[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:

> > 
> > 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.
> 
> Yes, this is correct, currently the entire network uses the
> same NSAP format (42.<mac addr>.00).
> 
> > 
> > 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.
> > 
> 
> So, if I'm reading your suggestion correctly, you're saying
> to have one isis area for the backbone (say with NSAP
> 42.<mac addr>.00), and a different area for each
> connected POP? 

Yes.  The NSAP address format is:

AFI.Area.Sys-ID.NSEL

Where AFI is two digits, and the NSEL is always 00 in a Cisco.  The precise demarcations of the Area and sys-ID differ between AFIs.  In AFI 49 (private internetworking) the Area ID is two 4-digit quads, and the Sys-ID is three 4-digit quads.  Check out RFC 1629 for more detail on that, but the basic answer is to encode the MAC or Loopback IP address into the Sys-ID rather than the area.


> Would the different area be created via a different
> instance (router isis pop1), or a different net address
> under the main isis instance? I may be getting confused
> between the different ways to segment an isis network, so
> I'm having trouble translating your suggestion to a config.
> 

BB1 -- BB2 -- BB3

BB1:
net 49.0000.0001.dead.beef.cafe.00
is-type level-1-2

BB2:
net 49.0bac.b05e.aced.1234.5678.00
is-type level-1-2

BB3:
net 49.ba53.ba11.1234.5678.1234.00
is-type level-1-2

Those three routers will only form L2 relationships.  In BB3, the other POP routers should have net 49.ba5e.ba11.XXXX.XXXX.XXXX.00, and be set to Level-1 only, and they will form L1 relationships and set default toward BB3.  BB3 can leak routes from L2 to L1 if you need something more specific than defaults.

David Barak
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