Posts Tagged ‘asbr’
What is the major difference in using an E1 route over an E2 route in OSPF?
From what I’ve observed, if you redistribute a route into OSPF either E1 or E2, the upstream router will still use the shortest path to get to the ASBR regardless of what is shown in the routing table.
The more I read about this, the more confused I get. Am I missing something?
This is actually a very common area of confusion and misunderstanding in OSPF. Part of the problem is that the vast majority of CCNA and CCNP texts teach the theory that for OSPF path selection of E1 vs E2 routes, E1 routes use the redistributed cost plus the cost to the ASBR, while with E2 routes only use the redistributed cost. When I just checked the most recent CCNP ROUTE text from Cisco Press, it specifically says that “[w]hen flooded, OSPF has little work to do to calculate the metric for an E2 route, because by definition, the E2 route’s metric is simply the metric listed in the Type 5 LSA. In other words, the OSPF routers do not add any internal OSPF cost to the metric for an E2 route.” While technically true, this statement is an oversimplification. For CCNP level, this might be fine, but for CCIE level it is not.
The key point that I’ll demonstrate in this post is that while it is true that “OSPF routers do not add any internal OSPF cost to the metric for an E2 route”, both the intra-area and inter-area cost is still considered in the OSPF path selection state machine for these routes.