15 redistributed from another protocol. Which route will the router install and use for forwarding?
- AThe O E2 route, because external routes are always preferred over intra-area routes
- BBoth routes will be installed and traffic will load balance between them
- CThe intra-area O route with metric 20, because intra-area routes have a lower administrative distance than external OSPF routes
- DThe O route with metric 20, because within OSPF, intra-area routes are always preferred over external routes regardless of the AD value assigned to each type
Show answer & explanationAnswer & explanation
Correct answer: D. The O route with metric 20, because within OSPF, intra-area routes are always preferred over external routes regardless of the AD value assigned to each type
OSPF has an internal route preference hierarchy independent of Cisco's overall AD table entry (which lists OSPF as AD 110 for all types): intra-area routes are always preferred over inter-area routes, which are preferred over external (E1/E2) routes, regardless of the numeric metric value. So the intra-area route wins even though its metric (20) is higher than the external route's metric (15).
Why the other options are wrong
- A. External routes are the least preferred type within OSPF's internal hierarchy, not the most preferred.
- B. OSPF does not load balance between different route types for the same prefix; only one type is selected based on hierarchy.
- C. Cisco's AD table lists a single AD (110) for OSPF; the preference here is due to OSPF's internal route-type hierarchy, not separate AD values.
OSPF Route Type Preference
Within OSPF, route selection follows a fixed hierarchy: intra-area > inter-area > external Type 1 > external Type 2, regardless of the numeric cost/metric.
- Intra-area routes always beat inter-area and external routes
- E1 costs include external+internal metric; E2 shows only external cost
- This is separate from Administrative Distance which is 110 for all OSPF routes
Memory trick: Home turf always beats foreign turf in OSPF, no matter the price