From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Subject: Re: bridged gateway
Summary: won't work
Organization: Silicon Graphics, Inc.  Mountain View, CA
Date: Fri, 17 Apr 1992 06:46:26 GMT

In article <1992Apr16.080429.1579@crow.omni.co.jp>, trich@crow.omni.co.jp (Timothy Richards) writes:
> Can some SGI network guru tell me if this will work ?

> I want to make my main server into a gateway so it feeds two networks,
> then bridge the two networks so that the server is not preoccupied
> with forarding packets.

> ...
>                       ___________                                       
>                      |           |
>          network A   |  server   |   network B
>                ______| (4D380)   |_____
>               |      |___________|     |
>               |        _________       |
>               |       | RETIX   |      |
>               |_______| Bridge  |______|
>               |       |_________|      |
>     host 1 ___|                        |___ host 4
>               |                        |
>     host 2 ___|                        |___ host 5
>               |                        |

I don't know and am not a guru in the SGI religion or any other, but I'm
confident it will not work.

As far as IRIX and most TCP/IP systems are concerned, a "network" is a set
of machines that can be reached by pumping packets out of one interface.
For every possible network, there is exactly one network-interface, driver,
ethernet connector, or call it what you will.  That unique interface is the
one that the system will choose to emit packets when sending to hosts
(including broadcast or multicast) on that network.  In the proposed
configuration, the two interfaces on the IRIS either have the same or
different network numbers.  If A == B, only one ethernet interface on the
server would be used.  (Actually I think other things will break, but
ignore minor details.)  If A and B were the same, one ethernet interface on
"server" would be idle and useless.

If the A differs from B, and if the left interface is to be used to talk to
host_1, then the network number (not host number) of host_1 must be A, and
similarly the network number of host_4 must be B.

However, that means that host_1 and host_4 cannot communicate using a
bridge.  A bridge does not "forward"; it just blindly copies packets.  For
host_1 or host_2 to communicate, you must use a "router", not a bridge.

Ethernet is slow enough today that pure bridges are bad investments.  You
can buy routers and "brouters" that can forward at ethernet speed for the
same price as a bridge.  A router, combined with the configuration settings
you propose for the server, can have the effect you wish.  There is one
gotcha.  Unless host_4, host_5, etc always refer to server by its network_B
address, or unless you run routed with the IRIX 4.0.1 option of "-m",
around 50% of the network-B traffic will go over to network-A and use the
network-A port of the server.

Bridges were reasonable investments only for a little while about 7 years
ago, when it was hard to make a router that could keep up.  Today, if you
are using reasonable protocols, bridges are nonsense.

Beware bridge salescritters; as times change, new technology** appears.
The latest are "bridging hubs."  These devices are ok if you have a
non-trivial network, where many machines exchange lots of traffic with many
machines.  If you have one or a few servers, such a hub will reduce the
effective throughput of your network, by hiding the collisions on the
server stubs.


Vernon Schryver,  vjs@sgi.com

**salescritters and manglers have changed English.  The word "technology"
 has become a synonym for male bovine excrement.  Every time hear one of the
 varients of "technology", try substituting, and see if the speaker's sense
 is changed.

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