From: Erick B. (erickbe@xxxxxxxxx)
Date: Fri Mar 02 2001 - 16:54:42 GMT-3
I did some reading into this (RFC) and it is
interesting details. Need to set up lab. Another thing
is the forwarding address which is mentioned in the
paragraph below how multiple E2 and E1 routes are
treated in RFC 2328.
>From RFC 2328:
When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used
to break the tie.
Both Type 1 and Type 2 external metrics can be present
in the AS at the same time. In that event, Type 1
external metrics always take precedence.
This section has assumed that packets destined for
external destinations are always routed through the
advertising AS boundary router. This is not always
desirable. For example, suppose in Figure 2 there is
an additional router attached to Network N6, called
Router RTX. Suppose further that RTX does not
participate in OSPF routing, but does exchange BGP
information with the AS boundary router RT7. Then,
Router RT7 would end up advertising OSPF external
routes for all destinations that should be routed to
RTX. An extra hop will sometimes be introduced if
packets for these destinations need always be routed
first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows
an AS boundary router to specify a "forwarding
address" in its AS-external-LSAs. In the above
example, Router RT7 would specify RTX's IP address as
the "forwarding address" for all those destinations
whose packets should be routed directly to RTX.
The "forwarding address" has one other application.
It enables routers in the Autonomous System's interior
to function as "route servers". For example, in
Figure 2 the router RT6 could become a route server,
gaining external routing information through a
combination of static configuration and external
routing protocols. RT6 would then start advertising
itself as an AS boundary router, and would originate a
collection of OSPF AS-external-LSAs. In each
AS-external-LSA, Router RT6 would specify the correct
Autonomous System exit point to use for the
destination through appropriate setting of the LSA's
"forwarding address" field.
--- Chuck Larrieu <chuck@cl.cncdsl.com> wrote:
> 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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