EVPN Broadcast, Unknown Unicast, or Multicast (BUM) Using Bit Index Explicit Replication (BIER)
Extracted elements (25)
EVPN BUM procedures are closely aligned with MVPN procedures (RFC 6514/RFC 8556), and an EVPN Broadcast Domain corresponds to a VPN in MVPN. This alignment is intentional so that BIER usage for EVPN can reuse most of the MVPN-with-BIER specification from RFC 8556 verbatim.
When only selective forwarding is used for all flows without tunnel segmentation, SMET routes suffice without S-PMSI A-D routes, reducing signaling overhead. S-PMSI/Leaf A-D routes are only needed when some traffic uses I-PMSI or when tunnel segmentation is required.
Three MPLS label-assignment rules from RFC 8556 Section 2.1 that handle AFI differences, MVPN extranet VRFs, and Extranet Separation do NOT apply to EVPN; they are explicitly excluded.
Tunnel segmentation with BIER is only applicable to EVPN-MPLS, not to EVPN-VXLAN/NVGRE/GENEVE. Additionally, the LIR-pF flag cannot be used with tunnel segmentation (same restriction as MVPN).
A PTA specifying BIER attached to an IMET, S-PMSI A-D, or per-region I-PMSI A-D route MUST NOT be distributed beyond the boundaries of a BIER domain; all receiving routers must be in the same BIER domain as the originator.
Egress PEs MUST be configured to receive and process packets addressed to the well-known multicast destination addresses (224.0.0.122 / FF02::14) used in the BIER PHP scenario.
If the determined tunnel requires leaf tracking (e.g., BIER) but no leaf-tracking routes exist, the BUM packet MUST NOT be forwarded onto the P-tunnel.
When BIER Penultimate Hop Popping (PHP) is used and the encapsulation after BIER header removal cannot indicate a VXLAN/NVGRE/GENEVE next header, the full VXLAN/NVGRE/GENEVE encapsulation MUST be included in the BIER payload.
An ingress PE pushes an upstream-assigned ESI label before the BD label only when all three conditions hold: the packet arrived on a multihomed ES, the encapsulation is EVPN-MPLS, and the ESI label split-horizon procedure is in use.
At a BFER egress PE, the BD is determined from the upstream-assigned MPLS label (EVPN-MPLS) or VNI/VSID (EVPN-VXLAN/NVGRE/GENEVE), after which standard EVPN forwarding per RFC 7432 or RFC 8365 proceeds. For EVPN-MPLS, an inner label following the BIER header is treated as the upstream-assigned ESI label for split-horizon.
At a P-tunnel segmentation point acting as BFIR, the upstream segment label is swapped to the upstream-assigned label from the PTA of the re-advertised downstream route, and a new BIER header is imposed for the downstream region.
For an inclusive PMSI, the set of BFERs is all originators of IMET routes for the BD. For a selective PMSI, BFERs are originators of matching SMET routes. SMET routes do not carry a PTA; the ingress PE uses the upstream-assigned label from its own IMET route.
For EVPN-MPLS with BIER, ESI label split-horizon procedures apply using two upstream-assigned labels: one identifying the BD (from the PTA) and one identifying the Ethernet Segment. The BFIR-id in the BIER header identifies the ingress PE that assigned these labels.
For EVPN-VXLAN/NVGRE/GENEVE at a BFER, the payload after BIER decapsulation is an NVO frame (with or without an IP header), and the VNI/VSID field in the NVO header identifies the corresponding BD.
For EVPN-VXLAN/NVGRE/GENEVE with BIER, the local bias split-horizon procedure still applies: the BFIR-id in the BIER header identifies the ingress PE, and a receiving PE does not forward the BUM packet to any multihomed ES that the ingress PE is also attached to.
For the BIER PHP case with NVO encapsulations, a well-known IP multicast destination address is used: 224.0.0.122 (IPv4) or FF02:0:0:0:0:0:0:14 (IPv6). These addresses are shared across all BDs; the inner NVO header identifies the specific BD.
IMET (Inclusive Multicast Ethernet Tag) A-D routes carry a PMSI Tunnel Attribute (PTA) to identify the provider tunnel for BUM flows in a BD. The same PTA structure defined in RFC 6514 and extended by RFC 8556 for BIER is reused for EVPN.
The Tunnel Type codepoint 0x0B, already assigned by IANA for BIER in the MVPN context (RFC 8556), is reused unchanged for EVPN. No new codepoint is required.
IANA assigned IPv4 multicast address 224.0.0.122 (in the Local Network Control Block 224.0.0/24) and IPv6 link-local multicast address FF02:0:0:0:0:0:0:14, both described as 'Network Virtualization Overlay (NVO) BUM Traffic' for the BIER PHP use case.
IANA registered three new values in the 'BIER Next Protocol Identifiers' registry: value 7 (VXLAN encapsulated, no IP/UDP header), value 8 (NVGRE encapsulated, no IP header), and value 9 (GENEVE encapsulated, no IP/UDP header), all referencing RFC 9624.
Using BIER as provider tunnels for EVPN is analogous to using BIER for MVPN and introduces no additional security implications beyond those discussed in the EVPN base specification (RFC 7432) and MVPN-with-BIER (RFC 8556).
At a BFIR ingress PE, four ordered rules determine the matched transmission route and leaf-tracking routes: (1) no selective forwarding → use IMET + all peer IMET routes; (2) all-IP-multicast SMET-based → use IMET + matching SMET routes; (3) S-PMSI A-D match → use S-PMSI + LIR/LIR-pF-derived Leaf A-D or SMET routes; (4) fallback → rule 1.
The MPLS Label field in the PTA carries an upstream-assigned MPLS label for EVPN-MPLS (assigned by the BFIR), or a 24-bit VNI/VSID of global significance for EVPN-VXLAN/NVGRE/GENEVE.
The Proto field in the BIER header is set to: 2 for EVPN-MPLS payload; 7 for VXLAN without IP/UDP header; 8 for NVGRE without IP header; 9 for GENEVE without IP/UDP header; 4 for IPv4 header (VXLAN/NVGRE/GENEVE PHP case); 6 for IPv6 header (PHP case).
The PTA Tunnel Identifier for BIER contains three subfields: (1) one octet for the BIER sub-domain-id, (2) two octets for the BFR-id within that sub-domain, and (3) the BFR-Prefix as either a /32 IPv4 or /128 IPv6 address (inferred from total PTA length).