Multicast MAC in Source MAC Address Field

Hi,

is it allowed/forbidden to send an Ethernet Source MAC address field with a multicast MAC address in it?

Any references? (RFCs, IEEE doks, other??)

I've been searching for a while now and the only thing I found out is that Cisco doesn't like it and calls it illegal but I did not find the law yet...

Thanks Mat

Reply to
Matthias Schaerer
Loading thread data ...

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.

Bert

Reply to
Albert Manfredi

Albert Manfredi wrote: (snip regarding multicast bit in source addresses)

And what about packets bridged between FDDI or TR and ethernet with such bit set?

-- glen

Reply to
glen herrmannsfeldt

IEEE 802.3-2002, Section 3.2.3(b):

The first bit (LSB) shall be used in the Destination Address field as an address type designation bit to identify the Destination Address either as an individual or as a group address. If this bit is 0, it shall indicate that the address field contains an individual address. If this bit is 1, it shall indicate that the address eld contains a group address that identies none, one or more, or all of the stations connected to the LAN. In the Source Address field, the first bit is reserved and set to 0.

The last sentence says it all; the first bit of the SA must be set to zero; multicast SAs are prohibited.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Reply to
Rich Seifert

The answer must be, the source MAC address lowest bit is returned to 0, the frame is coded up like a 802.1Q tagged Ethernet frame, and the CFI bit is set in the extended header, to show that source "routing" info is also added to the Ethernet header. And the routing info from the source-routed TR or FDDI frame is copied into the E-RIF of the extended Ethernet header.

But that might just be a textbook answer. I don't know whether switch manufacturers bother with this in practice.

Bert

Reply to
Albert Manfredi

Shucks. That's "the lowest bit of the first byte of the source MAC address ..."

Reply to
Albert Manfredi

In the days when bridges between FDDI/TR and Ethernet were being designed, there was no such thing as IEEE 802.1Q VLAN tagging, so these devices could not encapsulate source routing information for transport across an Ethernet network.

In a non-802.1Q environment, source-routed frames (i.e., with the first bit of the SA set to "1") were discarded at any Ethernet ports of a bridge; they could not be validly forwarded onto an Ethernet. Source routing could be used only within FDDI and TR networks.

There is an extensive discussion of this in Chapter 6 of "The Switch Book."

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Reply to
Rich Seifert

dont know about FDDI, but when i was doing L2 mixed Ethernet and T/R on IBM networks, the translation bridge has to take care of it.

basically - no source route info across Ethernet unless you use one of the proprietary Token ring tunnel protocols

FWIW a translation bridge is usually configured with a "ring number" for Ethernet, but the RIF doesnt propagate - which can be a good way to cause the loops source routing is supposed to resolve......

The same sort of thing happens with DLSw although the mechanisms are different.

Also you have to worry about 3 different interacting spanning trees - there were more good reasons for ditching Token Ring than just expensive NIC cards.

Reply to
stephen

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.