[c-nsp] Rationale for ISIS default origination behavior

[Saku Ytti]

On (2013-01-22 07:59 -0700), John Neiberger wrote:

> But thinking about that, it once again makes me wonder why we are
> redistributing the default into ISIS. If the default already exists in iBGP
> and the next-hop is in ISIS, that's going to converge pretty quickly. I'll
> have to think about this some more. There's probably some obvious factor
> that I'm overlooking.

In your edge boxes, advertise to full-mesh iBGP some eBGP route which you
consider important and stable, such as 8.8.8.8. You can pick few if you
want.

In your non-full-route boxes have static route to 8.8.8.8.

Now all your routers see all edge boxes as default destinations and will
choose which ever is closest, in terms IGP metric.
If one edge connection to INET, your 8.8.8.8 will be pulled, and static
route won't recurse to that edge anymore

To add robustness, in case you don't trust 8.8.8.8 to be there always you
can add another eBGP route, or fall back to ASBR loop0 by having static
route with worse admin distance.

-- 
  ++ytti