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I have two 3750 Cisco switches that have two fiber cables/sfp modules hooked up on each. The second fiber is just for redundancy.

I have installed two new 3750X switches on the rack for the upgrade, and I would like to use these existing fibers that have been pulled to link up these new switches.

What command can I run on the 3750 switches to determine which fiber is safe to pull, so I can then use it to connect the new switches - I don't want to take down an entire floor (even temporarily!)

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  • @KC hopefully I did not mangle your meaning. It sounds as if you want to connect the two 3750X switches only to each other. A line diagram -- indent with four spaces to preserve formatting -- might be very helpful in your question.
    – Craig Constantine
    Commented May 23, 2013 at 20:06
  • @CraigConstantine I want to connect the two NEW switches together, yes. What I am trying to avoid is a downtime, the old dusty switches are still on the rack, and I am loading these new switches with the config so I can simply plug and play - which is why I am asking about the fiber. And yes, I mean indiscernible to the end user as far as "temporarily" is concerned :)
    – Keven Culp
    Commented May 24, 2013 at 20:37
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin ♦
    Commented Jan 4, 2021 at 22:20

4 Answers 4

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As already suggested by Stefan and Ricky, if your fiber ports are aggregated in an etherchannel, then you can safely remove any of the fibers and all the traffic will use the remaining fiber. In order to check if they are aggregated you can run show etherchannel summary and check the output lookinf or any PortChannel interface made of both fiber ports.

Otherwise, if both fiber ports are not aggregated and STP or RSTP is in use, then you need make sure you are pulling off the fiber connected to the blocking port. Run show spanning-tree <interface_name> for both fiber ports in both switches and look for the ports in status blocking.

Just be careful if running PVST, RPVST or MST. Port costs could have been tuned in such way that one port is forwarding some VLANs and the other port is forwarding some other VLANs. In this case, whatever the fiber you pull off you will generate impact on your network.

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  • What Keven is asking is not really possible 'even temporarily' means no frames lost during convergence to me. Even LACP is not guaranteed safe to remove/add ports (the protocol allows for such guarantee, but does not force such implementation). STP certainly causes short outage.
    – ytti
    Commented May 24, 2013 at 11:51
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    I would not take "even temporarily" to mean no frames lost. I would take this as not noticeable to the end user or application. As such, it depends on flavor of STP.
    – YLearn
    Commented May 24, 2013 at 17:53
  • @YLearn Correct.
    – Keven Culp
    Commented May 24, 2013 at 20:43
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If you made an ether channel / port channel, don't worry, the traffic will go over the remaining fiber. Otherwise you could consider to make a channel.

To be on the safe side, run show interface <name> several times for interfaces to both fibers on one switch and watch the counters. The one which doesn't change at all, while the other one does, should be safer to pull.

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You'll have to look at the configuration of the switches to see how those ports are being used -- etherchannel/bonded, spanning-tree redundancy, etc. You'll need to look at the config of both ends to see if it's safe to use the "backup" fibers at all.

If it's just simple STP, then the blocked port will have an orange status led.

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Along with all my colleagues already told, I would recommend you to have a look at *show interfaces * then check input and output rate. Also you could reset counters using clear counters GigabitEthernet .. then have a look what interface is used.

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