From: Nigel Taylor (nigel_taylor@xxxxxxxxxxx)
Date: Fri Mar 02 2001 - 06:26:11 GMT-3
Fwells12,
I can't say that I've remember reading anything for the Tom
Thomas book which I have read but here's where I turned to since most of
these book are written around the protocol specific implementation(rfc's and
draft standards). RFC 2328 Section 11.
Moy Standards Track [Page 108]
RFC 2328 OSPF Version 2 April 1998
The set of paths to use for a destination may vary based on the OSPF
area to which the paths belong. This means that there may be
multiple routing table entries for the same destination, depending
on the values of the next field.
Area
This field indicates the area whose link state information has
led to the routing table entry's collection of paths. This is
called the entry's associated area. For sets of AS external
paths, this field is not defined. For destinations of type
"router", there may be separate sets of paths (and therefore
separate routing table entries) associated with each of several
areas. For example, this will happen when two area border
routers share multiple areas in common. For destinations of
type "network", only the set of paths associated with the best
area (the one providing the preferred route) is kept.
The rest of the routing table entry describes the set of paths to
the destination. The following fields pertain to the set of paths
as a whole. In other words, each one of the paths contained in a
routing table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in decreasing order of preference:
intra-area, inter-area, type 1 external or type 2 external.
Intra-area paths indicate destinations belonging to one of the
router's attached areas. Inter-area paths are paths to
destinations in other OSPF areas. These are discovered through
the examination of received summary-LSAs. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS-external-LSAs.
Cost
The link state cost of the path to the destination. For all
paths except type 2 external paths this describes the entire
path's cost. For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS. This
cost is calculated as the sum of the costs of the path's
constituent links.
NIgel.
----- Original Message -----
From: fwells12 <fwells12@hotmail.com>
To: <ccielab@groupstudy.com>
Sent: Friday, March 02, 2001 3:35 AM
Subject: Re: Friday Follies Returns - OSPF route preference
> In Tom Thomas' Cisco Press book 'OSPF Network Solutions Design Guide' is
> says exactly the opposite!! IE: a type 2 route is always preferred to a
> type one route to the same destination. Type 1 routes are those which
have
> their costs incremented as they cross the network. I would hazzard a
guess
> that on that basis alone they would be preferred due to the likelyhood of
> them reflecting more accurate costs.
>
> ----- Original Message -----
> From: Frank Jimenez <franjime@cisco.com>
> To: <ccielab@groupstudy.com>; Chuck Larrieu <chuck@cl.cncdsl.com>
> Sent: Thursday, March 01, 2001 10:20 PM
> Subject: Re: Friday Follies Returns - OSPF route preference
>
>
> > 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."
> >
> >
> > 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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