Forwarding issues related to MACs starting with a 4 or a 6 (Was: [c-nsp] Wierd MPLS/VPLS issue)

Nick Hilliard nick at
Fri Dec 2 16:59:37 UTC 2016

Job Snijders wrote:
> Dear IEEE, please pause assigning MAC addresses that start with a 4 or a
> 6 for the next 6 years.

Disagree that this is an IEEE problem.  This is problem that vendors
need to work around.  There is limited MAC space, and deprecating 1/8 of
it due to the inability of vendors to cope properly with it seems like a
really bad long term idea.

It seems that the problem that cropped up on cisco-nsp is that a layer 2
switch, the Nexus 92160 (and possibly everything else which uses the
same forwarding ASIC), cannot forward vpls frames with a 4 or 6 buried
at a specific location inside the contents of the frame.

This is an extraordinary bug which renders the hardware useless in
specific circumstances.  What makes it worse is that this is a well
known corner case which should have been shaken out during design, if
not found during QA.


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