MAC 11:11:xx:.... is not valid MAC address

On IXIA if you configured MAC/Eth address to be 11:11:00:02:01:01, the switch doesn't take them.

Thanks Paul Lavelle for the why :-) 

The least significant bit of the first byte is known as the IG bit. An IG bit of 0 indicates that this is a unicast MAC address, an IG bit of 1 indicates a multicast or broadcast address. It is illegal to use a multicast source MAC address in ARP replies.

References:

https://en.wikipedia.org/wiki/Organizationally_unique_identifier#Format

https://blog.ipspace.net/2012/05/arp-reply-with-multicast-source-mac-is.html

I use a simple trick: If you are manually setting a MAC address on Ixia, make sure the first byte of the address is an even number :)