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Date:      Sat, 4 Aug 2001 20:54:55 -0400
From:      Leo Bicknell <bicknell@ufp.org>
To:        Matt Dillon <dillon@earth.backplane.com>
Cc:        Leo Bicknell <bicknell@ufp.org>, Bernd Walter <ticso@mail.cicely.de>, sthaug@nethelp.no, oppermann@telehouse.ch, freebsd-hackers@FreeBSD.ORG, freebsd-net@FreeBSD.ORG
Subject:   Re: 303,000 routes in kernel
Message-ID:  <20010804205455.A94273@ussenterprise.ufp.org>
In-Reply-To: <200108050027.f750RkG77073@earth.backplane.com>; from dillon@earth.backplane.com on Sat, Aug 04, 2001 at 05:27:46PM -0700
References:  <20010804215529.C7176@cicely20.cicely.de> <32301.996956619@verdi.nethelp.no> <20010805002233.A7991@cicely20.cicely.de> <20010804184045.A87444@ussenterprise.ufp.org> <200108050027.f750RkG77073@earth.backplane.com>

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On Sat, Aug 04, 2001 at 05:27:46PM -0700, Matt Dillon wrote:
>     As far as I know routers on the edge tend to have full route
>     tables, but I was under the impression that modern core designs - that
>     is, the routers in the middle of a network, tended to use more of a
>     layer-2 tagged switching model.  The incoming edge routers with the full
>     route tables tag the packet, then intermediate routers switch the packet
>     using the tag (and ignore the IP address), and then when the packet
>     gets to the other edge of the network it gets run through a normal routing
>     table again.  This allows routers in the middle of the network to use
>     simple array lookups / layer-2 switched designs which are much faster
>     then full-route-table designs.

This can be done.  Some designs using MPLS implement this sort of routing,
and some ATM based designs come close to this idea.

That said, I'd venture about half of the largest ISP's still use plain old
routing in the core, every router has a full table and does a layer 3 lookup.
Of those that use MPLS and ATM, they probably only eliminate 10-25% of the
layer-3 lookups through the network.

It's the old cost problem, doing the full route table lookup is expensive
(usually in high speed-ram), which impacts the cost of the edge box more than
the core box.  When you're spending $500k-2.5M on a core router, the fact
that it has $100k of RAM vrs $70k of ram is just not your big worry. :-)

-- 
Leo Bicknell - bicknell@ufp.org
Systems Engineer - Internetworking Engineer - CCIE 3440
Read TMBG List - tmbg-list-request@tmbg.org, www.tmbg.org

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