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Fast Ethernet Topology Limitations

Posted on 2004-09-28
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I have a little experience with ethernet LANs. I usually adhere to the 5-4-3 rule when expanding my LANs. However, what are the limiting factors when cascading fast ethernet switches? Can the total length of Cat 5 ever excede 400 meters between any two nodes? I'm looking for guidelines / specs. I've been searching the internet and I seem to find conflicting opinions. Any general guidlines will help. Perhaps I need to incorporate a bridge???
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Question by:NAFiamengo
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by:Yan_west
ID: 12173988
Maximum length of a cat5 cable would be 100M.. you can add repeater or additional switches to do it..
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by:Yan_west
ID: 12173996
If you need something to go farther, I would use Monomode optic fiber..
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by:NAFiamengo
ID: 12173999
100 meters per segment - But isn't the maximum 4 segments at 100 meters each for a total of 400 meters?
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by:Yan_west
ID: 12174010
Yes, the maximum number I've seen is 4 cascaded hubs/switches..
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by:PennGwyn
PennGwyn earned 300 total points
ID: 12174745
Uh, that's hubs/REPEATERS.

Hubs and repeaters are (generally) line-speed devices, and the 3-4-5 rule is designed to limit the physical size of the collision domain to ensure that any collision is detected before the sending station thinks it has successfully completed transmission of the smallest legal packet.

Switches are w whole different story.  They buffer and/or stroe-and-forward, adding latency, but they also put each port in its own collision domain (which is, of course, subject to the same 3-4-5 rule as above).

The governing criterion with a switched network is the size of the *broadcast* domain, and of the spanning tree.  A typical good rule of thumb is that no node should be more than 9 bridges away from any other, and the spanning tree should be no more than 5 bridges deep.  If you need to exceed these numbers, it's almost certainly time to put in a routed backbone.

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by:trimixdiver
ID: 12174840
IF you are trying to get 400m using cat5 then every 100m you will need a switch. Better bet is using Multimode fiber optics. Put a switch on each end or a simple transparent media converter. With FX you can reach 2km and the fiber can also be used for gigabit
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by:TeeSeePeeEyePee
ID: 12176118
If you want to go a step further and extend the fabric of your switching backbone between closets (and eventually even between sites) it would be worth checking out 3Com's XRN technology:

http://www.3com.com/other/pdfs/products/en_US/500928.pdf
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bluelip earned 200 total points
ID: 12182132
The max length was designaed for more reasons than just signal degradation.

Most newer equipment can discern a signal on lengths longer than those given.

The problem comes about when a collision occurs. It takes too long for the 'blocking' signal to travel the length of the wire and reach each device in the collision domain.  In this case, some devices may not know that the data it just sent was part of a collision.

Rare? yes, but it's still there.
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