MPLS and Ethernet Operations, Administration, and Maintenance (OAM) Interworking
Extracted elements (23)
For Multi-Segment PWs (MS-PWs), Switching PEs (S-PEs) are not aware of the native service. Therefore, failure detection and notification at S-PEs is based solely on PW OAM mechanisms, and NS OAM to PW OAM mapping is performed only at Terminating PEs (T-PEs) to propagate failure notifications to EVC end points.
Scalability constraints may prevent running proactive monitoring such as CCMs with transmission enabled at a PE to detect EVC failures across the PW domain. This motivates the need for network-driven alarm mappings between the NS and PW domains rather than relying solely on end-to-end proactive OAM.
This document addresses Ethernet PW specifically because RFC 6310 covers ATM and Frame Relay emulation over PWs but explicitly does not describe defect state mapping for Ethernet due to its unique OAM characteristics (e.g., CFM MEG levels, AIS/RDI, CCM-based continuity check with 3.5× timeout).
Ethernet Link OAM (IEEE 802.3) and E-LMI (MEF16) are optional mechanisms. E-LMI is applicable only for single-hop Ethernet ACs (without any 802.1Q/802.1ad-compliant bridges in between); CFM and Y.1731 mechanisms are applicable in all types of Ethernet ACs.
When a PE cannot process NS OAM messages received on the PW (even when such messages are defined for Ethernet), fault notification mapping between the PW domain and the NS domain is required. This justifies maintaining both PW OAM and NS OAM notification paths in parallel.
For PWs established over MPLS or MPLS/IP PSN using mechanisms other than LDP (e.g., static configuration), in-band signaling using VCCV-BFD SHOULD be used to convey AC and PW status and defects; alternatively, mechanisms in RFC 6478 MAY be used.
Receive defect state SHOULD have precedence over transmit defect state in handling when both transmit and receive defect states are identified simultaneously at a PE.
Upon entry to the AC receive defect state, PE1 MUST notify PE2 via a forward defect notification (LDP PW status message indicating forward defect or VCCV-BFD forward defect diagnostic code). Additionally, if PE1 has a Down MEP with CCM transmission enabled, the MEP MUST set the RDI bit in transmitted CCM frames.
Upon entry to the AC transmit defect state, PE1 MUST notify PE2 via a reverse defect notification (LDP PW status message or VCCV-BFD reverse defect diagnostic code). Upon exit from this state, PE1 MUST clear the reverse defect notification to PE2.
Upon entry to the PW receive defect state, PE1 MUST assume PE2 has no knowledge of the defect and MUST send a reverse defect failure notification to PE2 via PW status notification message or VCCV-BFD diagnostic code (if CV type 0x08 or 0x20 was negotiated); NS OAM may also be used per Section 4.1. This requirement does not apply if the defect state was entered due to a forward defect notification from PE2 or loss of control adjacency.
Upon entry to the PW transmit defect state, if PE1 has a Down MEP with CCM enabled, the MEP MUST set the RDI bit in transmitted CCM frames or send a Status TLV with interface down to the peer MEP at CE1. If the PW failure was detected locally without receiving an RDI from PE2, PE1 MUST send an FDI to PE2.
Upon exit from the PW receive defect state, if PE1 has a Down MEP on the AC with CCM transmission enabled and Interface Status TLV support, the MEP MUST transmit CCM frames with the Interface Status TLV set to Up; without TLV support, it MUST resume CCM transmission; and PE1 MUST notify PE2 of cleared reverse defect via PW status or VCCV-BFD.
Upon exit from the PW transmit defect state, if PE1 has a Down MEP with CCM enabled, the MEP MUST clear the RDI bit in CCM frames or send a Status TLV with interface up to CE1, and PE1 MUST clear the FDI to PE2 if applicable.
When a MEP at a PE or CE receives a CCM with the wrong MEG ID, MEP ID, or MEP level, the receiving PE or CE SHOULD treat such an event as an AC receive defect; in any case, if such events persist for 3.5 times the local CCM transmission time, loss of continuity will be declared.
Four abstract defect states are defined at a PE: PW receive defect (impacts receiving traffic from the far-end PE), PW transmit defect (impacts sending traffic toward the far-end CE), AC receive defect (impacts receiving traffic from the local CE), and AC transmit defect (impacts sending traffic to the local CE). Arrows indicate the direction of user traffic impacted by each defect.
Native Service (NS) OAM notification for Ethernet PW uses three mechanisms: sending AIS frames from the local MEP when CCM transmission is disabled, suspending CCM frame transmission to convey PE receive defects when CCM is enabled, and setting the RDI bit in CCM frames when loss of CCMs from the peer MEP is detected or when conveying PW reverse defects.
PW defect status code points defined in RFC 6310 are used: forward defect is the logical OR of Local AC (ingress) Receive Fault, Local PSN-facing PW (egress) Transmit Fault, and PW Not Forwarding; reverse defect is the logical OR of Local AC (egress) Transmit Fault and Local PSN-facing PW (ingress) Receive Fault.
VCCV BFD CV types determine defect signaling scope: CV types 0x04 or 0x10 indicate BFD for PW fault detection only (other defects via LDP or equivalent); CV types 0x08 or 0x20 indicate BFD for both PW fault detection and AC/PW fault notification, so all defects are signaled via BFD.
The OAM interworking mechanisms do not change the security functions inherent in the actual messages. Generic PW traffic security from RFC 3985, VCCV security from RFC 5085, and PWE3 control protocol security from RFC 4447 all apply. Since these mechanisms enable propagation of OAM messages and fault conditions between native service networks and PSNs, continuity of the end-to-end service depends on a trust relationship between the operators of these networks, as discussed in RFC 5254.
AC Receive Defect State: PE1 enters upon physical-layer fault (LoS, Link OAM Dying Gasp/Critical Event, client Signal Fail), receipt of Ethernet AIS from CE1, absence of CCM frames for 3.5× the CCM transmission period, or a CCM with Interface Status TLV indicating interface down. PE1 exits when all triggering conditions are cleared (e.g., LoS cleared, AIS period expired, consecutive CCMs resumed, CCM Interface Status TLV up).
AC Transmit Defect State: PE1 enters upon physical-layer fault on the AC interface (LoS or Link OAM Link Fault indication) or receipt of a CCM frame with the RDI bit set from the peer MEP at CE1. PE1 exits when the physical fault is cleared and no subsequent CCM frames with the RDI bit set are received from the peer MEP.
PW Receive Defect State: PE1 enters when it receives a Forward Defect Indication (FDI) from PE2, detects loss of PSN tunnel connectivity upstream, or detects loss of PW connectivity via VCCV-BFD, VCCV-Ping, or NS OAM CC. PE1 exits when all previously detected defects clear and PE2 has cleared any FDI; exit may transition to the PW operational state or PW transmit defect state.
PW Transmit Defect State: PE1 enters when it receives a Reverse Defect Indication (RDI) from PE2 (indicating remote AC transmit fault or upstream PW fault) and is not already in PW receive defect state. PE1 exits upon receiving an OAM message from PE2 clearing the RDI or upon entering the PW receive defect state.