Chuck...... I talked to a good friend of mine that knows more of this than
I do...... and even HE wasn't clear on the line of switch -vs- router.....
so my following comments are strickly my thinking out loud...... mostly to
get feedback .....
I don't think there is a difference in a layer 3 switch that does 100,000
packets/sec -vs- a router that does 100,000 packets/sec. However,
respectfully submit that I don't know of any routers that can keep up with
Layer 3 switches because in multilayer switching the route processor only
has to route the first packet in a flow. At that point everything is
switched as fast as the switch can switch them (tounge twister), and even
the fastest routers would have trouble routing as speeds that switches can
switch at. IMHO, any switch-router that can do 100,000 packets/sec can't
reach it full potential unless Multilayer switching is enabled and
configured. Further, I submit that there isn't a router that can handle
the throughput of a switch-router utilizing Multilayer switching. With
switch backplanes reaching 32 to 256Gbps, I don't know of a pure layer 3
router than can keep up with this pace without using MLS.
I disagree with the assertion that ASICs and processor are equal because
they're both chips. Obviously we could compare the P4 chip to the 486 chip
and see that not all chips are equal. And I realize that's not what you are
claiming. =)
However, there is a distict difference between a general purpose CPU and an
ASIC and how fast they can do a specific job. General purpose CPUs have
instruction sets and need to be programmed with code. ASICs are indeed
chips, but they are chips whose entire circuitry is designed and dedicated
to do one thing and one thing only. This give the ASIC a definite advantage
over any general purpose CPU on a give task. Yes, I hate to make blanket
statements because there are things like the speed of the respective CPUs,
etc. But it is generally accepted that an ASIC performs a given task much
faster than a general purpose CPU. That's why 3D graphics cards use
specific processors (ASICs) instead of just another P3 on the video card,
etc......
Utilizing ASICs allows things like checking for MLS cache entries and
rewriting Layer 2 info, etc "at wirespeed". I can't say 100%, but I believe
this means that it can output a "routed" packet (assuming it's in the MLS
cache) in a single "cycle". I know know if that's the correct terminology,
but my point being that an ASIC is able to do this type of thing. A general
purpose CPU running IOS code, etc would take many clock cycles to perform
the same type operations and it's speed can't compare to an ASIC.
I have to say though, that I'm not all that familiar with the top end
routers (switch-routers, whatever) like the 12000 GSRs (or the new 10Gig
SRs), etc..... so perhaps they have some ASICs to perform their
routing...... dunno.........
My 2 cents......
Mike W.
"Chuck Larrieu" wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Very good, and thanks.
>
> But... to quote a sage, who made this point last time this topic came up,
> what exactly is the difference between a router that routes 100,000
packets
> per second, and a layer three switch that switches 100,000 packets per
> second?
>
> Cisco can talk about ASIC's versus processors all they want. Both are
chips.
> High end routers also have ASIC's as well as other means to optimize
traffic
> flow.
>
> Truth be told, layer 3 switch is a marketing concept, plain and simple.
>
> Chuck
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
> Michael L. Williams
> Sent: Wednesday, June 06, 2001 3:56 PM
> To: [EMAIL PROTECTED]
> Subject: Re: Layer3 switch vs Router [7:7406]
>
> For the record: Layer 3 switch = Multilayer switch (I say this because I
> like to use the term Multilayer switch rather than Layer 3 switch... dunno
> why... I guess because in the switch/routers, you actually use and
configure
> Multilayer switching)
>
> In reply to some of the other posts on this topic:
>
> >This is actually covered in under the switching portion of the CCNP..
> >Here is a link from Cisco for you to reference:
> >http://www.cisco.com/warp/public/cc/so/neso/lnso/cpso/l3c85_wp.htm
>
> Layer 3 switching (Multilayer switching) *is* covered on the CCNP
Switching
> exam and I don't know how one could pass it without an understanding of
how
> it works. (although that link that was provided pointed to a page that
> didn't explain MLS very well at all)
>
> > One differance is that a layer 3 switch does wire-speed switching
>
> AFAIK, *all* switches perform wire-speed switching, as long as the
backplane
> isn't oversubscribed (even then what gets switched is done at wirespeed
and
> other stuff is dropped). Can someone make sure I'm not fibbing or
confused
> on this?
>
> >Okay, two things -
> >
> >One - abduct a Cisco marketing rep, tie them to a chair, shine a bright,
> hot
> >light in their face and ask *them* what the difference is.
> >
> >Two - while they're tied up, ask them what "wire-speed" is supposed to
> mean.
>
> Wire-speed simply means that the data is switched across the backplane to
> the destination port ASAP, as fast as the wires can carry the data thru
the
> ASICs and to the destination port. I.E. the speed of light (minus a small
> fraction because the wires do actually have a non-zero resistance =)
>
> >Someone may have a product based answer for you but literally a router is
a
> >layer 3 switch. Just think of all of the functionality that a switch
> offers
> >you and add on the route switch module to sweeten the pot.
> >A router either bridges or has separate subnets on each of its
> >interfaces.(simplistic answer of course).
>
> This is very misleading. Although a router has a switching process within
> it, and that switching process can take on many forms, a router is NOT
> simply a Layer 3 switch. Although a router can bridge (including bridging
> VLANS using Integrated Routing and Bridging), even then it is not the
> equivalent to a switch because of the way it performs the process (in
> software on a CPU instead of with an ASIC). The router doesn't keep a CAM
> table like a switch, etc. and without something like a NetFlow Feature
Card
> or MLS processor, a router can't bridge (switch) at wirespeed like a true
> Layer 2 switch...... Even on a switch/router with a NFFC or the like,
> without Multilayer switching enabled, it's like a router with a ton of
> ethernet (or whatever) interfaces. The packets are NOT routed and
forwarded
> out at wirespeed like they are (after the first packet) when Multilayer
> switching is enabled and configured.
>
> I by no means am the expert on Multilayer switching, and I'm not trying to
> flame anyone for their answers. But there were alot of things being said
> that didn't answer the original poster's question (actually the one post
by
> Bob Salazar was correct, but he didn't mention that you have to actually
> turn on the Multilayer switching fuctions on the hardware he listed) or
that
> were misleading. I'm sure there are some things that I messed up or just
> don't have a full understanding of (like "wire-speed hehe), so comments
are
> welcome.
>
> Mike W.
>
> "Denton, Jason" wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > Can anyone tell me what the REAL difference is between a layer3 switch
and
> a
> > router?
> >
> > Jason
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=7469&t=7406
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
