[olug] OT: Cheap cisco routers?

David DeWester dewester at inebraska.com
Mon Aug 26 22:47:58 UTC 2002

I got confused by this once and went on a quest to find the answer.  What
I came up with said switches and routers are immune to the 5-4-3 rule, as
Jonathan pointed out.

The 5-4-3 rule is a way to approximate propagation delays so that the
first bit of a packet reaches the end of the network before the last bit
is transmitted.

The transmitting computer will listen to the wire to make sure no other
computer is trying to send packets at the same time (in CSMA/CD networks).  
The transmitting computer stops listening after it sends the last bit
which is why the first bit needs to have crossed the network before the
last bit is transmitted -- if it didn't the collision could occur after
the last bit has been transmitted and after the transmitting computer has
stopped listening.

The 5-4-3 rule approximates the more complex exact calculations by, for
example, counting a hub as a segment which takes into account the hub's
delay in relaying bits.  The 100 meter cable length on a segment and the
speed of light account for segment delays.  I never figured out why only
some segments can be populated.  Minimum packet sizes give you a lower
bound on how long it will take to transmit a packet.  All these combined
give you some idea of how long it takes to send a packet and therefore how
"long" your network can be, where "long" counts things like hubs.

My understanding is switches are not subject to this rule because they are
store-and-forward.  If a collision occurs (between two computers separated
by a switch) after the last bit of a packet is transmitted it won't break
anything because the switch will resolve the collision and retransmit any
lost packets.

So switches and routers segment a network into pieces and each such piece
is subject to the 5-4-3 rule.  The network as a whole isn't subject to the
5-4-3 rule because switches and routers will handle collisions without
involving the client computers.

I think the book I found this explanation in was Network+ Certification
All-in-one Exam Guide by Michael Meyers.  A highly recommended book, btw.  
Lots of pictures and understandable explanations.  Entry level stuff but
if you have gaps in the basics this is where to get them filled.

David DeWester

On Mon, 26 Aug 2002, Jonathan Warren wrote:

> Now I am really confused.  I am running two switches at home with no problems.
> A switched Ethernet network should be exempt from the 5-4-3 rule
> because each switch has a buffer to temporarily store data and all
> nodes can access a switched Ethernet LAN simultaneously.
> On Sun, Aug 25, 2002 at 04:58:50PM -0500, Nick Walter wrote:
> > http://www.webopedia.com/TERM/5/5_4_3_rule.html
> > 
> > Nick Walter
> > ----- Original Message -----
> > From: "Phil Brutsche" <phil at brutsche.us>
> > To: <olug at olug.org>
> > Sent: Sunday, August 25, 2002 4:01 PM
> > Subject: Re: [olug] OT: Cheap cisco routers?
> > 
> > 
> > > Nick Walter wrote:
> > > > Hmmm, I know the 5-4-3 hub rule, but that only applies with
> > hubs/repeaters.
> > > > Shouldn't apply to switched environments.  Is the 3 switch rule
> > something
> > > > different?
> > >
> > > I thought the 3-4-5 hub rule was the same for hubs and switches.
> > >
> > >
> > > Phil
> > >
> > >
> > > _______________________________________________
> > > OLUG mailing list
> > > OLUG at olug.org
> > > http://lists.olug.org/mailman/listinfo/olug
> > >
> > 
> > 
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