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Interpretation #11
IEEE Std. 802.1Q-2005

Topic: Definition of an MST region

IEEE Std 802.1Q defines an MST Region as follows:

3.87 MST Region: A set of LANs and MST Bridges physically connected
via Ports on those MST Bridges, where each LAN's CIST Designated
Bridge is an MST Bridge, and each Port is either the Designated Port
on one of the LANs or else a non-Designated Port of an MST Bridge that
is connected to one of the LANs,whose MCID matches exactly the MCID of
the Designated Bridge of that LAN.

NOTE-It follows from this definition that the MCID is the same for all
LANs and Ports in the Region, and that the set of MST Bridges in the
region are interconnected by the LANs.

The above definition was augmented in P802.1s/D13 (to include "and
each Port is either ...matches exactly the MCID .. of that
LAN"."). There doesn't appear to be any specific comment in the
disposition of comments in P802.1s/D12 for adding to the definition.

However IEEE Std 802.1Q also specifies an MST Region in clause 13.9 as

13.8 MST Regions

An MST Region comprises one or more MST Bridges with the same MST
Configuration Identifiers, using the same MSTIs, interconnected by and
including LANs for which one of those Bridges is the Designated Bridge
for the CIST and which have no Bridges attached that cannot receive
and transmit RST BPDUs.

These definitions are not the same or equivalent, because the
definition in 13.8 includes within an MST Region a LAN whose
Designated Bridge is within an MST Region but that may have RST
Bridges connected (which don't use an MCID), or indeed may have the
Root Port of a bridge in another region attached. Note that the
definition of fromSameRegion() does not help here, since that refers
to the bridge that transmiited the received BPDU, not the LAN it was
received on, and on any given LAN a Bridge Port can receive BPDUs from
bridges in the same region and not in the same region. Figure 13-2 LAN
'N" is an example of such a LAN and is shown within REGION 2 in the
figure and is decribed as such in 13.3.1 (f).

Am I correct in assuming that the definition 3.87 is wrong, and that
in 13.8 should be used?

I note in passing that 'MCID' is only ever used in clause 3 Definitions.

The clause 13.8 specification of MST Region takes precedence over the
3.87 definition. The latter will be changed in a future amendment or
revision of 802.1Q.

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Last modified by jlm, at 9:50am on Thu, 10 Sep 2009