From: Chuck Larrieu (chuck@xxxxxxxxxxxxx)
Date: Fri Mar 02 2001 - 13:03:12 GMT-3
This is an interesting discussion, no?
There is an answer to the original question. It is stated categorically in
the RFC, but without a clear explanation. In general I have had this problem
with RFC2328. I presume that an awful lot of the clarification may be found
in the draft documents and in Moy's book.
Cisco, as I have found in many cases, can be less clear about things. In
reading through one of the design guides on the CD I found a quite ambiguous
declaration, with no explanation Someone sent me a link to CCO TAC which
made the same categorical declaration as the RFC.
All of the behaviours discussed in this thread are documented in the couple
of the Cisco sources I checked. Ultimately, the RFC specifies them.
I post my traces and the quote from the FRC later today.
Chuck
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Brian Molinari
Sent: Friday, March 02, 2001 7:23 AM
To: Johnny Dedon; Jorge Mastrapa; ccielab@groupstudy.com
Subject: Re: Friday Follies Returns - OSPF route preference
I have seen this behavior in production networks.
If a router has two E2 LSAs for the same external network with the same
cost, it will use the route
coming from the closer ASBR (i.e use the E1 cost to break the tie). I don't
see this documented anywhere, but it works that way
consistently.
----- Original Message -----
From: Johnny Dedon <johnny.dedon@exodus.net>
To: Jorge Mastrapa <jmastrap@cisco.com>; <ccielab@groupstudy.com>
Sent: Friday, March 02, 2001 9:51 AM
Subject: Re: Friday Follies Returns - OSPF route preference
> Jorge,
> Is the following from you a quote from some source?
> 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..
>
> I would suggest that two equal cost paths of same external type would both
> be placed in the routing table. In fact by default OSPF will place up to
> four equal cost paths in the routing table and with maximum paths
statement
> can be 6 equal cost paths.
>
>
>
>
> Johnny Dedon
> Senior Staff Consultant
> Exodus Professional Services
> johnny.dedon@exodus.net
> www.exodus.net
> ----- Original Message -----
> From: "Jorge Mastrapa" <jmastrap@cisco.com>
> To: <ccielab@groupstudy.com>
> Sent: Friday, March 02, 2001 8:33 AM
> Subject: RE: Friday Follies Returns - OSPF route preference
>
>
> > 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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