Re: Routers vs. Bridges

Radia Perlman (radia@XX.LCS.MIT.EDU)
Thu 10 Dec 87 10:55:36-EST

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.


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