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Interpretation #9
IEEE Std. 802.1ag-2007

Topic: Support of priority tagged CFM frames

Clause 3.38 describes a Primary VID as "The VID... on which all CFM
PDUs generated by MPs except for forwarded LTMs are to be

Clause 22.1.6 describes how a MEP, positioned above or below the
Support of the EISS entity, can be used to transmit and receive
untagged CFM frames for the purpose of protecting all the traffic on
the port.

When a MEP is not associated with a VID, does it still transmit and
receive untagged CFM frames when the following attributes are

-  dot1agCfmMepCcmLtmPriority (clause
-  dot1agCfmMepTransmitLbmVlanPriority (clause

If the answer is yes, under what conditions can a MEP transmit and
receive priority tagged CFM frames?

[Note: The expectation is that a MEP can currently only transmit and
received tagged and untagged CFM frames, and that an update to 802.1ag
will be required to support priority tagged CFM frames.]

VLAN and/or priority tags are generated by certain functional elements
(e.g. that specified in subclause 6.7 "Support of the EISS" in
IEEE Std 802.1Q-2005) in the interface stack supporting a Bridge Port.
The CFM shims themselves do not generate VLAN or priority tags, nor
does tag generation by other elements depend upon configuring CFM
related priority attributes.  Whether a tag appears in a transmitted
CFM frame solely depends upon where the CFM shim is configured
relative to other functional elements that do generate these tags.

The only functional element defined in 802.1Q (including all current
amendments) that generates priority-tagged frames is specified in
subclause 6.9 "Support of the ISS for attachment to a Provider
Bridged Network" in IEEE Std 802.1ad-2005.  This is not a general
purpose priority-tagging function.  It is specifically constrained to
be at the ISS of a port on a C-component in a Customer Bridge
connected to a Port-based Service Interface to a Provider Bridged
Network.  In this scenario, a MEP could be configured on the ISS above
this element and the result would be the transmission of
priority-S-tagged (an S-TAG with a VID value of zero) CFM frames.  The
standard does not prohibit configuring a MEP at this location, but it
does not require that an implementation of a Customer Bridge support
this capability.

This response was approved by 802.1 at July 2008 plenary meeting.

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Last modified by jlm, at 3:41pm on Mon, 13 Oct 2008