[c-nsp] vs isis routing levels

[Adam Vitkovsky]

> We're right now rocking about 1500 nodes flat l2 core.
> But we are in situation where we're going to replace L2/LAN metro 
> networks with MPLS, and this is good situation to look into our 
> alternatives
>
>
> a) keep flat L2

If you plan on using (r)LFA than you can keep all the nodes in a common L2
as "fast" SPF computation is not that important anymore. 

Anyways even without LFA, on an up to date HW the SPF (full/incremental) is
in order of microseconds. 
So I'd checkout the spf logs to see how long it takes to complete full SFP
for 1500 priority 1 prefixes (i.e. loopbacks) multiply by 10 to account for
the metro networks and if that's still acceptable I'd keep it all L2. 
I guess programing the line-cards is still the biggest holdup and this is
solved by the (r)LFA.  

adam