EVPN Virtual Private Wire Service (VPWS) Flexible Cross-Connect (FXC) Service
Extracted elements (29)
For single-homed endpoints, individual AC failures are not signaled in BGP because there is no alternative path to that endpoint anyway. The trade-off is that traffic destined for a failed AC may traverse the MPLS/IP core and be discarded at the egress PE, wasting network resources until application-layer OAM detects the failure.
Local switching takes precedence over MPLS/IP forwarding in VLAN-Signaled FXC, aligning with baseline EVPN principles from RFC 7432 where locally switched preference is specified for MAC/IP routes. When a PE processes an Ethernet A-D per-EVI route whose ESI is a local ESI, it does not modify its forwarding state.
Service providers need to multiplex millions of ACs across multiple physical interfaces onto a single VPWS service tunnel to reduce the number of EVPN service labels, associated OAM monitoring overhead, and BGP signaling compared to RFC 8214 where each AC requires its own service tunnel.
The VPWS Service Tunnel is represented by a pair of downstream-assigned EVPN service labels advertised via Ethernet A-D per-EVI routes. A single tunnel can be associated with many VPWS service identifiers (normalized VIDs), enabling AC multiplexing while retaining the existing EVPN label distribution and fast convergence mechanisms of RFC 7432.
VLAN-Signaled FXC enables automatic cross-checking of normalized VIDs: if the same normalized VID is configured on three PE devices for the same EVI, the PE detects this when it receives a second remote Ethernet A-D per-EVI route with a different ESI and generates an error. This cross-checking is not feasible in Default FXC mode because normalized VIDs are not signaled.
When a single VPWS service tunnel carries multiple ACs across various ESs without BGP signaling of individual ACs, remote PEs cannot associate a failed ES with its ACs or redirect traffic via another multi-homing PE. This necessitated two distinct FXC mechanisms: Default FXC for single-homed endpoints and VLAN-Signaled FXC for multi-homed endpoints.
The M and V fields in the Control Flags are used only for consistency checking; a receiving PE MAY generate an alarm if it detects inconsistencies but will not disrupt the VPWS service. A thorough understanding of RFC 7432 and RFC 8214 are prerequisites for implementing this protocol.
24-bit VPWS service instance identifiers and 12-bit VPWS service instance identifiers representing normalized VIDs MUST be right-aligned in the Ethernet Tag ID field.
Both imposition and disposition PE devices MUST be aware of the VLANs through configuration in the Default FXC mode, since normalized VIDs are not signaled via EVPN BGP in this mode.
If both endpoints signal a matching normalized VID but one operates in single-tag mode and the other in double-tag mode, the V-bit facilitates detection and prevention of tunnel instantiation. The VPWS service tunnel must not be established in this mismatch condition.
Normalized VID lookup is REQUIRED at the disposition PE to forward packets to their proper egress endpoints, because the EVPN label alone is not sufficient for accurate forwarding when a single service tunnel multiplexes ACs across multiple ESs or physical interfaces.
Remaining bits in the Control Flags of the EVPN Layer 2 Attributes Extended Community MUST be set to zero when sending and MUST be ignored when receiving. The M and V fields are OPTIONAL and M is ignored at reception for forwarding purposes; it is used only for error notifications.
The 32-bit Ethernet Tag ID field MUST be set to the VPWS service instance identifier value. For FXC with a 12-bit ID, this identifier equals the single-tag normalized VID and must be the same on both VPWS service endpoints; for a 24-bit ID it equals the double-tag normalized VID.
The EVPN Layer 2 Attributes Extended Community with the new M and V flags SHOULD be sent with Ethernet A-D per-EVI routes for both Single-Active and All-Active redundancy modes.
VID normalization MUST be completed prior to MPLS encapsulation on the ingress PE because the VID lookup must be performed at the disposition PE. This requires both imposition and disposition PEs to support VLAN tag manipulation (rewriting, adding, or deleting single or double tags).
