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

[Mark Tinka]

On Sunday 08 November 2009 07:33:55 pm Richard A Steenbergen 
wrote:

> IMHO the rule of thumb for multiple areas in either ISIS
> or OSPF is "if you have to ask whether you should use
> them or not, the answer is you shouldn't". Their sensible
> use is so vastly exagerated in books and lab tests that
> it isn't even funny.

Speaking on my/our own behalf, there wouldn't be a doubt in 
our minds whether we needed the hierarchy or not.

In our case, coming from OSPF where Areas were in vast use 
(different for each PoP, and we had quite a few), it made 
sense, at the time, to maintain a similar hierarchy in IS-
IS, especially since what we wanted the most out of the 
migration was its "stretchy" property.

However, like I mentioned in an earlier post, it quickly 
dawned on us that since Route Leaking essentially adds all 
L1 routes from other PoP's into the L1 database in other 
PoP's, and you turn off the ATT bit to gain optimality, the 
point of running both L1 and L2 for scaling reasons quickly 
becomes moot.

However, having already gone down that path, in actual 
practice - operationally - it makes very little difference 
(to us) and doesn't add any undue complexity or burden. Only 
our core routers are L1/L2 capable, and those are beasts 
that forward only on MPLS labels. Everything else, i.e., all 
devices within each PoP (edge, peering, upstream, route 
reflectors, RTBH routers, aggregation switches, e.t.c.), 
speaks L1-only.

Cheers,

Mark.
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