Summary

  • RFC 9856 adds warm- and hot-standby procedures for redundant multicast G-sources carrying one Single Flow Group in an EVPN tenant.
  • A Single Forwarder election, ESI label or BFD session identifies control state; none alone proves one effective payload, duplicate-free delivery or an uninterrupted receiver.
  • Closure needs separate evidence for source health, selection epoch, ingress suppression, cutover, packet sequence, receiver branches and application acceptance.

Redundancy moves the decision; it does not remove it

Traditional multicast commonly assumes one source for a flow, or leaves duplicate handling to the receiver. RFC 9856 addresses a harder EVPN case: two or more source systems send what the tenant intends to treat as one Single Flow Group, expressed as (*,G) or a source-prefix form of (S,G). The sources may sit in different broadcast domains and may be single- or multi-homed.

This is not ordinary multihoming of one source. In that older case, an Ethernet Segment and its Designated Forwarder keep multiple attached PEs from forwarding the same source copy. Redundant G-sources are separate producers. The fabric needs an additional answer to a different question: which source copy should reach interested receivers?

Warm Standby places the answer upstream. Candidate upstream PEs advertise the SFG and elect a Single Forwarder. Non-SFs discard packets received from their local redundant sources; the SF accepts one local attachment circuit and discards other local copies. That reduces duplicate transport across the fabric. Yet the election proves only which PE won under the advertised preference and tie-break state. A stale winner, a silent source, an incorrect stream or data-plane leakage can coexist with a perfectly legible election.

Hot Standby moves the answer downstream. Upstream PEs forward their redundant flows, mark them with source-Ethernet-segment identity, and downstream PEs use that identity to accept one path and discard the others. The arrangement can shorten reaction because alternates are already arriving, but it multiplies the places where a decision must remain coherent. An advertised ESI label, a programmed DCB label and an up BFD session do not show that every downstream PE chose the same source epoch, discarded every extra copy, or preserved legitimate packets during a change.

Six receipts for one continuity claim

Source evidence identifies the actual producer, software/configuration epoch, stream identifier, freshness and payload canary. Reachability to its address is weaker: it says the path can answer, not that the intended multicast content exists.

Control evidence records the exact SFG, source advertisements, preferences, SF or downstream selection, and the BGP generation under which the decision was made. A later route snapshot must not be used to explain an earlier packet trace without an epoch join.

Suppression evidence belongs to the data plane. For Warm Standby it includes per-AC accepts and drops at every candidate PE. For Hot Standby it includes incoming label identity, selected path and duplicate-drop counters at representative downstream PEs. Counters need denominators and reset provenance; zero drops can mean no duplicates or no traffic.

Cutover evidence names both endpoints of the interval: the trigger, such as source health, BFD or route withdrawal, and the receiver-visible completion point. Control convergence, first accepted packet, stable sequence and restored application policy are different finishes.

Receiver evidence samples branches, not merely one convenient PE. Packet sequence, timestamps, loss, reordering and duplication establish transport behaviour. Application checks establish whether the payload was current, decodable and semantically accepted.

Finally, rollback evidence proves that a failed selection can be reversed without leaving mixed forwarding epochs, stale labels or a second active source. RFC 9251 membership state and RFC 9625 inter-subnet forwarding explain where interest and delivery state arise; they do not turn a selection record into receipt.

Sources