Date   : Wed, 15 Jul 2009 00:36:44 +0100
From   : pete@... (Pete Turnbull)
Subject: Is it a bridge? Was: Master Ethernet upgrade

On 14/07/2009 18:33, Darren Grant wrote:

 > On 14/07/2009 18:17, "Pete Turnbull" <pete@...> wrote:

 >> No, that would actually be a gateway or router.  It *does* need to know
 >> about the protocols to do PPP over ATM (or whatever), and it doesn't
 >> just use MAC addresses.  The devices that talk to it don't need to know
 >> about the protocols, and may only need to know about MAC addresses
 >> (depending on how it's done), of course.  Actually, of course, what you
 >> describe is a pair of devices; definitely not a bridge.
 >
 > Sorry Pete but I disagree, I have used 3com bridges to do exactly this, I
 > have set-up 2 offices on a single IP Subnet the bridges connect over 
an X25
 > connection to each other and no nothing of the protocol used, IP, IPX and
 > AppleTalk all work over the bridge as the bridge only works by MAC 
Address.

 From end to end, true, the *client* devices *on each LAN* only need to 
know about MAC addresses.  It's transparent to those devices.  Not so in 
between.  What you did overall could loosely be described as bridging, 
especially since what you set up was a single point to point link and 
you don't care what's in the middle, but I would describe (and other 
companies such as Cisco would describe) those *network devices* as 
gateways or routers, and technically they are not bridges, because the 
protocol in between does not share the same link layer protocols at all. 
  You're doing encapsulation and decapsulation from A to B to C, which 
is tunnelling, not bridging A to B.  Your PPP and ATM examples weren't 
either.  If I set up a VPN tunnel from home to work, that's not a 
bridge, even though it's transparent to devices at the endpoints.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York