Path: utzoo!mnetor!uunet!husc6!hao!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 (CORRECTION) Message-ID: <12357633164.33.RADIA@XX.LCS.MIT.EDU> Date: 11 Dec 87 15:06:24 GMT References: <8712101941.AA23821@braden.isi.edu> Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 24 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. Radia -------