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Connection issue between 2960x and 3750G, never seen this before


This is a very weird issue and I'd like to see what you all think about it.

I'm attempting to connect a 2960x 24p switch to a pair of 3750G 12p SFP switches running HSRP.  The 2960 has a fiber link (GLC-LH-SMD modules in all devices) between each switch:

2960 1/0/25 --> 3750G 1/0/11

2960 1/0/27 --> 3750G 1/0/11

Each interface is configured as a trunk with dot1q and a native vlan.  This is where it gets interesting.  After connecting the fiber, link lights on the 2960x are green and the interfaces show up/up.  CDP neighbors shows nothing.  Spanning Tree shows FWD.

On the 3750G side (both switches same output), no link light, down(not connect) and no spanning tree info.  CDP neighbors shows nothing on int 1/0/11.  

2960x running 15.0(2a)EX5

3750G running 12.2(58)SE2

The 3750G is working fine, no other issues and a number of dual homed switches connected on other interfaces.

I've never experienced one side of a link show green while the other is not connect.  I shot light over the fiber connection to visually verify that the connections are correct and they are indeed connected to the correct interfaces. 

Any Ideas on what I can check?   Thank you in advance.

18 Replies 18

Tagir Temirgaliyev
Frequent Contributor
Frequent Contributor

this can be unidirectional link. one side of duplex pach cord is bad. and so optical signal go in one direction but not in opposite. try to change duplex pairs.

check optical links with OLTS or OTDR or both


We had this issue as well. We found that we had a bad SFP that was causing this issue.


was these two 3750 switches in a stack or not?

Yes Two 3750 switches stacked  connecting to 2960x switch stack

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