XFP 10G Direct-Attach Cable Flapping

This is quite a common problem with 10G XFP Transceivers. Link is flapping UP/DOWN/UP/DOWN … In this case, it is with an XFP 10G DAC cable.

As we know, the XFP optical transceiver specification was published in 2002, but the SFP+ optical transceiver's first publication was in 2006. This means that XFP modules have been in business for a lot more time than SFP+ modules. These roughly four years are a lot of time for technologies to develop, as, for example, network devices like routers and switches have adopted a lot of new functionality during these years.

The problem happens because there are older devices (switches, routers) that support XFP modules and newer devices that support SFP+ and XFP modules. The main difference between XFP and SFP+ modules is their form factor. XFP is slightly larger. This difference in size is because of fact the XFP transceiver has a built-in clock and data recovery (CDR) chip. On the contrary, SFP+ modules do not have this CDR chip built in. The newer network devices, like switches, have a built-in CDR chip in the upstream ASIC or in a special chip called – signal conditioning chip.

XFP 10G DAC cables differ. There are Active (with CDR compensation) and Passive (without CDR compensation) XFP 10G DAC cables. Active type cables have a built-in CDR chip in transceivers, whereas Passive they don’t. As a result Passive technology DAC cable can be used only in newer type of network devices, which provides the CDR function by themselves. Older network devices which does not have a CDR chip on their platform must use Active XFP 10G DAC cables, which have a built-in CDR chip.

This is the reason for the problem. The Passive type XFP 10G DAC cable is used in older types of equipment which does not have a CDR chip on board. As a result, CDR is not being implemented, and received and sent signals are getting out of phase, which leads to a flappy link. To evade this problem, choose the correct XFP 10G DAC cable wisely!