Copper GB - crossover needed?

Discussion in 'Cisco' started by Academic Technology Services, Feb 27, 2004.

  1. I was under the impression that copper GB ports were auto detecting wrt
    crossover cables -- ie, that one could connect a pair of GB switches
    with a straight-through Cat5e cable. But I just fought with a pair of
    3750 switches. If I use a cat5 x-over, they connect fine, but they
    won't connect with a regular cable (yes, I tried multiple cables).

    Is there some interface setting that affects this behavior? Like maybe
    it needs CDP to be active? Or am I just wrong in expecting a non x-over
    cable to work?

    /Chris, UCLA
    Academic Technology Services, Feb 27, 2004
    1. Advertisements

  2. Academic Technology Services

    Jeff C Guest


    "The Auto-MDIX feature is enabled by default on switches running Cisco IOS
    Release 12.2(18)SE or later. For releases between Cisco IOS Release 12.1
    (14)EA1 and 12.2(18)SE, the Auto-MDIX feature is disabled by default. For
    configuration information for this feature, refer to the switch software
    configuration guide or the switch command reference"

    -Jeff C
    Jeff C, Feb 28, 2004
    1. Advertisements

  3. Academic Technology Services

    hktco Guest

    The funny thing is that for 1000Base SFP module (fig. B-11), it
    actually crosses the pins 4,5,7 &8. Normally we can only buy Cat5
    crossover cables with pins 1,2,3&6 crossed.

    So is it a specially cable that the article is referring to, or I
    misunderstood it? :)

    hktco, Feb 28, 2004
  4. Academic Technology Services

    Chris Thomas Guest

    Thanks, Jeff. To summarize, there is an interface-level command
    which controls whether auto detection is operative -- the command is
    mdix auto (or no mdix auto). For some reason, no auto was chosen as
    the default in 3750 IOS releases prior to 2/04. Note that upgrading
    to the most current release may leave auto off since it appears to
    leave a no mdix auto statement on each interface.

    6500 IOS supports the same command. The default appears to be auto.
    My 4500-sup IV doesn't seem to support it. Note that support depends
    on both IOS level and on the hardware implimentation in the line

    The weird (to me) thing is that for interfaces set to no mdix auto,
    connectivity can be enabled by using an FE crossover cable. An FE
    crossover has the two used pairs crossed, and the two unused pairs
    straight through. A true GB crossover has all four pairs crossed
    (and is rare as hens teeth). It surprizes me that the GB interface
    is able to understand a half-crossed cable, but I guess the designers
    figured that might happen a lot and so planned for it.

    Does anyone know if if having mdix auto on one side of a link is
    sufficient, or whether it needs to be on both sides to work? I'll go
    test it and post the result.

    /Chris, UCLA
    Chris Thomas, Feb 29, 2004
  5. Academic Technology Services

    Chris Thomas Guest

    It appears that having mdix auto operating on one side only is
    sufficient to allow the link to connect via a straight cable.

    Chris Thomas, Mar 2, 2004
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.