Multicast Source Redundancy in EVPNs
Extracted elements (30)
In the Hot Standby solution, the ESI label is repurposed for ingress filtering at the downstream PE, which is the opposite of its use in RFC 7432 where it identifies which egress ACs to exclude for BUM traffic split-horizon filtering.
The anycast address workaround used in conventional PIM/MVPN networks is unsuitable for Hot Standby redundancy and introduces challenges when sources must be reachable via IP unicast or when multiple sources with the same IP address share a Broadcast Domain. RFC 9856 avoids these problems by using ESI labels instead.
The Hot Standby solution triggers S-PMSI A-D route advertisement based on SFG configuration rather than on traffic reception (unlike Warm Standby). This ensures downstream PEs learn RPF check parameters before traffic arrives, enabling immediate fast failover without waiting for route convergence after the first packet.
The Warm Standby solution operates entirely on upstream PEs, so downstream PEs that are not connected to redundant G-sources require no upgrades. This makes WS deployment easier in incremental rollout scenarios at the cost of longer failover times.
In the Hot Standby solution with IR or AR P-tunnels, upstream PEs push S-ESI labels on packets destined for all PEs in the tenant domain (not just PEs sharing the ES as in RFC 7432), extending baseline split-horizon ESI label usage to ingress filtering.
System support for both the Warm Standby and Hot Standby solutions is not mandated. If one solution is implemented, support for the other is OPTIONAL. Operators select between them based on failover time requirements and bandwidth constraints.
The use of variable-length source prefixes in S-PMSI A-D route advertisements is permitted in this document only for the specific application of redundant G-sources; it is not a general extension to EVPN S-PMSI A-D procedures.
This document extends RFC 9573 DCB label usage: DCB-allocated ESI labels MAY be used with Ingress Replication (IR) tunnels, and MAY be used by PEs that do not themselves use DCB-allocated PMSI labels. RFC 9573 originally restricted DCB labels to MP2MP, P2MP, and BIER tunnels.
A consistent DF Election Algorithm MUST be used across all upstream PEs for the Single Forwarder election. In OISM networks, the Default Designated Forwarder Election Algorithm MUST NOT be used if redundant G-sources are attached to Broadcast Domains with different Ethernet Tags.
All packets for a given G-source MUST carry the same S-ESI label. Multi-homed PEs sharing an S-ES MUST allocate the same ESI label for that S-ES, and labels for distinct S-ESes MUST be different.
An upstream PE MUST withdraw the S-PMSI A-D route when SFG traffic ceases. A timer is RECOMMENDED to detect inactivity and trigger route withdrawal.
If an SBD exists in an OISM tenant, the EVPN A-D per EVI and A-D per ES routes MUST include the SBD-RT so that they are imported by all PEs in the tenant domain and can trigger mass withdrawal of RPF checks.
In the Hot Standby solution, an upstream PE MUST advertise S-PMSI A-D routes for each SFG based on SFG configuration (not traffic reception), including ESI Label Extended Communities for all S-ESI labels associated with the SFG.
In the Hot Standby solution, each redundant G-source MUST be associated with an ES (S-ESI) on the upstream PEs, for both single-homed and multi-homed G-sources. S-ESI labels are used for RPF checks on downstream PEs.
Non-SF upstream PEs MUST discard all (*,G) or (S,G) packets received on local ACs. The Single Forwarder MUST forward (*,G) or (S,G) packets received on one and only one local AC.
PEs attached to the SBD of an SBD-RT (but not to the source BD) MUST import EVPN A-D per ES/EVI routes and use them for redundant G-source mass withdrawal. Upon importing A-D per ES routes for different S-ESes, a PE MUST select a primary S-ES and add an RPF check that discards packets not carrying the primary S-ESI label.
The Flags field within the ESI Label Extended Community MUST be set to 0x00 on transmission and MUST be ignored on reception when the community is used with S-PMSI A-D routes for the Hot Standby solution.
The withdrawal of the last EVPN S-PMSI A-D route for a given (*,G) or (S,G) SFG SHOULD result in the downstream PE removing the S-ESI label-based RPF check for that flow.
Upon receiving the first IP multicast packet for a configured SFG on a Broadcast Domain, an upstream PE MUST advertise an S-PMSI A-D route ((*,G) or (S,G)) that includes the BD-RT (and SBD-RT if applicable), the SFG flag in the Multicast Flags Extended Community, and the Designated Forwarder Election Extended Community.
When IP multicast source multi-homing is needed, EVPN multi-homed ESes MUST be used. EVPN multi-homing ensures that only one upstream PE forwards a given multicast flow at a time, preventing packet duplication at downstream PEs.
A Single Flow Group (SFG) is a multicast group carrying a single flow where multiple redundant G-sources may transmit identical traffic. An SFG is represented as (*,G) for any source or (S,G) where S is a prefix of any length; a source is deemed redundant if its address matches the prefix.
BFD MAY be used in the Hot Standby solution to monitor multipoint tunnels carrying SFG packets from redundant G-sources. The BGP-BFD Attribute is advertised alongside S-PMSI A-D or IMET routes, and multipoint BFD sessions are bootstrapped per RFC 9780.
DCB-allocated S-ESI labels are used in the Hot Standby solution. An ESI label is considered a DCB label if either the ESI-DCB-flag is set in the ESI Label Extended Community or the label is signaled by a PE that has advertised a DCB-allocated PMSI label.
The Hot Standby (HS) solution is downstream-PE-based: upstream PEs forward SFG traffic from all redundant G-sources simultaneously, each appending a unique ESI label (S-ESI label). Downstream PEs perform RPF checks keyed on the ESI label of the selected primary G-source and discard packets from other ESIs.
The S-ESI (Multicast Source Ethernet Segment Identifier) is the ESI associated with a redundant G-source. Single-homed G-sources require a dedicated ESI on the upstream PE; multi-homed G-sources use their all-active multi-homing ES as the S-ESI.
The Warm Standby (WS) solution is upstream-PE-based: all upstream PEs connected to redundant G-sources for an SFG elect a Single Forwarder (SF) among themselves. Non-SF PEs discard all (*,G)/(S,G) packets from local ACs; the SF forwards from exactly one local AC.
IANA allocated bit 4 in the 'Multicast Flags Extended Community' registry (introduced by RFC 9251) as the 'Single Flow Group' flag. This flag in an S-PMSI A-D route indicates the (*,G) or (S,G) entry is associated with an SFG.
IANA allocated bit 5 in the 'EVPN ESI Label Extended Community Flags' registry (introduced by RFC 9746) as the 'ESI-DCB' flag. When set, this flag indicates that the ESI label in the ESI Label Extended Community is a DCB (Domain-wide Common Block) label.
The security considerations of RFC 9625 apply. The Warm Standby method has low impact on PE processing capabilities due to its lightweight control plane, while the Hot Standby method adds significantly more control-plane burden on all PEs of the tenant that have sources or receivers.
The ESI Label Extended Community (Type=0x06, Sub-Type=0x01) is extended with an ESI-DCB-flag at bit 5 of the Flags octet. When set, this bit indicates the ESI label is a Domain-wide Common Block (DCB) label. The community is 8 octets: Type (1), Sub-Type (1), Flags (1), Reserved (1), Reserved (1), ESI Label (3).