3750G and GLC-SX problem

[AndyBurtchaell]

Setup

I have a 3750G-24TS-S switch (1.5u version) and want to use the G1/0/28, I have installed a GLC-SX-MM and the switch sees it correctly. At the other end is a slightly more modern 3750 (1u version) which also has a GLC-SX-MM installed in G1/0/28. This is configured as a routed/Layer 3 connection. Both switches are running a 12.2 load and both switches have other SFP/GLC-SX-MM links active and working properly.

Problem

When I plug both ends of the fibre into the G1/0/28's at both ends I only see a link on the 3750G switch, no link light appears on the 3750. I have tried swapping out the SFP's, Fibres, even the port used at the 3750G end but without success. Just for info, the other three (G1/0/25-27) ports are all setup and working properly.

Extra information

No this is where it gets weird!. If I remove the MMF fibre and SFP from the 3750G interface and plug it into an ASA (expansion card) and setup the SFP interface settings correctly, both ends see each other and work perfectly.

It appears to me that the 3750G G1/0/28 interface cannot drive the SFP with the same power as the ASA... but that does not make any sense!

Any help will be greatly appreciated as I am running out of ideas!

[Reza Sharifi]

On the 3750 side, can you try and switch the fiber strands around?

[AndyBurtchaell]

Thanks for the idea Reza, I di try this but it didn't help and afterwards, as I thought it through, if the fibres were the wrong way round the ASA wouldn't of worked!!

BUT.... I have found the solution (which is equally odd!).

One end 3750 interface had the "Speed Nonegotiate" command on and the other end interface didn't. For some reason, even though the GLC-SX SFP's can only run at 1Gb, the would not sync. One I entered "No Speed Nonegotiate" at both ends all worked fine.

This does confuse me as the GLC-SX are 1Gb inetrfaces only and can't run at any other speed, why it would not work with the "Speed Nonegotiate" on on one end???

I assume it is an "Undocumented Feature" that I will remember from now on.

Thanks 

[Reza Sharifi]

Andy,

Glad to hear you found the solution. As you noted correctly with fiber ports the speed is set and usually you don't have to worry about speed negotiation command. You usually plug in the fiber ports and as long as the optics and cables are good the connection comes up.  With so many different Cisco switches and IOS versions, you never know what "feature" you run into :)

HTH