BESS Working Group K. Ananthamurthy Internet-Draft A. Sajassi Intended status: Standards Track L. Krattiger Expires: 23 January 2027 Cisco 22 July 2026 Per-Flow EVPN Designated Forwarder Election draft-kriswamy-bess-evpn-perflow-df-02 Abstract The Ethernet Virtual Private Network (EVPN) solution offers procedures for electing a Designated Forwarder (DF) for multihomed Ethernet Segments. In this context, the Provider Edge (PE) router is responsible for sending Broadcast, Unknown Unicast, and Multicast (BUM) traffic to a multihomed device or network. This applies in cases of an all-active multihomed Ethernet Segment (ES) as well as for BUM and unicast traffic in the case of single-active multihoming. While the Default Algorithm provides an efficient and automated mechanism for selecting the DF across Ethernet Tags within an Ethernet Segment, it does not provide Per-flow DF selection within an EVPN Instance (EVI). This document defines a new DF Election Algorithm that performs Per- flow DF selection by defining a new DF Algorithm value. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Ananthamurthy, et al. Expires 23 January 2027 [Page 1] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 23 January 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Designated Forwarder Election Extended Community . . . . . . 3 3. Per-Flow DF Election Operation . . . . . . . . . . . . . . . 4 3.1. Per-Flow DF Election Example . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. Normative References . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction The EVPN solution offers procedures for electing a Designated Forwarder (DF) for multihomed Ethernet Segments. In this context, the PE is responsible for sending BUM traffic to a multihomed device or network. This applies in cases of an all-active multihomed ES as well as for BUM and unicast traffic in the case of single-active multihoming. While the Default Algorithm provides an efficient and automated mechanism for selecting the DF across Ethernet Tags within an Ethernet Segment, it does not provide Per-flow DF selection within an EVPN Instance (EVI). Ananthamurthy, et al. Expires 23 January 2027 [Page 2] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 [RFC8584] and [I-D.ietf-bess-evpn-per-mcast-flow-df-election] enhance the default DF election by introducing the Highest Random Weight (HRW) algorithm and the tuple consisting of Ethernet Segment Identifier (ESI), VLAN, and multicast flow to provide more deterministic and stable DF selection respectively. Existing DF election procedures maintain state in both the control plane and the forwarding plane. These procedures improve the distribution of BUM traffic across the PEs attached to a multihomed Ethernet Segment. However, the forwarding responsibility is assigned at the Ethernet Tag or multicast-group granularity and does not provide per-flow load distribution. [I-D.ietf-bess-evpn-per-mcast-flow-df-election] extends the DF election procedures to support per-flow load distribution for multicast traffic. In that approach, a multicast-group-specific DF election is performed when a host attached to a multihomed Ethernet Segment joins a multicast group using IGMP or MLD. As a result, the participating PEs maintain multicast-group-specific DF election state in the control plane. This document addresses the remaining gap for BUM forwarding by defining a per-flow DF election algorithm that does not require maintaining per-flow control-plane state. Instead, all participating PEs independently derive the same Designated Forwarder for a given flow by applying a common hashing function to the flow key and selecting the DF from the ordered PE list defined in [RFC8584]. This approach distributes flows across the available PEs while ensuring deterministic and consistent forwarding decisions. This document updates [RFC8584] by defining a new DF Algorithm value. 2. Designated Forwarder Election Extended Community To advertise support for the Per-flow DF election algorithm, this document reuses the Designated Forwarder Election Extended Community defined in [RFC8584] that is advertised along with the Ethernet Segment route. This document defines a new DF Algorithm type Per- flow. A PE can notify other participating PEs in the redundancy group about its support for the Per-flow DF Algorithm by signaling a DF Election Extended Community. A PE that supports the procedures specified in this document MUST advertise the corresponding DF Alg in its Ethernet Segment Route (Route Type 4). This enables all PEs participating in the redundancy group to discover the capability and consistently apply the enhanced DF election procedures in a backward- compatible and interoperable manner. The format of the DF Election Extended Community that is used in this document follows: Ananthamurthy, et al. Expires 23 January 2027 [Page 3] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0x06 | Sub-Type(0x06)| RSV | DF Alg | Bitmap ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Bitmap | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: DF Election Extended Community * The DF Alg (5 bits) field identifies the DF Election Algorithm used for the Ethernet Segment. - This document defines a new DF Algorithm value, Per-flow. IANA is requested to assign the value 6 from the "DF Alg" registry defined in [RFC8584]. The assigned value is encoded in the DF Algorithm field of the DF Election Extended Community. * The procedures defined in [RFC8584] to handle Designated Forwarder Election Extended Community are applicable. * A PE SHOULD attach the DF Election Extended Community to the ES route and the Extended Community MUST be advertised if the ES is locally configured for DF type Per-flow DF election. * A PE MUST use the Per-flow DF Algorithm only if all participating PEs advertise the same DF Algorithm value for the Ethernet Segment. Otherwise, the procedures of [RFC8584] apply. Once all participating PEs advertise the Per-flow DF Algorithm, they perform DF selection as specified in Section 3. 