Per-Flow EVPN Designated Forwarder Election

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Table of Contents

1. Introduction

The EVPN solution offers procedures for electing a Designated Forwarder (DF) for multihomed Ethernet Segments. In this context, the PE router 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).¶

This document defines a Per-flow DF Election Algorithm to provide fine-grained load sharing. This document updates [RFC8584] by defining a new DF Algorithm value.¶

2. Designated Forwarder Election Extended Community Extensions

This solution reuses and extends 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. The format of the DF Election Extended Community that is used in this document follows:¶

 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                                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The DF Alg 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 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.¶

3. Operation

A PE that supports the Per-flow DF Algorithm MUST advertise the ES route with the DF Election Extended Community, with the DF Algorithm field set to Per-flow.¶

A PE that supports the Per-flow DF Algorithm SHOULD perform DF election on a Per-flow basis when it receives an ES route containing the DF Election Extended Community with the DF Algorithm field set to Per-flow.¶

A PE that does not recognize the advertised DF Algorithm SHALL process the DF Election Extended Community according to [RFC8584].¶

The ordered PE list defined in [RFC8584] SHALL be used. When operating in Per-flow Designated Forwarder (DF) mode, the hash computation SHALL use 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. 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 (CRC) 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 PE routers participating in the ES, all such PE routers 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 PE routers in the ordered PE list defined by [RFC8584]. The number of participating PE routers MUST be 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 defined by [RFC8584] 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 PE routers 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 PE routers changes, the value of N changes, and the resulting DF assignment for existing flows may also change.¶

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
  

  ¶

Authors' Addresses