Network Toolkit – Poor Man’s way of testing an SFP

When I walked into a company recently, they had a stack of SFP’s that were not in any type of anti-static container or bag. Needless to say, there was no way of telling at first glance if the SFP’s were good or not. Since that isnt a piece of test gear that I carry with me or own at this point, I had to come up with a way of doing some basic go/no go tests before trying to use them in a production environment. I started setting up my test bed with getting a console cable connect to a switch that was powered up and not connected to the network. If you havent worked with fiber optic SFP’s before, please take this next statement VERY seriously – DO NOT look into the SFP when it is plugged into a switch or any other device. The SFP contains a laser transmitting device in it that even at low levels could damage your eye or possibly blind you in one or both eyes. I will address a way to see if the external part of the SFP is functional in a bit.

I inserted the first SFP and do a sh inventory command. If you get a response back like the following, this is a basic indication that most of the SFP’s electronics are functional to the point that the switch can identify that something is plugged in, what type of SFP is plugged in and the serial number of the SFP. A side note here – if the SFP displayed on the console screen doesnt agree with the serial number on the sticker, you might have a counterfeit SFP on your hands.

Switch#sh inv
NAME: “GigabitEthernet1/0/1”, DESCR: “1000BaseSX SFP”
PID: Unspecified , VID: , SN: FNS1002J1AG

If you get a message similar to this when plugging in the SFP –

*Mar 1 01:48:20.457: %GBIC_SECURITY_CRYPT-4-VN_DATA_CRC_ERROR: GBIC in port Gi1/0/1 has bad crc
*Mar 1 01:48:20.457: %PM-4-ERR_DISABLE: gbic-invalid error detected on Gi1/0/1, putting Gi1/0/1 in err-disable state

You either have a SFP with bad electronics, is a counterfeit unit or is one that isnt compatible with a Cisco switch/router. From what I have read, there is a small EEPROM that contains a serial number, vendor name, vendor ID, a security code and a CRC. When the SFP is installed, all that info is read and the switch/router recomputes the security code and CRC. One of the possible outcomes is that the SFP port you are using will be put in a err-disabled state. Remove the SFP and do a shut, followed by a no shut to reset the port for the next SFP to be tested. Put this one in the suspect pile. One possible solution for this error is to upgrade the IOS on the switch you are testing with to a slightly newer version. If other SFP’s work in the same slot, I would be more prone to having a bad SFP.

In researching this error, I found a command that isn’t documented or that shows up with using our old friend ?. type service unsupported-transceiver and press enter. You will get several lines with a message warning you that Cisco may withhold support if a problem is found with a third-party or unsupported SFP. This is one command that I would only use in an emergency or if you didn’t have a spare SFP on hand.

If you haven’t had a problem so far, you will need to create a loopback connector to fool the SFP into thinking it is connected to another SFP. Another option is to put a known good SFP into available slot on the same switch or another switch that also isn’t connected to your production network. If the external part of the transceiver is good, you should see the link light come up on the port you are using. You should also see the following on the console screen –

*Mar 1 02:06:41.890: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/1, changed state to down
*Mar 1 02:06:42.897: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/1, changed state to down

Fiber LoopbackIf you don’t have another switch handy, I usually can find a LC to ST patch cable. If you have a spare port in a fiber distribution panel, you should be able to find a “barrel” connector which is a device that will join the two ST fiber cable ends together making the cable a loopback connector. If you don’t have the parts handy, here is a loopback connector for a very reasonable price I found on Amazon (LC Fiber Optic Multimode 62.5/125 Loopback Adapter). Click on the graphic to go to the Amazon page for this item. They also showed having other connector types and types of cable (i.e. SC, single mode fiber, etc) that you may want to add to your collection depending on the types of fiber and connectors that you have on your network.

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