Multicast MAC in Source MAC Address Field
This is discussed in Radia Perlman's Interconnections (ISBN 0-201-56332-0), Chapter 4, and in IEEE 802.1D Annex C. In short, an address that looks like a MAC multicast source address *can* be used, but it's meant for a very special purpose. It is used by a host to denote that "source routing" instructions are included in the MAC frame. (Note: this is Layer 2 source "routing" we're talking about, not the IP option). The "multicast bit" is set in the MAC source address, to indicate source routing. But instead of being called G/I bit, as it is when it's associated with the MAC destination address, in this case it is called RII, for Routing Information Indicator. Thing is, though, this applies to IEEE 802.5 and FDDI (respectively sections C.2.5.1 and C.2.5.2). No mention of Ethernet support. (If you want to use source routing with Ethernet, don't despair. You don't code the RII bit in the MAC source address. Instead, you use the IEEE 802.1Q "tagged frame" format, and set the CFI bit. When the CFI bit is set in Ethernet tagged frames, that means that there is E-RIF included in the extended Ethernet header, i.e. routing information field.) So that was a bit of a side trip. I think, in short, the RII bit is not set in Ethernet MAC source addresses. But it can be set in Token Ring or FDDI, and it makes the MAC source address look like a multicast address.
