Can only the 100Mbs part of a 10/100Mbs router fail?
Travis Roy
travis at scootz.net
Wed Dec 22 06:28:01 EST 2004
Just catching up on your email now :)
Most people have forgotten about the threads you're replying to tonight :)
> I realize this thread is old, but I'm catching up and I might be able to
>offer some insight here...
>
>On Wed, 8 Dec 2004, at 8:32am, travis at scootz.net wrote:
>
>
>>Most consumer level 10/100 "switches" or "routers" are switching hubs.
>>That is there's a 100mb hub and a 10mb hub and the switching happens
>>between the 10 and 100mb parts, not between each port. So that part might
>>also be broken.
>>
>>
>
> I assume you're using "hub" to mean "repeater" like most do. (The term
>"hub" properly means "the center of any star topology network", which is
>damn confusing since *BASE-T /is/ a star topology.) What you're describing
>as a "switching hub" is really the same as any ordinary repeater, just with
>a learning bridge instead of a dumb bridge.
>
> You see, in reality, all Ethernet repeaters are single-speed. That's
>inherent in the nature of a repeater: Electrical signal comes in, is
>re-timed, and repeated on all the other ports. There's no buffering to
>handle speed changes. 10BASE-T and 100BASE-T are two different signals,
>after all.
>
> A "dual-speed repeater" is really two repeaters: A 10 megabit repeater and
>a 100 megabit repeater. A dumb bridge connects the two, so that traffic
>that comes in on the 100 megabit repeater will be bridged and transmitted on
>the 10 megabit repeater. Physically, this is all one unit, of course (if
>not one IC chip), but conceptually, that's how it works.
>
> It is very possible for the bridge in a dual-speed repeater to fry while
>leaving the individual repeaters working. One particular model of 8-port
>repeater from LinkSys seemed to have a tendency to fail in this way. It
>once manifested itself to us as all the 100 megabit workstations in an
>office working fine, but the 10 megabit network-attached printer was
>unreachable. Damn, but that took forever to figure out.
>
> The difference between a dumb bridge and a learning bridge: A dumb bridge
>just forwards every data frame it gets. Frame comes in on one interface, it
>goes out all the others. These are rather rare, and are used mainly for
>"media conversion". A learning bridge tries to keep track of what layer two
>addresses (MAC addresses) it sees on which interface. If it gets a frame
>for an address it knows, it only sends it out the interface it saw that
>address on.
>
> Back in the days when Ethernet ran on coax instead of twist-pair, learning
>bridges were used to keep busy parts of the network well-isolated at layer
>two. These days, of course, we have switches: Multi-port learning bridges.
>
> I can also say that many of the consumer switches being sold at Staples,
>Best Buy, and the like act like "real" switches, in terms of how they
>forward frames. Traffic on one port does not come out the other ports once
>the destination address is learned. This I've seen with a sniffer. I have
>no doubt that some "switches" work this way, but "most" might be a bit of a
>stretch.
>
>
>
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