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Cascading Switches: how many switches can we cascade?

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Hello,
One of our responsibilities here is to set up networks at big conferences hosted in hotels all around the world.
For a long time we have had a golden rule which I never understood: “never put more than 3 switches in cascade in the same network segment”.
1. Why no more than 3?
2. What’s the risk?

I searched Google everywhere for a canonical limitation in the number of switches cascading in the same segment but I only found very vague indications and none of them ever said why. Some said 7, some 8 and another guy 12 (still far from our golden rule of 3).

Personally I recognize 3 risks in having many switches in cascade:

  1. Bottlenecks: a switch allows communication among its ports using an very large internal bandwidth (aka backplane bandwidth). Connecting two switches over ethernet limits the speed on that segment to the speed of the port.
    Obviously there’s a lot more to the performances of a switch than the simple difference between internal speed and port speed but this is not the scope of this article.
  2. Fault tolerance: a line of switches in cascade means that if any of those fails (or if any of the connecting cables fails) a whole section of the network goes down. The more switches you introduce in your network the more chances you have of having a failure.
  3. Monitoring, administration and negotiation: if you have many switches, it is more difficult to keep them under control and especially in the event of different brands and models, bad negotiation is always behind the corner.

In some forums (please note that I found no official document supporting this theory) some people say extensive cascade switching could lead to Broadcast Storm.
As far as I know, Broadcast storm is usually caused either by an eccessive number of clients in the same broadcast network or by malicious clients attempting a DoS attack through Broadcast. Neither of these causes is consequence of the number of switches that you have in a network.

Last but not least I would like to state clearly that the 5-4-3 rule does not apply here because that rule does not apply to ethernet switched networks:

Written by zantoro

September 19, 2012 at 10:16 pm

Posted in Networking

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