From: Faryar Zabihi \(fzabihi\) (fzabihi@cisco.com)
Date: Sat Apr 15 2006 - 21:25:26 GMT-3
I am sure you have done this...but just in case...i would reset the ospf
process on all the pods. The ios shouldn't matter. If it still
differs...post the configs.
Faryar
-----Original Message-----
From: James Simons [mailto:ccie.jimmy@gmail.com]
Sent: Sat Apr 15 19:57:19 2006
To: Cisco certification
Subject: Re: OSPF route preferencing
thanks Brian,
That helps in terms of solving the problem with the lab. But I still don't
understand why I would need to create a virtual link and someone else, with
the same configuration, would not. It really feels like OSPF is implemented
slightly differently depending on the IOS version.
Jimmy
On 4/15/06, Brian McGahan <bmcgahan@internetworkexpert.com> wrote:
>
> 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
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> 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
> > >
> > >
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