[c-nsp] Cisco Layer 3 Switch Recommendation

[Jon Allen Boone]

On Nov 11, 2004, at 16:35, Randy Bush wrote:

> ok, i gotta ask.  'switches' do layers 3 & 4 (bgp, ...).  'routers
> do layer 2 (atm, atmv2, ...).  so what is the technical difference
> between a router and a switch?
>

I think that definition has changed over time (and will continue to do 
so).

   It used to be that Layer 3/4 switches were *really* layer 2 switches 
with bags on the side to support some limited amount of Layer 3 
inter-operability.  They allegedly offered "expedited" forwarding by 
looking deeper into packet structures prior to making the forwarding 
decisions.  Routers, conversely were *really* layer 3 devices with 
various bags on the side to support limited Layer 2 switching 
functionality.  They offered more robust layer 3 inter-operability 
features and performed layer 2 switching functionality in a 
low-performance way.

   This model was, I believe, driven by the economics of chip 
production.  Switches used more specialized chips that were expensive, 
unless produced in bulk.  Routers tended to use more general purpose 
chips that were already produced in bulk due to their general purpose 
nature.  By GP here, I really mean that someone other than the router 
vendor was using them, as the volume used by the router vendor didn't 
make it cost effective to use customized chips extensively.

   With the advent of low-cost special-purpose ASICs, we began to see 
the convergence of routers and switches.  At the same time, we saw an 
enormous amount of vendor consolidation, so one company would tend to 
sell both routers and switches.  So the definition changed to be:  
switches were hardware oriented around special purpose ASICs with a 
thin software shim and big interface memory buffers, while routers were 
hardware oriented around special purpose ASICS with lots of software 
running on top and limited interface memory buffers.

   In the future, I think the big differentiator will be the quality of 
the software, as the hardware will be virtually indistinguishable.  
Switches will have very robust, but low-feature software.  Routers will 
have lots of features and lots of bugs.  The future is NOW!

--jon