From: Jorge Mastrapa (jmastrap@xxxxxxxxx)
Date: Fri Mar 02 2001 - 11:33:07 GMT-3
Brian
The reason is the following..
For example, suppose you have two routers (cost 10 and 80, respectively)
advertising the same external route, which one do you take? OSPF will
determine the link metric going to those external networks. In this case of
10 and 80, because 10 is lower, that is the route that will be chosen. But
what if the cost was equal? Then OSPF will use the internal metric to
determine the lowest cost, thus breaking the tie.
Easier,.. if we receive an E2 route with different costs ( two or more
paths ) we take the lower cost, but if we receive an E2 with equal costs (
two or more paths ), we will include E1 costs to brake the tie..
OSPF Metrics ( from the OSPF Network Design Documents )
" Types of External Metrics: E1 and E2
Routes that originate from other routing protocols (or different OSPF
processes) and that are injected into OSPF via redistribution are called
external routes. There are two forms of external metrics: type 1 (E1) and
type 2 (E2). These routes are represented by O E2 or O E1 in the IP routing
table. They are examined after the router is done building its internal
routing table. After they are examined, they are flooded throughout the
Autonomous System (AS), unaltered. External information could come from a
variety of sources, such as another routing protocol.
E1 metrics result in routes adding the internal OSPF metric to the external
route metric; they are also expressed in the same terms as an OSPF
link-state metric. The internal OSPF metric is the total cost of reaching
the external destination, including whatever internal OSPF network costs are
incurred to get there. These costs are calculated by the router wanting to
reach the external route.
E2 metrics do not add the internal OSPF metric to the cost of external
routes; they are also the default type used by OSPF. The E1 metric is
generally preferred. The use of E2 metrics assumes that you are routing
between AS; therefore, the cost is considered greater than any internal
metrics. This eliminates the need to add the internal OSPF metrics. Figure
5-12 shows a nice comparison of the two metrics. "
J.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Brian
Sent: Friday, March 02, 2001 9:08 AM
To: Frank Jimenez
Cc: ccielab@groupstudy.com; Chuck Larrieu
Subject: Re: Friday Follies Returns - OSPF route preference
On Fri, 2 Mar 2001, Frank Jimenez wrote:
> Chuck,
>
> As fate would have it, I was just pondering the same thing at a customer
site a few days ago.
>
> The answer is a), given that the routes are both for the same destination.
>
> From: http://www.cisco.com/warp/public/104/3.html under the heading 'E1
vs. E2 External Routes'
>
> "A type 1 route is always preferred over a type 2 route for the same
destination."
I read that too.......and thats fine and great..........but when they
follow that sentence with: "because the cost will always be lower" thats
when I get confused. And I quote from Coriolis Exam Cram p.208 para.1
"OSPF type 1 routes are always preferred over type 2 routes for the same
destination, because the cost will always be lower"
I don't see how the cost is lower, thats what confuses me
>
>
> Frank Jimenez, CCIE #5738
> franjime@cisco.com
>
>
>
>
> At 10:06 PM 03/01/2001 -0800, you wrote:
> >A less fun reprise of my once famous Friday Follies questions:
> >
> >Given the following information, which route will ospf prefer, and
therefore
> >place into the routing table?
> >
> >a) E1 ( external type 1 ) with a cost of 84
> >b) E2 ( external type 2 ) with a cost of 20
> >
> >For extra credit - why?
> >( I don't know either and I can't find a good explanation on the doc CD )
> >( but this time I have traces and captures to prove the point )
> >
> >Chuck
> >----------------------
> >I am Locutus, a CCIE Lab Proctor. Xx_Brain_dumps_xX are futile. Your life
as
> >it has been is over ( if you hope to pass ) From this time forward, you
will
> >study US!
> >( apologies to the folks at Star Trek TNG )
> >
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