3. Per-Flow DF Election Operation Once all PEs in the redundancy group have advertised support for the Per-flow DF Algorithm, they independently compute the Designated Forwarder for each flow using the procedures described in this section. No additional control-plane signaling is required beyond the ordered PE list. When operating in Per-flow Designated Forwarder (DF) mode, the ordered (ordinal) PE list defined in [RFC8584] SHALL be used. This is the only control plane state maintained. When a BUM packet arrives, the hash is computed using a set of packet header fields that are applicable to the packet being processed. * The selection of fields is dependent on the packet format and protocol type. Ananthamurthy, et al. Expires 23 January 2027 [Page 4] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 * The exact choice of fields is outside the scope of this document. * However, all participating PEs MUST use a consistent set of packet header fields for a given packet format to ensure that identical packets produce the same hash value across all participating PEs. The hash function MAY use a Cyclic Redundancy Check 32 (CRC-32) or any other implementation-specific hashing algorithm. * The choice of hash function is outside the scope of this document. * To ensure consistent Per-flow DF selection across all PEs participating in the ES, all such PEs MUST use the same hashing algorithm. Let H denote the hash value computed over the flow key, and let N denote the number of active PEs in the ordered PE list. The value of N is used in the DF selection algorithm. The DF index, I, MUST be computed as follows: I = H mod N The PE at position I in the ordered PE list SHALL be elected as the Designated Forwarder for the corresponding flow. * This approach allows multiple flows within the same VLAN to be forwarded by different PEs elected as the Designated Forwarder for a given Ethernet Segment. * As long as the ordered PE list remains unchanged, a given flow is always mapped to the same DF. * Whenever the set of active PEs changes, the value of N changes, and the resulting DF assignment for existing flows may also change. 3.1. Per-Flow DF Election Example Ananthamurthy, et al. Expires 23 January 2027 [Page 5] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 BUM Source | | | | +---------+ +--------------+ PE-4 +--------------+ | | | | | +---------+ | | | | EVPN | | | | | | | +---------+ +---------+ +---------+ | PE1 +--------+ PE2 +---------+ PE3 | | EVI-1 | | EVI-1 | | EVI-1 | +---------+ +---------+ +---------+ |__________________|___________________| AC-1 ESI-1 | AC-2 AC-3 +---------+ | CE-1 | | | +---------+ | | | | BUM Receivers Figure-1 : Multihomed network Figure-1 illustrates Ethernet Segment ESI-1 multihomed to PE1, PE2, and PE3. Assume that all three PEs advertise support for the Per- flow DF Algorithm and derive the following ordered PE list according to [RFC8584]: Ordered PE List = {PE1, PE2, PE3} N = 3 PE1, PE2 and PE3 receive BUM traffic from the EVPN core for the same VLAN (e.g., VLAN 101) and sends the traffic toward the multihomed Ethernet Segment. For each flow, every participating PE computes the same hash value over the selected packet header fields and compute the DF index as specified in this section. The following example uses representative hash values: Ananthamurthy, et al. Expires 23 January 2027 [Page 6] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 Flow VLAN Hash Value (H) DF Index (H mod 3) Elected DF ---- ----- -------------- ------------------ ---------- Flow-A 101 101 2 PE3 Flow-B 101 205 1 PE2 Flow-C 101 300 0 PE1 Flow-D 101 401 2 PE3 Because all participating PEs use the same flow-key fields, hashing algorithm, and ordered PE list, they independently elect the same DF for each flow without requiring per-flow control-plane signaling. Consequently, Flow-A and Flow-D are forwarded toward CE-1 only by PE3, Flow-B only by PE2, and Flow-C only by PE1. This distributes different BUM flows within the same VLAN and Ethernet Segment across the available PEs while ensuring that each individual flow is forwarded by only one DF. If the membership of the redundancy group changes, the participating PEs update the ordered PE list and recompute the DF index using the new value of N. For example, if PE2 is no longer active, the ordered PE list becomes: Ordered PE List = {PE1, PE3} N = 2 As a result, some existing flows may be assigned to a different DF. All remaining participating PEs nevertheless continue to derive identical forwarding decisions because they use the same ordered PE list, flow-key fields, and hashing algorithm. 4. Security Considerations This document introduces no new security considerations beyond those discussed in [RFC7432] and [RFC8584]. 5. IANA Considerations This document solicits: * The allocation of one new value in the "DF Alg" registry created by [RFC8584] as follows: Alg Name Reference ---- ----------------------------- ------------- 6 Per-flow This document 6. Normative References Ananthamurthy, et al. Expires 23 January 2027 [Page 7] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 [I-D.ietf-bess-evpn-per-mcast-flow-df-election] Sajassi, A., Mishra, M. P., Thoria, S., Rabadan, J., and J. Drake, "Per multicast flow Designated Forwarder Election for EVPN", Work in Progress, Internet-Draft, draft-ietf-bess-evpn-per-mcast-flow-df-election-13, 2 March 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A., Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8584] Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake, J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet VPN Designated Forwarder Election Extensibility", RFC 8584, DOI 10.17487/RFC8584, April 2019, . Authors' Addresses Krishnaswamy Ananthamurthy Cisco 170 W. Tasman Drive San Jose, CA 95134 United States of America Email: kriswamy@cisco.com Ali Sajassi Cisco 170 W. Tasman Drive San Jose, CA 95134 United States of America Email: sajassi@cisco.com Ananthamurthy, et al. Expires 23 January 2027 [Page 8] Internet-Draft Per-Flow EVPN Designated Forwarder Elect July 2026 Lukas Krattiger Cisco 170 W. Tasman Drive San Jose, CA 95134 United States of America Email: lkrattig@cisco.com Ananthamurthy, et al. Expires 23 January 2027 [Page 9]