Re: Routers vs. Bridges (CORRECTION)

Radia Perlman (radia@XX.LCS.MIT.EDU)
Fri 11 Dec 87 10:06:24-EST

Just a clarification -- bridges don't HAVE to forward multicasts (broadcasts)
in order to be a bridge. The Vitalink and DEC bridges have the capability
to manually set certain multicast addresses as "don't forward". This
is very important for various reasons. The default is "forward", but
some finite number (large enough for all practical purposes, I claim)
of multicast addresses can be manually configured to be localized, i.e.
not forwarded by the bridge.

Once people agree on what a bridge vs a router is, I'd summarize the
tradeoffs as:
  1) bridges don't require a standard layer 3 (win for bridges unless
     suddenly layer 3 standards crystallized and universalized)
  2) routers can do much fancier stuff because of the extra layer
     of header and explicit cooperation from the stations, like
     utilizing better routes, or utilizing hierarchical addresses
  3) even if layer 3 standards crystallized, a mixture of bridges and
     routers might be desirable because it gives an extra level
     of hierarchy "for free". In other words, it might be efficient
     to clump LANs into big LANs, and then the layer 3 protocol
     doesn't have to trouble itself with the inner topology of
     the bridged extended LANs.


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