Cisco Catalyst Gigabit Fiber Port Not Working

Posted on 2014-01-13
Last Modified: 2014-01-16
Hello - we have installed a new Cisco Catalyst 3750 V2 switch in our warehouse, and need to connect it via fiber to another switch.  We have a GLC-SX SFP MMF module installed in port 1, and a Duplex Multimode fiber patch cable connecting into that module.  The switch was processed through express mode, sertting the correct IP configuration for the network, and reinitialized the switch.  We also confirmed that the port 1 was not shutdown, with a NO SHUT command. It shows the port as down, but not off.  We get no activity light on the port, however.  The switch and port on the other side of the connection are active.  Not sure what the problem could be.

Thanks for any help
Question by:Damian_Gardner
  • 2
LVL 45

Assisted Solution

by:Craig Beck
Craig Beck earned 250 total points
ID: 39777993
That sounds like one of the fibre cores is broken.

When you connect a fibre link you have a TX and RX from each end.  Obviously the TX links to the RX, and vice-versa.

With a break in the cable, one switch sees the TX from the opposite switch so its link-light comes on, but at the other end the TX isn't seen as that core is broken, so the link-light stays off.

You'll need a fibre tester to perform a definitive test, but if you want to prove continuity across both cores you can shine a LED torch down each core.  If you can see the light at the other end on each core your fibre is probably working.

Accepted Solution

gbotts earned 250 total points
ID: 39786515
On the fiber port:

Conf t
int g0/1
switch trunk encap dot1
swit mode trunk
switch trunk all v all (or only the vlans you want across)

speed nonego
************* If this is on, make sure BOTH sides have it*************
wri mem
copy run start

Let me know if this helps,

Author Comment

ID: 39786522
Turned out the new switch was accidentally on the wrong VLAN.  put it on the matching VLAN and it works now.  thanks guys

Expert Comment

ID: 39786557
Glad to hear it's good to go!!   Thanks for the update!!

You may want to do the following as well:
<----This would be more secure, but more work as far as adding vlans to your network.  As you will have to add to both switches---

vtp mode transparent  
vtp domain  (add a domain of your choosing - make sure it's the same on both switches)

<------- If you want less security but less work on configuring vlans.  This would allow you to create the vlan on the one switch and have it propagate to the other. -----------------

Sw1# vtp mode server
Sw1# vtp domain (add a domain of your choosing)

Sw2# vtp mode client
Sw2# vtp domain (add the same domain as you did above)

<------- If you want less security but max ease on configuring vlans.  This would allow you to create the vlan on the either switch and have it propagate to the other. -----------------

Sw1# vtp mode server
Sw1# vtp domain (add a domain of your choosing)

Sw2# vtp mode server
Sw2# vtp domain (add the same domain as you did above)


Also, look into changing the native vlan....

Hope this helps,

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