OSPF types of routes

If you look at an OSPF routing table ("sh ip route ospf", for instance), there are a couple of different types of routes that might be displayed.

• "O" -- An OSPF route from within the same OSPF area.
• "IA" -- An OSPF route from a different OSPF area.
• "E1" -- An OSPF route that is a Type 1 external route.
• "E2" -- an OSPF route that is a Type 2 external route.

An External route is one that is learned from another routing protocol and redistributed into OSPF.  Type E1 is an external route that includes the path metrics from the redistribution other routing protocol and the cost to get to the router that's doing the redistributing.  Type E2 is an external route that only includes the cost from the redistributed routing protocol and does not pay attention to the cost to get to the border router that is doing the redistributing.