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 transmitted". 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 configured? - dot1agCfmMepCcmLtmPriority (clause 12.14.7.1.3:h) - dot1agCfmMepTransmitLbmVlanPriority (clause 12.14.7.3.2:e) 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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