quote of the day... when e1 and e2 matter...

from odom, ccnp route...

The benefits of the different external route types apply mostly to when multiple ASBRs advertise the same subnet. For example, imagine two ASBRs, ASBR1 and ASBR2, between OSPF and another routing domain. If the goal is to always send traffic through ASBR1,
you could use E2 routes and set the metric for ASBR1’s redistributed routes to a lower metric than ASBR2. Because routers ignore the internal metrics when calculating the E2 metrics, then every router will choose ASBR1 as the better ASBR. Conversely, if the goal were
to balance the traffic, and make each router pick the closest ASBR, both ASBRs could set the same metric to their redistributed routes, but make the routers Type E1. As a result, routers closer to each ASBR choose best routes based on the lower OSPF internal costs. Also, note that for a given prefix/length, OSPF always prefers an E1 route over an E2 route.

e1 v e2...

very nice... the ellipses are pretty good too...

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

E1 vs. E2 External Routes

External routes fall under two categories, external type 1 and external type 2. The difference between the two is in the way the cost (metric) of the route is being calculated. The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route. A type 1 route is always preferred over a type 2 route for the same destination. This is illustrated in the following diagram:

As the above diagram shows, RTA is redistributing two external routes into OSPF. N1 and N2 both have an external cost of x. The only difference is that N1 is redistributed into OSPF with a metric-type 1 and N2 is redistributed with a metric-type 2. If we follow the routes as they flow from Area 1 to Area 0, the cost to reach N2 as seen from RTB or RTC will always be x. The internal cost along the way is not considered. On the other hand, the cost to reach N1 is incremented by the internal cost. The cost is x+y as seen from RTB and x+y+z as seen from RTC.
If the external routes are both type 2 routes and the external costs to the destination network are equal, then the path with the lowest cost to the ASBR is selected as the best path.
Unless otherwise specified, the default external type given to external routes is type 2.


Remember that if the subnet keyword is not used, only routes that are not subnetted will be redistributed.