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Posted by Matthias Schaerer on August 30, 2005, 5:13 pm
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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
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Posted by Albert Manfredi on August 31, 2005, 9:38 am
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Matthias Schaerer wrote:
> 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...
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
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Posted by glen herrmannsfeldt on August 31, 2005, 12:15 pm
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Albert Manfredi wrote:
(snip regarding multicast bit in source addresses)
> 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.
And what about packets bridged between FDDI or TR and ethernet with
such bit set?
-- glen
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Posted by Albert Manfredi on August 31, 2005, 1:04 pm
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glen herrmannsfeldt wrote:
> Albert Manfredi wrote:
> (snip regarding multicast bit in source addresses)
>
> > 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.
>
> And what about packets bridged between FDDI or TR and ethernet with
> such bit set?
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
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Posted by Albert Manfredi on August 31, 2005, 1:10 pm
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> > And what about packets bridged between FDDI or TR and ethernet with
> > such bit set?
>
> The answer must be, the source MAC address lowest bit is returned to 0,
Shucks. That's "the lowest bit of the first byte of the source MAC
address ..."
> 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
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