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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