The Ethernet Performance Monitoring on untagged EFPs feature enables sessions to run on untagged Ethernet flow points (EFPs). If an EFP is configured as untagged, then the EFP handles any frames without a dot1q tag, that it receives. Any frames sent using this EFP do not have a dot1q tag.
The dot1q tag contains class of service (CoS) bits, which are used by EPM to test delay or loss of packets with a specific CoS. This support is unavailable when using EPM over untagged EFPs but all other performance monitoring functionality is supported.