From: McCallum, Robert (Robert.McCallum@let-it-be-thus.com)
Date: Thu Jun 12 2003 - 04:36:51 GMT-3
Howard you could have a scenario like this quite often in fact when you are connecting customer routers via a vrf within an MPLS environment. I agree that it is a design issue but hey you know what customers are like ;-> I have seen this on a few occasions when I place a customer into a vpn with
OSPF between the PE and the CE. The customer seems to forget that the way they used to connect their routers together has now completely changed. But it still works. The other beauty when doing ospf is that the customer then decides at a later date to interconnect two of his sites together. Then
moans the face of us when the traffic still goes via the MPLS core.....enter SHAM link.
> -----Original Message-----
> From: Howard C. Berkowitz [mailto:hcb@gettcomm.com]
> Sent: 12 June 2003 03:59
> To: ccielab@groupstudy.com
> Subject: Re: OSPF partitioning issue
>
>
> At 8:42 AM +0800 6/12/03, huang gang wrote:
> >hi,
> > ospf don't require that all areas except area 0 should
> be continuous.
> >huangg
>
> Only in a very, very limited sense, that there's nothing violated by
> having more than one area with the same ID. Another limited sense is
> that a nonzero area can be discontiguous with area 0.0.0.0 if it uses
> a virtual link.
>
> But WHY would anyone deliberately want to have multiple area x's
> (other than as a failure mode as a result of a partition)? There
> have been a lot of questions about doing this, and I am utterly
> mystified why anyone would WANT to do this deliberately. If nothing
> else, it's a nightmare for documentation. If you moved a router from
> one area 0.0.0.2 to another 0.0.0.2, do you really want the same
> network statements?
>
> Even if one is exploring the effect of nonzero area partitioning, set
> it up as an single area with two ABRs, and perhaps one critical
> internal link. By taking that link up and down, you'll get better
> understanding of what happens when an area is partitioned.
>
> The idea of having multiple areas with the same number comes up often
> enough on the list that I wonder if some practice lab somewhere is
> using it as a horrible example. I'd never permit it in any
> operational network for which I had responsibility.
>
> I've heard of many strange configurations rumored to be on the CCIE
> lab, but this would be beyond my worst imagination if Cisco actually
> asked you to do it.
>
> >
> >> Hi all,
> >>
> >> In the following scenario-
> >>
> >>
> >> Area0---------Area2
> >> \ |
> >> \ |
> >> \ |
> >> \ |
> >> Area2
> >>
> >> If the link between Area2 fails, it becomes discontiguos,
> this makes the
> >> intra area routes to be shown as inter area routes in
> both the area 2
> >> routers, I tested this in the lab and found no visible
> reachability issues.
> >> Does this disconinuity create any hidden problems?
> >>
> >> The other question is, that if we need to repair this,
> can i use a virtual
> >> link between Aree2 to Area 0 and then to Area 2
> >> or should I use a tunnel interface on each router putting
> them into area 2??
> >> I actually tried it using tunnel but was not able to make
> it work , as I was
> >> still seeing some of the Area 2 routes as O IA routes.
> >>
> >> Thanks as always for your inputs.
> >>
> >> Smiles,
> >>
> >> Mohit.
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