From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sat Apr 15 2006 - 21:17:06 GMT-3
Core functionality like this does not change between IOS
versions. If you can reproduce the behavior they saw post the
configuration and verification, otherwise I wouldn't necessarily trust
it.
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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> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> James Simons
> Sent: Saturday, April 15, 2006 6:55 PM
> 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
> > 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