The 5-4-3 "rule" starts over with the switch. A switch port is the "demarc"
for a collision domain.
Think of your network in hierarchical terms or in blocks. Design the shared
hub-based blocks using the 5-4-3 rule. Design the switched blocks with a
concern for scalability of the Spanning Tree Algorithm and broadcast
traffic. Switches forward broadcasts. Too many stations sending numerous
broadcasts in a switched network is a scalability issue. Cisco recommends
no more than a couple hundred stations be in a single broadcast domain.
A "rule" for switches is that you shouldn't have more than 7 connected
together. But I don't think that's an actual rule you'll find in any IEEE
documents. However, it is a good idea to limit the size of a large switched
network to avoid the Spanning Tree Algorithm taking a long time to converge
and broadcast traffic becoming a concern. You can subdivide your switched
network with routers and VLANs. With VLANs, you can have one spanning tree
per VLAN.
And a few more words from my basic Networking 101 "lecture," that I often
give to K-12 network engineers who are usually too busy to step back and
think about fundamentals!? ;-)
A hub is just a way of making wiring less confusing. Instead of connecting
stations in a bus coax cable, we learned from the telephone industry that
we could use structured cabling, hubs and twisted-pair cabling. But that's
all a hub really does. It simplifies cabling (and amplifies the signal,
since it is a repeater.) A hub makes it possible for stations to share
bandwidth even though they are physically cabled in a star topology with an
active repeater (hub) between them.
Although placed in topologies in the same place as hubs, switches are
completely different beasts. A switch has the opposite purpose of a hub. A
switch divides up the number of stations that share bandwidth. It reduces
the size of collision domains. A switch is a high-speed multi-port bridge.
A hub is a repeater.
Priscilla
At 12:05 PM 6/11/01, Hartnell, George wrote:
>A couple of 5-4-3 refinements:
>
>Try this picture:
> MORE HOSTS
> > \.../
> > hosts HUB(s) hosts
> > \|/ \|/ \|/
> >host --- hub --- repeat --- SWITCH --- repeat-- hub --- hub---- etc...
> > /|\ /|\ /|\
> > hosts hosts hosts
> >
>The question of 'slot time' becomes confusing to me with the addition of
>one, or more, switches into the (10,100,1000) circuit(s). The 'etc' above
>asks, "What is the practical limit of cascaded switch/hub combinations in a
>10 Mb/s ethernet? A combination 10 and 100 Mb/s ethernet? Yes, even with a
>1Gb/s backbone?" Please, don't laugh, we here in K-12 education need to
>lash up all kinds of stuff. Sometimes, if you can believe it, people will
>even sneak an extra hub into the building!
>
>No problem is insurmountable with the comparatively limitless private sector
>resources. Network design is easy (easier) without the albatross of old
>equipment never, ever, being surplused out, just moved out closer to the
>edge. That's why we like the theory, as below. We frequently need to figure
>out what flies ourselves.
>
>Very best, G.
>
>
>"Be strict in what you send, and forgiving in what you receive."
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> George Hartnell, Network Supervisor
> Bellingham School District, 1306 DuPont St.
> Bellingham, Wa. 98225-3198 (360)647-6860
> [EMAIL PROTECTED] Internet Mail
>
>
>
>-----Original Message-----
>From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]]
>Sent: Thursday, June 07, 2001 12:38 PM
>To: [EMAIL PROTECTED]
>Subject: Re: Rule 5-4-3 [7:7578]
>
>
>At 02:17 PM 6/7/01, Aleksey Loginov wrote:
> >Hi!
> >Question about rule 5-4-3.
> >How it's work for 10BaseT?
> >This scheme correct?
> >
> > hosts hosts hosts
> > \|/ \|/ \|/
> >host --- hub --- repeat --- hub --- repeat --- hub --- host
> > /|\ /|\ /|\
> > hosts hosts hosts
> >
>
>Great job on the ASCII art! It's quite pleasing to the eye. ;-)
>
>A 10BaseT hub is a repeater. Dare I say that there's no difference between
>a Layer-1 repeater and a hub? Just like there's no difference between a
>Layer-2 switch and a bridge, or a Layer-3 switch and a router. Argh.
>
>One topology that became popular for explaining propagation delay on an
>Ethernet network was the 5-4-3 topology. With a 5-4-3 topology, you can
>have up to five segments in series, with up to four repeaters, and no more
>than three mixing segments. If three mixing segments are used, then the
>remaining two segments must be link segments. A mixing segment is one that
>can have more than one attachment, such as a coax cable. A link segment is
>a point-to-point link. It could be a fiber link between hubs, for example.
>A 10BaseT hub connection to an end station is also a link segment.
>
>The 5-4-3 topology is just one of many models that work actually. It is a
>simplification of the actual rule that says that the round-trip propagation
>delay in one collision domain must not exceed the time it takes a sender to
>transmit 512 bits, which is 51.2 5s for 10-Mbps Ethernet, and is also known
>as the slot time.
>
>A single collision domain must be limited in size so that a station sending
>a minimum-sized frame (64 bytes or 512 bits) can detect a collision and jam
>signal reflecting back from the opposite side of the network, while the
>station is still sending the frame. Otherwise, the station would be
>finished sending and not listening for a collision, thus losing the
>efficiency of Ethernet to detect a collision and quickly retransmit the
>frame. For a higher layer to notice that the frame needs retransmitting
>takes much more time. Ethernet retransmissions occur within nanoseconds.
>
>Priscilla
>
>
> >Any comments, please.
> >
> >Best regards,
> >Aleksey
>________________________
>
>Priscilla Oppenheimer
>http://www.priscilla.com
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8048&t=7578
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