[c-nsp] IS-IS Multiarea on 12.2 SR

[Mark Tinka]

On Friday 06 November 2009 04:09:58 pm Mikael Abrahamsson 
wrote:

> This is probably the biggest problem, the few people
> doing L1-L2 separation are those into academia/theoretics
> (passing a test/exam), when you go into the real world
> it's no longer in major use.
>
> I've never bothered to learn about ISIS L1, never needed
> to, see no use for it in real life. L2-only is the way to
> go.
>
> I'd also recommend against it from a sw standpoint. Sure,
> the sw supports it, but it hasn't been exposed to real
> life as much as L2 only because of above reasons.

Well, we switched from OSPF to IS-IS in 2008, and we're 
running:

	* L1-only for all routers/switches in a PoP.
	* L1/L2 on all core routers.
	* L2-only for all PoP-to-PoP core links.

The above has been stable, runs very well - helps us manage 
a multi-Gbps transport network :-).

I will say one thing, though. Dividing the IS-IS domain into 
L1 and L2 levels accordingly is meant to help you scale. 
However, in this case, we trade scaling for optimality (even 
with an L1 and L2 network) by performing Route Leaking on 
all core routers. So if you think about it, it sort of moots 
the point, and perhaps makes an L2-only network an obvious 
choice.

However, we still went ahead to deploy a multi-level IS-IS 
backbone, because there could be some day where we only need 
L1 routes in a specific PoP (which, to be honest, I can't 
see now - but as with anything else in network operations, 
better to be prepared).

Cheers,

Mark.
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