OSPF External Routes: E1 and E2 Routes

Today I am going to talk about the OSPF external routes which is defined as E1 and E2. We will discuss about both types of routes. Whenever you get the route from ASBR it can be E1 or E2 routes. Both E1 and E2 routes are external routes (routes redistributed into OSPF).

Before understanding OSPF Routes, you guys needs to understand the OSPF protocol. For understanding the OSPF in details you can check the below written articles which helps you to understand the OSPF.

OSPF Basics
Quick Tips on OSPF Protocol
OSPF Basic Configuration
OSPF prefix-suppression- Configurations and Verification
OSPF Distance External Command Behaviour Configuration Guide
Configure Redistribution of iBGP Routes Into OSPF
Introduction to OSPF NSSA Area
Configuration OSPF To Filter Type-5 LSAs
Cisco IOS-XE: OSPF stuck in INIT - LLS TLV

What is E1 Route in OSPF?
E1 route is an external route that takes the combined costs of all paths in account when it is routed through the OSPF domain. This permits you the ability to preference the path with the best route to the exit point (closest exit. Hot potato). In other, words, as far as OSPF considers it, the external metric useful to the route and the internal metrics are the same.

Fig 1.1- OSPF E1 and E2 Routes
What is E2 Route in OSPF?
E2 route is an external route that only takes the redistributed metric (seed metric) in account when being communicated. In the other words you can say that this route is "an order of magnitude larger" than any other external route. This is to say that the E2's redistributed metric (seed metric) is not equivalent to the metrics used in the OSPF domain and should be measured after all routes that are E1 routes.

OSPF default routes are the same as other default routes. They are typically flooded as Type-5 LSAs. You have the choice of only originating a default route if one exists in the routing table (excluding the always command in your default-information originate statement) or to originate a default route regardless of the existence of a default route in the local router's routing table (default-information originate always)