From: Faryar Zabihi \(fzabihi\) (fzabihi@cisco.com)
Date: Sat Apr 15 2006 - 19:34:34 GMT-3
What image are you runnin on the pods?
-----Original Message-----
From: James Simons [mailto:ccie.jimmy@gmail.com]
Sent: Sat Apr 15 15:25:26 2006
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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