Technical Note: Error 'mac address should not be multicast' seen when modifying MAC address

[rodrigogarcia_FTNT]

Description

Sometimes it is required to modify the MAC address of an interface.  However, not all the MAC addresses are valid since there are specific rules specified by the IEEE.

On occasions, the error message 'mac address should not be multicast' may be seen.

For example: When modifying the MAC address of an interface, the FortiGate CLI shows the following:

FGT90D (interface) # edit wan1
FGT90D (wan1) # set macaddr 07:5b:0e:XX:XX:XX
mac address should not be multicast
node_check_object fail! for macaddr 07:5b:XX:XX:XX
value parse error before '07:5b:XX:XX:XX'
Command fail. Return code -33

Solution

There is a reason for this error:

By definition, Ethernet frames with a value of 1 in the least-significant bit of the first octet of the destination address are treated as multicast frames and are flooded to all points on the network.

With Ethernet, it is the least-significant bit of an octet that is the first to be transmitted. A multicast is indicated by the first transmitted bit of the destination address being 1.

As an illustration, a MAC address with  odd values  (1,3,5,7...) have ON the least significant bit for the first octet (in red):

DECIMAL -- BINARY

0 -- 0000 0000
1 -- 0000 0001
2 -- 0000 0010
3 -- 0000 0011
4 -- 0000 0100
5 -- 0000 0101
6 -- 0000 0110
7 -- 0000 0111

Either change the MAC address with this bit OFF (even numbers at the first octet).