[HECnet] DECnet et al

[Johnny Billquist]

On 2011-07-17 20.22, hvlems at zonnet.nl wrote:
A bridge (hw) maintains a list of mac addresses it sees at each port. A bridge will never forward a packet with a destination address on that list.
The bridge ( program) learns which mac addresses live behibd what port and thus knows what traffic to forward and what not. Broadcst, multicast traffic is always forwarded.
The only difference between a DEBET and the program is that the program ignores all other protocols

This is essentially what a switch does as well.
I think in the old DEC nomenclature, a bridge only connected two segments, so it becomes a very binary "the address is either on this segment, or on the other side". A switch can, and should, be more clever, since it should either not forward the packet at all, if the destination is on the same port as the packet in on, or just forward it to the specific port where the destination exists, if it is on another port. Unknown destinations needs to be treated the same as multicast, in that it needs to be sent to all ports.

I might have called my program a switch as well, except for other reasons I thought a bridge was a better name for it. Also, in the beginning, it did not keep any destination cache around. Improvements along the way. :-)

	Johnny


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-----Original Message-----
From: "Bob Armstrong"<bob at jfcl.com>
Sender: owner-hecnet at Update.UU.SE
Date: Sun, 17 Jul 2011 11:10:11
To:<hecnet at Update.UU.SE>
Reply-To: hecnet at Update.UU.SESubject: RE: [HECnet] DECnet et al

The term segment is used because each node that runs the
bridge program does filter packets that should stay local.

    Ah, so the bridge program is not really a bridge (at least not in the way
I use the word).   Usually I think of a bridging two networks as meaning to
copy all traffic from network A to network B and vice versa.   If the box or
program does something smart about deciding which traffic should go where,
then it's a "router" (or at least a "switch").

    How does the bridge program decide what DECnet messages to bridge and what
to drop?

Thanks,
Bob