Shortest Cat5/6 cable from firewall to switch?

  • Ive been trying to find a definative answer to this and it seems that everyone has a differnet opinion. What is the shortest cable I should use to go from a gigabit switch to a gigabit interface on a pfsense?


  • I've always heard 3 feet as a minimum between devices (segments of the run could be shorter, say for switch to patch panel), but I just spent an hour reading the 802.3 specs.  There is no mention of a minimum length.  The maximum lengths are caused by signal integrity and collision detection propagation.

  • I've used ~25cm cables before on Gbit without problems.

  • We use a 5cm selfmade cable here to connect two devices stacked on each other.
    There is no shortest distance the cable has to have. Only a longest distance (100m).
    If you want you could solder out the connectors and connect the two devices directly via an even shorter cable.
    Or remove the PHY out of the equation and connect the GMII of the MACs directly.
    Although if you would want to do that you should consider to decouple the link with capacitors.
    (If you really need that PM me and i could send you some application notes regarding this)

  • Thanks guys. Thats what I figured. I have always heard 3 feet/1 meter minimum between devices but no one could back it up.

  • This minimum length still is true if you have a hub somewhere.
    The collision detection cannot work if the cable is too short.
    I think the exact value for the length is somewhere in the CSMA/CD standard.
    Wiki has a short notice to this too:

    But with current equipment you will have to look very hard to find a situation where a colission is possible.
    As long as you dont use hubs (or rings even) you wont have problems with the minimum cable length.

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