23 Feb XFP 10G Direct-Attach Cable Flapping

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

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

The problem happens because there are older devices (switches, routers) which support XFP modules and newer devices which 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 that XFP transceiver has built in clock and data recovery (CDR) chip. On the contrary SFP+ modules does not have this CDR chip built in. The newer network devices like switches has built in CDR chip in upstream ASIC or in special chip called – signal conditioning chip.

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

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