OSPF External Routes E1 vs E2 and Forward Metric [Explained]
What is the difference between OSPF external type 1 (E1) and external type 2 (E2)
- E1 routes have a cumulative cost to reach the destination: cost to reach ASBR + cost to destination from ASBR.
- E2 route cost is only from the ASBR to destination. This is the default type used by OSPF for redistribution.
In simple words, External route type E1 takes into consideration the costs of each hop to reach the ASBR (accumulation of costs to the ASBR is added to the route cost) while E2 metric is by default 20 with no consideration of the costs after redistribution in the ASBR.
Forward Metric
- The forwarding metric represent an internal cost to the ASBR. This metric is independent from external type 2 (the E2 cost is still non accumulative) but the Forward metric will be used to determine the best path in the case you have more than one E2 route to the same network for the same prefix.
the E1 route is preferred over the E2 route due to the OSPF path selection state machine preference. Even if we were to raise the metric of the E1 route so that the cost is higher than the E2 route, the E1 route would be preferred.
If multiple E2 routes exist with the same redistribution metric, the path with the lower forward metric (metric to the ASBR) is preferred. If there are multiple E2 routes with both the same redistribution metric and forward metric, they can both be installed in the routing table.
