[c-nsp] Rationale for ISIS default origination behavior

[Adam Vitkovsky]

Basically it doesn't matter much whether you inject 8.8.8.8 or 0.0.0.0 from
all network edges and let IGP to find best path to nearest NH for that
route. 

I guess the question turned out to be 
What would be the best trigger for a default route advertisement either from
the edge(PE) routers or core(RR) routers. 
-eBGP peer's interface might be reachable though the eBGP session might be
down. 
-ability or inability to see a single route (or couple of routes for that
matter), might not reflect the ability of a particular edge to perform
full-table routing. 

I believe we lack the knobs in our route-maps/policies to do the default
advertisement decision wisely
We would need something like: 
Is the eBGP session operational to this eBGP peer? If yes than originate
default [advertise it to iBGP peers]. 
Or did I receive certain number of prefixes from this eBGP peer? If yes I
can advertise a default route. 


adam