From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sat Apr 15 2006 - 20:33:50 GMT-3
Ahhh... now it makes sense :) The question does *not* say that
you want to prefer inter-area routes over intra-area routes. The
question says "R4 and R5 should route across the Frame Relay connection
as opposed to the PPP connection to reach each other's Loopback0
networks." The logic of the question and answer is as follows:
The frame relay link is area 1245 and the PPP link is area 0.
The Loopbacks are advertised into area 0. Since the routers always
prefer intra-area routes over inter-area routes they will route over the
PPP link. Changing the cost of the PPP link or the distance wont help
because they still prefer the intra-area route. So... you need to make
the route via the frame relay link intra-area also.
This is accomplished by configuring a virtual-link over area
1245. Since a virtual-link is an area 0 adjacency now both routes are
intra-area via area 0. Now you can change the cost on the PPP interface
because you are comparing an intra-area route to an intra-area route. :)
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
24/7 Support: http://forum.internetworkexpert.com
Live Chat: http://www.internetworkexpert.com/chat/
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> James Simons
> Sent: Saturday, April 15, 2006 2:25 PM
> To: Schulz, Dave
> Cc: ZeroFlash; Brian Dennis; Aaron Pilcher; Faryar Zabihi (fzabihi);
Cisco
> certification
> Subject: Re: OSPF route preferencing
>
> Ok,
>
> It seems that my original post was not clear enough in terms of the
> specific
> problem I was facing. I was originally just looking for the theory
behind
> preferencing inter and intra-area routes. But I made some new
discoveries
> and I figure I should share them.
>
> The original problem comes off of Internetwork Experts core lab #9.
In
> this
> lab, we have two routers, R4 and R5. These routers are directly
connected
> with a ppp connection (we will call these interefaces S1). they are
also
> connected via a second interface that goes into a frame-relay cloud
(these
> interfaces will be called S0). both routers have frame-relay map
> statements
> for the layer 3 to layer 2 mappings for this connection. I was told
to
> apply ospf to both routers and have the ppp connection (S1) and the
> loopbacks of both routers in area 0. Then I had to put the frame
> connections (S0) in area 1245. After all that, I was told that each
> router
> should route to the other router's loopback, not through the PPP
> connection,
> but through the frame cloud. For the life of me I could not get this
to
> work
>
> I didn't understand how you were supposed to route from area 0 through
a
> transit area back to area 0 and have that prefered over the intra-area
> route
> through the PPP connection. The IE solution was to increase the cost
of
> the
> PPP interfaces (S1). I tried that and it didn't work. It also made
no
> sense how that it could work anyway. So I had some colleagues test it
out
> on their pods and they got it to work fine by changing the ospf cost.
For
> some reason, the inter-area route through the frame cloud was listed
in
> the
> routing table (even though it shouldn't be) as an equal cost path to
the
> loopback interfaces. So increasing the cost of the S1 interface
> accomplished the task. After going back over this several times as a
> group,
> we all made sure that our configs were exactly the same. We all felt
that
> the behavior of my routers is correct according to how OSPF should
work
> but
> their router's behavior was correct according to the IE solution.
>
> All of this leads me to believe that OSPF is not implemented exactly
the
> same from IOS version to IOS version. I would have thought that the
code
> for OSPF is very stable and is not prone to changing but since the
only
> difference between the pods we tested it out on is IOS, then that is
our
> only conclusion.
>
> Hope this helps. And let me know if you have any other theories for
why
> this occured.
>
> cheers,
>
> Jimmy
>
> On 4/15/06, Schulz, Dave <DSchulz@dpsciences.com> wrote:
> >
> > If we are looking to change the preference for the route (intra and
> > inter-area routes), why not simply change the distance between the
type
> in
> > the specifc router? Just a thought.
> >
> > Dave
> >
> > ------------------------------
> > *From:* nobody@groupstudy.com on behalf of ZeroFlash
> > *Sent:* Fri 4/14/2006 10:49 AM
> > *To:* 'Brian Dennis'; Aaron Pilcher; ZeroFlash; Faryar Zabihi
(fzabihi);
> > James Simons; Cisco certification
> >
> > *Subject:* RE: OSPF route preferencing
> >
> > Brian --
> >
> > PBR does not answer the question as you stated, but creating a
tunnel
> via
> > tunnel interfaces or virtual-links can change the route decision
process
> > inside the OSPF process, correct?
> >
> > Depending on the area that the tunnel interfaces are..
> >
> > ZeroFlash
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com<nobody@groupstudy.com>]
> > On Behalf Of
> > Brian Dennis
> > Sent: Friday, April 14, 2006 10:39 AM
> > To: Aaron Pilcher; ZeroFlash; Faryar Zabihi (fzabihi); James Simons;
> Cisco
> > certification
> > Subject: RE: OSPF route preferencing
> >
> > Of course you can use all kinds of methods to have the "routing
table"
> > prefer one route over another but that wasn't the question. The
> > question was basically asking if inter-area routes can be preferred
to
> > intra-area routes. With PBR you are overriding the routing table
but
> > not changing the route selection process within OSPF itself. The
> > problem I have with your answer is that people will now assume that
they
> > can get PBR to have OSPF select inter-area routes over intra-area
routes
> > which isn't true.
