Alex Bligh <amb@gxn.net> wrote:

...

Yes.

Not disagreeing with Sean, which whilst amusing is often unproductive:

Heh.

One of the higher clue arguments in favour of maintaining an L2 mesh (and 'routing' protocol) is that it converges quicker than than L3 as in theory it has less nodes. The normal counter arguments are: 1. that L1 (SONET/SDH) converges quicker still.

But the reason ISPs adopted NBMA L2 networks in the first place is because the telcos convinced them that L2/L3 integration was going to happen (or because the ISPs got a cheap deal in support of the telcos' agenda of making that happen). The ATM Forum has done a great job on the IP/ATM integration front (I don't think). As you say, L1 converges quicker still, which is a very good argument for running IP on top of Packet Over SONET/SDH and using MPLS in support of IP virtual-circuits to do QoS routing.

2. as MCI/Worldcom may well have aptly demonstrated, L2 routing protocols have their own problems.

MCI/Worldcom is just a crappy bureaucracy - that's the real reason.

3. L3 routing can be hierarchicalized (sp?) and tweaked to converge just as effectively - witness sending your loopback IP's round as next hops in BGP and running an underlying IGP such as OSPF/ISIS/EIGRP which cuts out many (though unfortunately not all) recomputations when the underlying fabric changes.

But L3 routing is not the issue...

4. Implementation of L2 heavy protocols on routers is often poor. Witness how long it took Cisco to implement OAM management. So just how long does it take to notice a VC's gone down?

Telcos like nailed-up QoS products because they understand them. ISPs like best-efforts QoS products because they understand them. Cisco likes both (and buzzwords like 'integration') because it's into aquiring market share in both markets. The Internet is never (IMHO) going to become a fully circuit-switched QoS network because of all the cooperation that has to happen to prevent it melting down when operating as a best-efforts network (which is what it basically still is don't forget). MPLS (despite what the telco spin-doctors might claim) is not really about integration with ATM - it's about introducing levels of QoS appropriate to the services ISPs want to provide over their own networks (and perhaps, given a year or so, in conjunction with peer networks as well). VoIP doesn't need the predictably precise levels of jitter (with associated protocol overheads) and suchlike that the telco ATM (reinvented voice-switching) weenies claim will be required to provide a decent QoS for voice traffic.

A more convincing argument for buying ATM circuits is that they (are) / (used to be) dirt cheap. Recent experience suggests there may be a reason for this.

The telcos are flogging a dead horse with IP over ATM QoS integration. Best pretend that's why you're buying ATM if you want a cheap deal (or just invoke your MCI/Worldcom contractual QoS guarantees ;-) ).

Alex Bligh GX Networks (formerly Xara Networks)

I think it's high time you dropped the 'formerly' stuff and learn to live with the fact that you sold out. ;-) M.