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

[Jared Gillis]

On 06/23/2011 05:37 PM, David Barak wrote:
> 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.
> 

Ahh, okay. For simplicity, we implemented a minimal NSAP, with no area tag. It shouldn't be a problem to convert to a more detailed NSAP if it gets what we need.

> 
> 
> 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.

Okay, I see what this is doing. I set this up in the lab, and it worked as you described, however, if I have two POPs connected to one BB router, both POP routers see each other's L1 announcements (I assume because they are in the same area). This config should limit the "leaked" routes to only other POPs directly connected to the same BB, but we really need to get the POPs to only ever receive default, as we've got upwards of 30 POPs connecting to each BB router. Also, to throw another monkey wrench in the works, each POP is redundantly homed off two BBs, so it would learn double the number of prefixes learned by the POP.

Here's a quick diagram of what the network needs to look like:
BB1---BB2
|\     /|
| \   / |
\  POP1 /
 \     /
  \   /
   POP2

The BBs should be learn all routes in the network (L2), and the POPs can be whatever level, but they should only learn default, ever.

> 
> David Barak
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