Technical Tip: OSPF External Route Metrics Types explanation

[bkarl]

Description	This article describes the OSPF redistribution routes related to the route metrics types.
Scope	FortiGate.
Solution	Type 1: Considered close to the AS. Metric is based on the sum of the internal and external costs. Type 2: Considered far away from the AS.  Metric is based on the external cost only. OSPF router prefers a type 1 external router over a type 2. Each external route is assigned a metric. There are two types of external-route metrics. A type 1 metric is the sum of the external cost plus the internal cost to reach the ASBR. A type 2 metric is only the external cost (the internal cost is not considered). If there are two external routes to the same destination, one type 1 and one type 2, an OSPF router selects the type 1 route over the type 2 route. That is represented in E1 and E2. Type 1 Typical use: E1 routes are useful in networks where internal cost (within the OSPF domain) is relevant to determining the best route to the destination. They are preferred in networks with multiple ASBRs where internal costs may vary. Type 2 Typical use: E2 routes are useful when internal cost is not important or when it is preferred to maintain a fixed cost for all external routes, regardless of the receiver's location. They are often used in networks where access to external routes is uniform.