When a single normalized VID is used, the lower 12 bits of the Ethernet Tag ID field in EVPN routes MUST be set to that VID. When a double normalized VID is used, the lower 12 bits MUST be set to the inner VID and the higher 12 bits to the outer VID.
When cross-connection occurs between two ACs belonging to two multi-homed ESs on the same set of multi-homing PEs, traffic between the two ACs MUST be locally switched within the PE rather than forwarded via the MPLS/IP network.
At the disposition PE, an EVPN label lookup identifies a VID-VRF (VPN Routing and Forwarding for normalized VID lookup), and a subsequent lookup of the normalized VID within that table identifies the appropriate egress endpoint or interface. Tag manipulation from normalized to local VID SHOULD occur as part of this lookup or at the egress interface.
EVPN-VPWS service failure detection can be performed via OAM mechanisms such as BFD for pseudowire VCCV (RFC 5885). Upon such failure detection, the switchover procedure to the backup PE follows the same multi-homing convergence procedures defined in RFC 8214.
In Default FXC mode, a single Ethernet A-D per-EVI route is sent with ESI=0 when the first AC is configured; additional ACs are only locally associated with the pre-established tunnel without generating additional BGP routes. The route includes two new flags in the EVPN Layer 2 Attributes Extended Community indicating Default FXC mode and normalized VID type.
In Local Switching scenarios, the Ethernet A-D per-EVI route must be advertised with the MPLS label associated with either the destination AC or the destination ES, rather than the VID-VRF label, to avoid forwarding ambiguity when the same normalized VID is reachable via two different ESs.
In VLAN-Signaled FXC mode, the single EVPN-VPWS service tunnel is represented by multiple VPWS service IDs (one per normalized VID), and each normalized VID is signaled via an Ethernet A-D per-EVI route where the ESI represents the ES ID, the Ethernet Tag is set to the normalized VID, and all ACs share the same dynamically generated EVPN label.
VID normalization translates potentially overlapping VLAN IDs from multiple physical interfaces into unique normalized VIDs. When more than 4095 VLANs are multiplexed onto a single tunnel, single-to-double tag translation is performed, resulting in a 24-bit double normalized VID.
IANA has allocated bits 8-11 in the 'EVPN Layer 2 Attributes Control Flags' registry: M (bits 10-11) for signaling mode of operation (Default FXC vs. VLAN-Signaled FXC vs. RFC 8214 baseline), and V (bits 8-9) for VLAN-ID normalization type (single vs. double vs. RFC 8214 baseline).
RFC 9744 describes a multiplexing capability that leverages EVPN-VPWS signaling without adding functionality beyond the muxing service itself. No additional security considerations are needed beyond those already specified in RFC 8214.
A PE port failure in Default FXC mode causes withdrawal of the service tunnel route, updating the adjacency list for all ACs on that port. In VLAN-Signaled FXC mode, a port failure triggers withdrawal of both the Ethernet A-D per-EVI routes for all normalized VIDs on that port and the Ethernet A-D per-ES route for the port's ES.
In Default FXC mode, an AC or VLAN failure is not signaled in BGP; the egress PE with the failed AC drops packets but remote PEs continue directing traffic to it. In VLAN-Signaled FXC mode, an AC failure triggers withdrawal of the Ethernet A-D per-EVI route for the corresponding normalized VID, causing remote PEs to remove the failed PE from their outgoing adjacency list.
In the event of a PE node failure, EVPN route withdrawals are performed by the route reflector instead of the PE itself; subsequent switchover behavior mirrors PE port failure handling for both Default FXC and VLAN-Signaled FXC modes.
The EVPN Layer 2 Attributes Extended Community (Type 0x06, Sub-type 0x04) is extended with two new 2-bit fields in the Control Flags: M (bits 10-11) encodes the FXC mode of operation (00=RFC8214, 01=VLAN-Signaled FXC, 10=Default FXC), and V (bits 8-9) encodes VID normalization type (00=RFC8214, 01=single-VID, 10=double-VID).