Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!umd5!ames!ucbcad!ucbvax!XX.LCS.MIT.EDU!radia From: radia@XX.LCS.MIT.EDU (Radia Perlman) Newsgroups: comp.protocols.tcp-ip Subject: Re: Routers vs. Bridges Message-ID: <12357379979.12.RADIA@XX.LCS.MIT.EDU> Date: 10 Dec 87 15:55:36 GMT References:Sender: usenet@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 32 The January, 1988 issue of IEEE Network magazine has multiple articles on bridges vs routers. I wrote one of them. The main problem with the "bridge vs router" issue is defining what a bridge vs a router is. You could define an entire network architecture and declare that to be your data link layer, and claim then that the box that forwards packets is a "bridge" because it operates at the "data link layer". My contention, though, is that a Data Link layer header only has in it information to deal with one hop -- one pair of addresses, (source and destination), no route, no hop count, etc. Since the Network Layer handles multiple hops, if a header is defined with two pairs of addresses (ultimate source, ultimate destination plus immediate source and immediate destination), hop counts, routes, etc, then I claim it's a Network Layer protocol. For instance, I claim the DEC bridge is clearly a bridge (although it allows store and forward) because as far as the end stations are concerned, they are dealing only with a Data Link protocols -- the header they see fits my description of a Data Link header. The "source routing bridge", on the other hand, I'd claim is clearly a router and not a bridge, because it requires end stations to discover and place a route in the header. You may want to wait until the January IEEE Network comes out for many different viewpoints on this issue. If you'd like to study the issue independently, I'd suggest starting with a rigorous definition of "routers" and "bridges" that clearly places a box in one category or another, before trying to argue the merits of each kind of box. Radia -------