Jay Hennigan wrote:
>
> On Tue, 3 Oct 2000, NetEng wrote:
> > What are the benefits of layer3 switching (versus layer2)?
> Layer 3 switching is really just marketingspeak for routing.
> > Can I create VLAN's w/o using a router on a layer3 switch?
> Yes, because a layer 3 switch is a router. They just don't call it a
> router because the selling point is: routing = slow, switching = fast.
Let me jump out on a limb and oversimplify...
Layer 2 switching is MAC to MAC with no regard for any higher layer
in the stack. At layer 2 everything looks like the same layer 3 stuff.
So, the *really really* cheap L3 switches are just L2 switches that
ignore (for the most part) the L3 part of the packet and above. You
have to accomodate for broadcast/multicast propagation (or blindly
flood it), but essentially there isn't a whole lot more to it.
But throw in access lists, or QOS, or policy routing, or anything
above and beyond straightforward routing, and now it gets more and more
difficult to cram that into an ASIC or whatever.
So a layer 3 switch does the absolute minimum (in most all cases) to
act like layer 2, but something has to "approve" the intermediate skip
of layer 3. The best example is the NFFC in the Cat5000 series; it
will use an imbedded RSM or an external router to "get permission" to
allow it to cross the layer 3 boundary (approve access lists, etc),
and then it treats it essentially like a layer 2 switching path for
that flow. The NFFC handles header rewrites of MAC addresses (you
have to fake the router's presence) or with newer blades the rewrite
is handled onboard by the ASIC.
There are very few "pure" layer 3 switches, in that they setup a flow
and then pretend it is a layer 2 path. In contrast, there are also
very few "really slow and dumb" layer 3 routers; with cacheing, fast
and/or distributed switching, CEF, etc., they try to bypass the full
overhead of L3 routing.
So if "L3 switching" is a misnomer, it is because unless there is some
new switch with more bells and whistles than I've ever heard of, there
is some overhead to establish "L3 permission" to turn it into a "L2
path".
Jeff Kell <[EMAIL PROTECTED]>
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