GLC-SX-MM and 1000 Base-SX

Unanswered Question
Aug 24th, 2010


I am working on a lab at work before doing some changes on the cisco network.

I have a 3750 and a 2960. They are connected and configured.

When I do a connection between those switch on their Giga interface with a GLC-LH-SM everything works fine. On our network we want to use the GLC-SX-MM but this is KO. I even went to a location on our network where noone  works in present time. I took the 1000 BASE-SX off and replace them with the GLC-SX-MM and it was also KO.

I am wondering if there is a problem with compability (but it doesn't look like it based on documentation I have read on the cisco website). I have checked the fiber and the RX, TX and still the same problem.

Based on what I have read, it should work but this is not the case either in lab or on our network (where we have 1000 Base SX).

Thanks for any advices.

I have this problem too.
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Nagaraja Thanthry Tue, 08/24/2010 - 05:57


When you changed the SFP, did you change the underlying Fiber as well?

GLC-LH-SM uses a single mode fiber whereas GLC-SX-MM needs a multimode


Hope this helps.



emanjamin2002 Tue, 08/24/2010 - 06:20

Thanks for your answer. Well I am using MultiMode fiber. This is only what we have here on site

. We do not use single mode.

But the multi mode is functionning on the LH-SM but not on the SX-MM.


Jerry Ye Tue, 08/24/2010 - 06:16

I first want to understand what SFP is at both end of the fiber and what type of fiber. Here is the example that it will work


GLC-LH-SM <-MMF-> GLC-LH-SM (require mode conditioning patch cord)


Any other combination other than the above will not work.



emanjamin2002 Tue, 08/24/2010 - 06:28

Thanks for your answer.

I am using

GLC-LH-SM << multimode fiber >> GLC-LH-SM   and this is working (with connector in between).

GLC-SX-MM << Multimode fiber >> GLC-SX-MM and this is not working

Note: on the network (not the lab I am on), we use 1000 Base -SX and this is working. Basically we want to increase the number of switches on the network with some 2960 and some 2960S and we want to use SX-MM.

So as our business doesn't use Single mode fiber, and only has multi mode fiber, we should not buy SX-MM, isn't it ?

Shall we use the LH-SM (but the distant required is not more than 300 meters) or are the 1000 Base-SX still available ? (I will check that with our suppliers).

Thanks again. Hopefully I explained the above correctly.


Jerry Ye Tue, 08/24/2010 - 06:35

GLC-SX-MM should work if you are using it within the spec (the link in my previous post). If you are using 62.5 micron fiber, it needs to be within 275 ft. 50 micron can go much longer. Have you test that with only a patch cord? Have you check your switch's CLI to see if the switch sees the SFP module?



emanjamin2002 Tue, 08/24/2010 - 06:45

Thanks Jerry,

Yes I have read the link you sent me. I am doing an internship here in the business for 1 week now so I don't have all the detail. But on lab, I have a fiber with a connection in between. The technical service told me that this is multimode they have here. Now I am starting to doubt about it as this is working with the LH-SM and not with the SX-MM.

On the fiber this is not showing what it is. Is there a way to find out easily if this is multi or mono mode. Sorry I am not an expert in fiber and just discovering about it now.


Jerry Ye Tue, 08/24/2010 - 06:54

I see. On the fiber, usually the spec is printed on it, when you find it just put them here so we can take a look.

Another way to ID the fiber is tell me the color. Usually, MMF is in ORANGE (62.5 micron) or AQUA (50 micron), SMF is in Yellow.



emanjamin2002 Tue, 08/24/2010 - 07:01

Thanks so much jerry,

Here is what I have on the fiber :

"optronics 62.5/12.5 mm d62 zipro"    (so I think this is multi mode = mm ??) - the cable is orange.

If this is multi mode, then I don't see why the SX-MM won't work ?


Jerry Ye Tue, 08/24/2010 - 07:09

Yes, it is a MMF and it should work with the GLC-SX-MM. Have you look at the switch's CLI or change to a different SFP to see it works or not? BTW, are those Cisco branded SFP?



emanjamin2002 Tue, 08/24/2010 - 07:16

Thanks for your answer.

Yes they are all Cisco's ones. I have 10 of them and tested all of them but nothing !!!! So maybe I should get new ones from the supplier and check again. I don't know if those are good ones or not as I don't have another place to check them. I have checked few ones on a switch 2960 connected on the network but it was still KO. (we have 1000 Base SX on the network and working well).

Well if nothing more ?? I can check, I appreciate your replies and help. I did learn quite a bit with this.


Jerry Ye Tue, 08/24/2010 - 07:23

It is very odd that 10 of them are not working. I would suggest you to check your fiber also. I am assuming the switch configuration works with the GLC-LH-SM SFP?

emanjamin2002 Tue, 08/24/2010 - 07:33

I agree that this is odd that 10 of them don't work.

Yes the config for both switch are fine and the connection is OK with the GLC-LH-SM. I have not checked the fiber yet but this is not working neither on site where connection is OK with the 1000Base SX.

So, I am a bit confused here !!!???? Maybe I should reboot the switch ? what do you think ?

(By the way, do you know of a link for a good dictionnary for cisco terms. I am having some hard time some times finding the meaning for abrevation like SX ou GLC and others).

Jerry Ye Tue, 08/24/2010 - 07:52

You can give it a try on rebooting the switch. The SFP is hot swappable and usually there is no need to reboot the switch.

Those are really part number, it should be in the link from my previous post.



emanjamin2002 Wed, 08/25/2010 - 00:04


Good morning from France.

I checked few more thing. With GLC-SX-MM connected, I did a show interface .... and it did not see it (normal as there is no connexion). Then it says media type 10/100/1000 Base TX for the type of media that can be connected. My question is then   "the SX-MM should be fine isn't it ?"

Apparently there is a switch with the SX-MM connected somewhere on the network so I am going to try to replace them with the LH-MM temporaly and try them on my lab here. I will let you know.


emanjamin2002 Wed, 08/25/2010 - 11:03

Hi Jerry,

Thanks for the advices. I had already check this out.

I think I handled the problem. It was stupid but it is OK now (I hope so . I managed to find one GLC-SX-MM on the network. At the other end, it was an 1000T Base SX. The connexion was working so I managed to take them as the part of the network was not being used. I installed them on my lab and it was all GOOD. Then I took one out and tried the other SX-MM I had. The interface went down and and erro message came up. I had to shutdown the interfaces and put if back up to make it work again with the other SFP. Basically all the ones I had created that error. Checking on the net, I understood that those SFP are KO.

AFter putting back up the interface, I took again the ones functionning and it was OK.

So I am ordering a bunch of SX-MM and about 10 switches and one 2060S for the network. Hoping I won't have more problem. I will test everything in lab before connecting any to the network.

Thanks again for ALL your help. I think I learned quite a bit with your guidance.



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