Juniper EX4500 CoS Error on VTY console

What if you get the errors below on a Juniper switch?
[Sat Aug 02 15:01:00 2015 LOG: Err] Unable to find cos_cn_t for port 10 on dev 0
[Sat Aug 02 15:01:00 2015 LOG: Debug] mrvl_cos_cn_disable_pfc_by_ifd_idx: COS-Unable to find cos_cn_t for port 10 on dev 0”

I recently ran into the above error appearing every 30 seconds when logged into the VTY console of a Juniper EX4500 running JunOS 11.4. Searches on the internet showed no results. JTAC to the rescue!

What does this error mean?

The source of this error message actually comes from having Data Center Bridging Capability Exchange protocol (DCBX) turned on.

DCBX is a discovery and capability exchange protocol that is used for conveying capabilities and configuration of the above features between neighbors to ensure consistent configuration across the network.

Why do I get this error?

In my case it was because the aggregated interface ae10 had a child interface that was down which results in the System identifier for that interface showing as 00:00:00:00:00:00. As such DCBX is not able to discover the Class of Service (CoS/QoS) settings on that interface.

This message is actually informational, but until JunOS 12.1R1 it was coded at the error level.

• If you do not use Fiber Channel over Ethernet (FCoE) with a Converged Network Adapter (CNA) you can ignore the message.
• You can get rid of the message by turning off DCBX.
• You really should fix the interface that is down ;)