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).
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.
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Fig 1.1- OSPF E1 and E2 Routes
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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)