RE: Re: FAQ 10/100 Hubs Block Other Speed Traffic

[Rich Adamson <radamson () routers com>]

> The bad thing about hubs that _don't_ have this "feature", is that
> in order to support 10bt devices, they throttle the entire hub speed
> down to 10bt if there is one or more 10bt only devices hooked up to it.
> I have seen this behavior (and did the bandwidth tests to proove it) on
> old 3com office connect 10/100 hubs (newer ones do the 2 hubs with a switch
> thing.)  So, the point of what I am saying is, since these old hubs have
> no switching capabilities, and they don't know which port the traffic is
> supposed to go to (no switch=no arp table), they have to throttle bandwidth.

None of the hubs and switches have any significant amount of storage
on the ethernet chip sets, and therefore _any_ non-layer-three box that 
has 100 -> 10 capability can only handle small amounts of traffic before
the chip set drops incoming packets on the floor. Guess one might call
that throttled bandwidth, but at the expense of retransmission timeouts
and retransmissions at the end nodes.

If the box has a backplane, multiple cards and some network management
functions, there is a higher _probability_ the manufacturer has some
additional buffering going on to keep dropped packets from happening
on at least small bursts of traffic.

In the most generic of terms, if a box supports 100 "full-duplex", then
its a switch (regardless of what the manufacturer calls it). If it
supports 100 -> 10, there is 50-50 chance the box has some MAC address
awareness. If a box only supports 10 -> 10 or 100 -> 100, there is a
high probability it is not MAC address aware and therefor functions
like a hub.




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