Understanding Polarity in MPO System

MPO/MTP technology has led to the adoption of 40/100GbE, however on of its challenges is with regards to  proper polarity of these array connections.  Maintaining  the correct polarity across a fiber network enables signals  from any type of active equipment to be  directed to the receive port of a second piece of active equipment – and vice versa. To ensure the MPO/MTP systems work with correct polarity, the TIA 568 standard suggests several methods.

MPO Connector

First on the list is the  MPO connector usually consisting of  12 fibers. 24 fibers, 36 fibers and 72 fibers  Each MTP connector has a key on one of the flat side added by the body. When the key sits on the bottom, this is called key down. When the key sits on top, this is referred to as the key up position.  In this orientation, each of the fiber holes in the connector is numbered in sequence from left to right and is referred as fiber position. The orientation of this key also determines the MPO cable’s polarity.

Three Cables for Three Polarization Methods

The three methods for proper polarity defined by TIA 568 standard are named as Method A, Method B and Method C. To match these standards, three type of MPO truck cables with different structures named Type A, Type B and Type C are being used for the three different connectivity methods respectively. In this part, the three different cables will be introduced firstly and then the three connectivity methods.

MPO Trunk Cable Type A: Type A cable is also known as straight cable, is a straight through cable with a key up MPO connector on one end and a key down MPO connector on the opposite end. This makes the fibers at each end of the cable have the same fiber position.

The issue of polarity with MPO cables can be easily addressed by selecting the correct type of MPO cables, connectors, cassettes and patch cables. Various  polarity settings/methods can be applied  to satisfy the requirements of the 40G environment.  

The issue of polarity with MPO cables can be easily addressed by selecting the correct type of MPO cables, connectors, cassettes and patch cables. Various  polarity settings/methods can be applied  to satisfy the requirements of the 40G environment.  

As a manufacturer of 3rd Party Certified Optical Transceivers, I’m often barraged with questions regarding the difference between Cisco approved SFPs and third party SFPs (Cisco Compatible). Inevitably the discussion starts going down the slippery slope of vendor lock in and high-profit racketeering. I’m going to try to explain the differences and ways to circumvent “lock in”.

Cisco uses OEMs (original equipment manufacturers) to produce all their SFPs, XFPs, SFP+, SFPs manufactured under the OEM model are packaged up in Cisco sealed bags and called “Cisco approved”.

Being Cisco approved means the SFPs have undergone rigorous testing with Cisco products and are guaranteed to have 100% compatibility and complete support. Third party SFPs (aka Cisco Compatible) are manufactured by companies not on the Cisco AVL  (approved vendor list) and, therefore, are not deemed Cisco approved. These manufacturers will offer 100% compatibility guarantees but Cisco will not support them. Cisco may threaten breach of SmartNet and refuse support. Cisco reserves the right to refuse service and/or support if the problem is determined to be related to third party SFPs. From personal experience I’ve had plenty of customers using third party SFPs call in for other hardware problems and the SFPs go unnoticed. But if you are trying to bring up a fiber connection and it won’t come up and need help from Cisco you won’t get far with 3rd party transceivers.

The third party SFPs won’t work by default. Cisco-approved SFP modules have a serial EEPROM that contains the module serial number, the vendor name and ID, a unique security code, and cyclic redundancy check (CRC). When an SFP module is inserted in the switch, the switch software reads the EEPROM to verify the serial number, vendor name and vendor ID, and recomputes the security code and CRC. If the serial number, the vendor name or vendor ID, the security code, or CRC is invalid, the software generates this security error message and places the interface in an error-disabled state.

Here is a common log message indicating the hardware platform has detected an invalid SFP:

SYS-3-TRANSCEIVER_NOTAPPROVED:Transceiver on port Gx/x is not supported

These commands will differ from platform to platform. Fortunately, there are some undocumented (and unsupported) commands to circumvent this issue. From configuration mode enter the following commands. Note that since the first command is undocumented you can’t “tab” and “?” your way to the command. You can only type the full command in.

switch(config)# service unsupported-transceiver

switch(config)# no errdisable detect cause gbic-invalid

The first command will yield the following:

Switch(config)#service unsupported-transceiver

 Warning: When Cisco determines that a fault or defect can be traced to the use of third-party transceivers installed by a customer or reseller, then, at Cisco’s discretion, Cisco may withhold support under warranty or  a Cisco support program. In the course of providing support for a Cisco networking product Cisco may require that the end user install Cisco transceivers if Cisco determines that removing third-party parts will assist Cisco in diagnosing the cause of a support issue.

The above command should make it clear that you run the risk of losing support. I’ve used the above commands on Cisco 3750, 3560, and 2960 platforms.

Ultimately it’s the decision of the customer to make the call. Only they can ultimately decide risk versus reward. It’s our job as technology partners to explain the advantages and disadvantages of either approach.

Here are some reference links for additional information:

Third Party Policy:

http://www.cisco.com/en/US/prod/prod_warranty09186a00800b5594.html

SFP Invalid Error:

http://www.cisco.com/en/US/products/hw/modules/ps4999/products_tech_note09186a00807a30d6.shtml