Johnson, Tracy wrote:
> Actually, no one ever told me how a hub degrades a switch.  I mean,
> isn't the port smart enough to understand you've plugged a 10MBit item
> into it?  Isn't that WHY the the ports on a switch dual-speed?  Isn't
> that WHY ports are configurable?
>
> When ever I confront them with that, I get are mumbled expletives, and
> how dare I ruin their perfect port configs.
>
> (Methinks they're just too lazy to configure a single port differently
> than the others.)

Alright, for the sake of network nerds around the world, I can't take
that one lying down :-)

Hubs have two fundamental problems:  they are half-duplex shared media,
and they replicate ingress traffic on every egress port.  The first
hurts performance, the second hurts security.

Most hubs are fixed speed, 10 or 100, and have no autonegotiation logic
or circuitry.  This is fine for hubs, but if you plug it into an
intelligent switch that "expects" to negotiate, you run into
autonegotiation issues (which is a separate topic that I think I've
covered at length in the past).  Typically the switch will make it's
best guess, or least-common-denominator default. 

If it is a configurable switch, or it decides correctly, then the switch
will setup half-duplex at the proper speed.  And by "configured
correctly" this implies that not only do you have an accomodating
network nerd, but that you also bothered to inform him that you were
plugging a hub into that switchport ahead of time :-)

Now, in the best case scenario, you have a properly communicating hub,
albeit half-duplex, and duplicating all of it's traffic on the remaining
hub ports.

The problem of 10/100/1000Mbps smart switch having the 10Mb hub plugged
into it is the possibility of dropped packets as the traffic rates go up
and the available switch buffers go down.  The degree performance
degradation depends on whether the switch buffers are allocated
per-port, or has a globally shared buffer pool.  If the latter, you'll
potentially cause packet loss across the whole switch.

With all that said... get the 3000 and anything related to it at
full-duplex if humanly possible.  To add a half-duplex DTC, either buy
your network nerd lunch and ask him to setup a port specifically for it,
or buy your own small dual-speed switch to put between the DTC and the
real network switch to absorb the "half-duplex" issues.  That will go a
lot further than a $0.99 eBay hub.

Jeff

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