RE: ARP Spoof Question

["David Gillett" <gillettdavid () fhda edu>]

> > Switches are layer 2 devices, IP begins at layer 3. A 
> > -switch- usually doesn't understand a single ip bit. 
> > The management side of the switch (snmp, http, telnet, 
> > whatever) are to be considered as any other networked host.
> ------------------------
> How would that apply to a layer 3 switch/router? Actually the 
> packaging says that I have a Residential Gateway/Router/Firewall. 
> Aren't gateways layer 7 devices? While switches are layer 2 
> devices, they deal with MAC addresses right? Maybe a "smart" 
> switch knows which MAC addresses are allowed on the network? 
> Or am I missing it all here?
> --Rivera--

1.  "Residential Gateway/Router/Firewall"?  I don't see "switch"
in that list, do you?

2.  However, many small home routers are now incorporating a 
switch on the LAN side.  It's all in one box, but for purposes
of understanding, it's more useful to think of it as two separate
devices, one at layer 2 and one at layer 3.

3.  "Gateway" is a generic term.  A layer 2 gateway is a bridge
(a switch is a multiport bridge).  A layer 3 gateway is a router.
A layer 7 gateway is a proxy.  A protocol converter might
sometimes get called a gateway.

4.  Some switches do have some layer 3 awareness.  The sort you
will find bundled into the box with a home router (see #2, above)
are not among them, however.
  Switches that are layer-3-aware can be useful, because it's
easier to find the port associated with a given IP address than 
if you have to search by MAC address.
  Switches that are layer-3-aware can be a pain, because if you're
not careful with your configuration, they'll start listening to RIP
(from misconfigured clients...), or (I've seen this happen) sending
ICMP and UDP traffic one way and TCP traffic another, or generating 
ICMP unreachable messages for packets that were delivered via another
switch....
  A switch that knows about layer 3 can serve as a router, but in
my experience they rarely make very good routers.  (I make an 
exception for the Cisco 5000/5500/6500 line, where the layer 3 
intelligence amounts to a *good* router on a blade that fits in 
the switch chassis.)

David Gillett




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