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RE: Caching a sniffer
From: "Shawn Jackson" <sjackson () horizonusa com>
Date: Thu, 25 Mar 2004 08:36:31 -0800

A switch is not a hub/router. In fact it is a micro segmented bridge.

It's nice I get so much attention, you all sure know how to make a man
feel 
wanted :-). Now let the rebuttal begin.

As my response to Fernando, which hopefully will get posted to the list
soon 
then this message, states that switches have numerous functionality and 
systems that operate at higher layers of the OSI model then just 2. As
with 
most network connected devices they need to have functionality on other
layers 
besides the one where their core functionality resides. Case in point,
bridges 
merely forward traffic destined to address on other interfaces, limiting
broadcast 
in each segment, this is the core functionality of a switch. I will
retract my 
earlier statement in place of this one which better suites the core
functionality 
of the switch, in this case you guys win.

With that stated, switches can make decisions on traffic based on IP 
information which resides at the Network layer (layer 3) of the OSI
model, as 
opposed to MAC address information which resides at the Data-Link layer 
(layer 2) of the OSI model. Most switches provide IGMP functionality for
systems 
to announce they multicast group membership, which is all layer 3 data.
Some 
switches allow you to set ACL's or security based on IP information, not
just 
MAC or layer 2 information. Some switch can also make routing or 
'gateway of last resort' decisions.

A switch operates at layer 2 of the OSI model i.e. MAC address layer. 

Layer 2 of the OSI model is the Data Link layer. LLC and MAC are logical
parts 
of that layer and not layers themselves.


Therefore if someone has plugged a scanner into a network point they
will not be 
able to sniff any useful information from the network unless that
person has admin 
access to the switch. You can check this by ensuring that none of the
ports on the 
switches are in span mode

Through ARP poising, brought up by David, and MacOf which I mentioned
allow people 
to sniff traffic on a switched network. You don't need to be using SPAN
to sniff 
traffic, that just the way us admins do it.

Shawn Jackson
Systems Administrator
Horizon USA
1190 Trademark Dr #107
Reno NV 89521

www.horizonusa.com
Email: sjackson () horizonusa com
Phone: (775) 858-2338
       (800) 325-1199 x338

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