[c-nsp] Rationale for ISIS default origination behavior

[Saku Ytti]

On (2013-01-20 09:41 -0700), John Neiberger wrote:

> This is sort of a follow-up to a question I had a few weeks ago about how
> to configure conditional default origination in IOS XR. It seems that ISIS
> default origination in both IOS and IOS XR behaves in a pretty suboptimal
> way. I don't have a lot of history with IS-IS, so I'm curious about this.

There is little point advertising default routes in dynamic routing
protocols, if you have recursing static routes available.

You could pick some fairly important/stable Internet address, say 8.8.8.8
then you could add loop42 with 10.42.42.42/32 in RD and RA. Then all
routers could have

ip route 0.0.0.0 0.0.0.0 8.8.8.8
ip route 0.0.0.0 0.0.0.0 10.42.42.42 200

Now if 8.8.8.8 is received from anywhere, you're using nearest exit to it,
from point of view of local router.
If 8.8.8.8 is gone for one reason or another, you're using nearest edge
router, hoping it has working route to where ever you're going.

If you have core boxes, with full view to BGP RD+RA, then you could just
add loop42 there and always just route to the nearest core box with full
view.
Typical setup could be

            Edge2
            |
PE1----P----P---P---Edge1
|           |   |
+-----------+   PE2

If each P has full view, just put 10.42.42.42/32 on their loop, and each PE
has static route there.

-- 
  ++ytti