OSPF has two external route types. Type 1 External routes are advertised into the OSPF domain by an ASBR allowing subsequent routers to add their internal cost to reach the external network to the prefix. Type 2 External routes are advertised into the OSPF domain by an ASBR not allowing any router to manipulate or add their internal costs to reach the external network.
Type 1 External (E1) routes are useful for multi-homed OSPF domains where there are multiple outlets to the external networks, by allowing the internal routers to add their cost to reach that network allows for differentiating best paths through each of the available external paths.
For E1 routes the cost to reach the external network ASBR is added to the original cost as advertised by the ASBR, in order to compute this path cost routers add the following:
- Cost to reach the ABR
- Cost to reach the ASBR that advertised the route, ABRs announce this cost in a Type 4 LSA
- The external metric for the route, as listed in the Type 5 LSA created by the ASBR
Type 2 External (E2) routes are more deterministic, and controlled by network administrators by polarizing external routes to one ASBR or another. By not allowing the metric to change, an admin could set the cost to a specific external path through an ASBR to be preferred over all others, this could potentially create inefficient routing scenarios, however more predictable routing of traffic as a whole.
When external routes are advertised into Area 0 by the ASBR, they are advertised as Type 5 LSAs, ABRs then propagate their LSDBs with these routes documenting the ASBRs RID that advertised those external routes, and creates Type 4 LSAs to flood those external networks to nonbackbone areas along with the RID of the ASBR that sent the external route.