> >
> > Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
> > bdennis@internetworkexpert.com
> >
> > Internetwork Expert, Inc.
> > http://www.InternetworkExpert.com
> > Toll Free: 877-224-8987
> > Direct: 775-745-6404 (Outside the US and Canada)
> >
> >
> > -----Original Message-----
> > From: Aaron Pilcher [mailto:apilcher@itgcs.com <apilcher@itgcs.com>]
> > Sent: Friday, April 14, 2006 4:54 AM
> > To: Brian Dennis; 'ZeroFlash'; 'Faryar Zabihi (fzabihi)'; 'James
> > Simons'; 'Cisco certification'
> > Subject: RE: OSPF route preferencing
> >
> >
> >
> > I 110% agree, but you could force a single ospf router to route over
an
> > inter-area route when the ospf process would naturally select an
> > intra-area
> > route. I mean, making "unnatural" stuff happen is what PBR is all
> >
> > about.
> >
> >
> >
> >
> > -----Original Message-----
> > From: Brian Dennis
> [mailto:bdennis@internetworkexpert.com<bdennis@internetworkexpert.com>
> > ]
> > Sent: Thursday, April 13, 2006 11:37 PM
> > To: Aaron Pilcher; ZeroFlash; Faryar Zabihi (fzabihi); James Simons;
> > Cisco
> > certification
> > Subject: RE: OSPF route preferencing
> >
> > To have inter-area routes preferred over intra-area routes, policy
based
> > routing (PBR) will not work. PBR can not alter the route selection
> > process within OSPF itself. So this means that if the requirement
is to
> > prefer inter-area over intra-area OSPF routes, PBR will not meet the
> > requirement.
> >
> > HTH,
> >
> > Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
> > bdennis@internetworkexpert.com
> >
> > Internetwork Expert, Inc.
> > http://www.InternetworkExpert.com
> > Toll Free: 877-224-8987
> > Direct: 775-745-6404 (Outside the US and Canada)
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com<nobody@groupstudy.com>]
> > On Behalf Of
> > Aaron Pilcher
> > Sent: Thursday, April 13, 2006 7:45 PM
> > To: 'ZeroFlash'; 'Faryar Zabihi (fzabihi)'; 'James Simons'; 'Cisco
> > certification'
> > Subject: RE: OSPF route preferencing
> >
> > To add to your points, PBR would also fit the bill.
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com<nobody@groupstudy.com>]
> > On Behalf Of
> > ZeroFlash
> > Sent: Thursday, April 13, 2006 9:27 PM
> > To: 'Faryar Zabihi (fzabihi)'; James Simons; Cisco certification
> > Subject: RE: OSPF route preferencing
> >
> > Creating a tunnel would certainly help in learning routes from one
area
> > to
> > another to influence route selection based on inter/intra area
routes.
> > The
> > only with this is to be careful not to learn your tunnel routes
through
> > the
> > tunnel or you'll get recursive routing and the tunnels will bounce.
> >
> > I would also think about another type of tunnel, perhaps a
virtual-link
> > here
> > might help.
> >
> > Just some thoughts off the top o the head.
> >
> > Later...
> >
> > ZeroFlash
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com<nobody@groupstudy.com>]
> > On Behalf Of
> > Faryar Zabihi (fzabihi)
> > Sent: Thursday, April 13, 2006 10:20 PM
> > To: James Simons; Cisco certification
> > Subject: RE: OSPF route preferencing
> >
> > As you might well know...AD will not work here. You might be able
to
> > accomplish this through having another OSPF process but haven't
tried.
> > You can also change routing protocol for those routes(but you
probably
> > don't want that) Also if you want to get yourself into a mess read
> > this(I have and never implemented)
> >
http://mirrors.isc.org/pub/www.watersprings.org/pub/id/draft-mirtorabi-o
> > spf-tunnel-adjacency-00.txt
> > I suggest you take a step back and see what the requirement really
wants
> > you to do.
> >
> > Faryar Zabihi
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com<nobody@groupstudy.com>]
> > On Behalf Of
> > James Simons
> > Sent: Thursday, April 13, 2006 6:51 PM
> > To: Cisco certification
> > Subject: OSPF route preferencing
> >
> > Hello all,
> >
> > Another OSPF question for you all. OSPF always prefers intra-area
> > routes over inter-area routes, regardless of the route cost right?
Is
> > there anyway to get a router to prefer an inter-area route? If
there
> > are multiple methods, I would like to know as many of them as
possible
> > since you never know what you will be allowed to do in the lab.
> >
> > thanks,
> >
> > Jimmy
> >
> >
This archive was generated by hypermail 2.1.4 : Mon May 01 2006 - 11:41:57 GMT-3