[c-nsp] How to effect a totally stubby area in IS-IS

[David Barak]

--- On Thu, 6/23/11, Jared Gillis <jared.a.gillis at gmail.com> wrote:
> 
> Our current config is that the backbone routers are
> L2-only, and the POP routers are L1/L2, and the result is
> that all routes are propagated across the entire network. In
> the lab, I've tried switching backbone to L1/L2 and POP
> routers to L1, with the same effect.
> 
> I thought I may have found the solution in IS-IS multi-area
> (multiple IS-IS instances on the backbone routers), and
> moving each POP into it's own sub-connection. This
> accomplishes stopping other network routes from going down
> to the POP router, however, routes from the POP router stay
> local to the directly-connected backbone router no matter
> what I do. There is a command for redistributing routes from
> one IS-IS instance to another, but it doesn't seem to work.
> 
> Does anyone have any suggestions on how to accomplish
> this?

Hi Jared,

You didn't specify this, so I'll hazard a guess that all of your NSAPs are in the same area, and that would cause the problem you're describing.  Routers that have NSAPs in the same area can form L1 relationships, and if they are in different areas they will not.

Put each of your POPs in a different area, use a completely different area for your backbone routers, and the L1/L2 routers will set the ATT bit toward the L1-only POP routers, and you'll see the result you're looking for.  Remember, L2 will see everything, so the L1/L2 boundary needs to be thoughtfully chosen.  If you have a two-layer hierarchy, then the backbone itself are the L1/L2 routers (but each one is in a different area).  If you have a three-layer hierarchy, then L1/L2 belongs in the middle.

Hope that helps,

David Barak
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