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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