From: Brian (signal@xxxxxxxxxx)
Date: Fri Mar 02 2001 - 11:05:29 GMT-3
On Thu, 1 Mar 2001, Chuck Larrieu 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 )
I would say the E2 route would be preferred, but something tells me your
going to say otherwise :)
Here is a rambling from me back in October, when I first became confused
by what I was reading regarding E1/E2 routes in OSPF:
Date: Wed, 11 Oct 2000 20:27:09 -0500 (CDT)
From: Brian <signal@shreve.net>
To: cisco@groupstudy.com
Subject: OSPF external routes
I have seen more than a handfull of books confusingly explain E1 vs. E2
routes........and unless this is a case of the blind leading the blind, I
can't make sense of what they are trying to say.
I have used OSPF for years, and on ASBR's I basically just leave it as the
default type-2 external routes. But I think I have a handle on the
difference between the two:
E1 routes add the internal and external cost to a destination.
E2 routes consider only the external cost to a destination.
So, obvious to me, if you have an internal cost of 30 and an external cost
of say 20, then the E1 cost will be 50 and the E2 cost would be 20. I
would think that E2 would always be preferred to a destination (from the
same source) over E1. But I see book after book say (and I quote):
"OSPF type 1 routes are always preferred over type 2 routes for the same
destination, because the cost will always be lower."
Now if you have a E1 and E2 route with EQUAL costs, I know the E2 route
will be preferred. But if anyone can make sense of the above sentence I
would appreciate it. Like I said, to me it seems vivdly clear that E2
cost will always be less than E1 cost for a given destination, and
therefore always preferred (for the same path).
Let me say this now, the statement:
"OSPF type 1 routes are preferred over type 2" routes, I can agree
with. Because from a design perspective.......it would be preferrable
(generally) to take into account both the internal and external cost of a
route.........so i agree with that.
But when book author reads into that further and then tacks onto the end
of the sentence "....because the cost will always be lower." Then I think
to myself a) either they don't understand what they read, or b) I don't
understand what I think I know.........and it worries me :)
Brian
>
> 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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