Per-flow based EVPN DF Election

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Table of Contents

1. Introduction

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 multi-homing. While the Default Algorithm provides an efficient and automated way of selecting the DF across different Ethernet Tags in the Ethernet Segment, but lacks in providing per flow based DF within the given EVPN Instances (EVIs).¶

This document proposes a Per-flow DF Election algorithm to provide optimal load sharing. This document updates RFC8584 by modifying the definition of the DF Election Extended Community with new DF Algorithm type.¶

3. 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                                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DF Alg will carry the DF alg type.¶

New DF Alg type called per-flow df with value 6 (to be allocated by IANA) will be defined in the draft and same will be encoded in DF algorithm EC.¶

The procedures defined in [RFC8584] to handle Designated Forwarder Election Extended Community are applicable.¶

When a PE receives the ES routes from all the other PEs for the ES in question, it checks to see if all the advertisements have the Extended Community with the same DF Alg, then it will operate in per-flow DF algorithm.¶

4. Operation

When PE supports the Per-flow DF algorithm, the ES route is sent with the DF Election Extended Community, with DF-Alg set to Per-flow.¶

When PE supports the Per-flow DF algorithm, it should use the DF election as per the flow when it receives the ES route with the DF Election Extended Community and the DF-Alg set to Per-flow.¶

PE, which does not support Per-flow DF, will revert to the default modulo-based DF.¶

When a Provider Edge (PE) router operates in Per-Flow Designated Forwarder (DF) mode, it computes a hash based on the five-tuple of the packet header. The hashing algorithm can use a Cyclic Redundancy Check (CRC). The number of PEs involved in redundancy multihoming will be used in the calculation as follows: I, which is HASH mod N. This approach allows multiple flows within a given VLAN to be forwarded by different PEs acting as DF for a specific Ethernet Segment (ES).¶

6. IANA Considerations

This document solicits:¶

• The allocation of two new values in the "DF Alg" registry created by [RFC8584] as follows:¶

  Alg         Name                               Reference
  ----        -----------------------------      -------------
  6           Per-flow Algorithm       This document
  

  ¶

Authors' Addresses