BESS Working Group                                          P. Brissette
Internet-Draft                                                A. Sajassi
Intended status: Standards Track                         LA. Burdet, Ed.
Expires: October 29, 2021                                  Cisco Systems
                                                                D. Voyer
                                                             Bell Canada
                                                          April 27, 2021

       EVPN Multi-Homing Mechanism for Layer-2 Gateway Protocols


   The existing EVPN multi-homing load-balancing modes defined are
   Single-Active and All-Active.  Neither of these multi-homing
   mechanisms adequately represent ethernet-segments facing access
   networks with Layer-2 Gateway protocols such as G.8032, (M)STP, REP,
   MPLS-TP, etc.  These loop-preventing Layer-2 protocols require a new
   multi-homing mechanism defined in this draft.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 29, 2021.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Single-Flow-Active redundancy mode  . . . . . . . . . . .   6
     3.2.  Fast-Convergence  . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Handling of Topology Change Notification (TCN)  . . .   7
       3.2.2.  Propagating L2GW Protocol Events  . . . . . . . . . .   7
       3.2.3.  MAC Flush and Invalidation Procedure  . . . . . . . .   8
     3.3.  Backwards compatibility . . . . . . . . . . . . . . . . .   8
       3.3.1.  The two-ESI solution  . . . . . . . . . . . . . . . .   8
       3.3.2.  RFC7432 Remote PE . . . . . . . . . . . . . . . . . .   9
   4.  ESI-label Extended Community Extension  . . . . . . . . . . .  10
   5.  EVPN Inter-subnet Forwarding  . . . . . . . . . . . . . . . .  10
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Existing EVPN Single-Active and All-Active multi-homing mechanisms do
   not address the additional requirements of loop-preventing Layer-2
   gateway protocols such as G.8032, (M)STP, REP, MPLS-TP, etc.

   These Layer-2 Gateway protocols require that a given L2 flow of a
   VLAN be only active on one of the PEs in the multi-homing group,
   while another L2 flow may be active on the other PE.  This is in
   contrast with Single-Active redundancy mode where all flows of a VLAN
   are active on a single multi-homing PEs and it is also in contrast
   with All-Active redundancy mode where all flows of a VLAN are active
   on all PEs in the redundancy group.

   This draft defines a new multi-homing mechanism "Single-Flow-Active"
   specifying that a VLAN can be active on all PEs in the redundancy
   group but each unique L2 flow of that VLAN can be active on only one

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   of the PEs in the redundancy group at a time.  In fact, the
   Designated Forwarder election algorithm for these L2 Gateway
   protocols, is not per VLAN but rather for a given L2 flow.  A
   selected PE in the redundancy group must be the only Designated
   Forwarder for a specific L2 flow, but the decision is not taken by
   the PE.  The loop-prevention blocking scheme occurs in the access
   network, by the Layer-2 protocol.

   EVPN multi-homing procedures need to be enhanced to support
   Designated Forwarder election for all traffic (both known unicast and
   BUM) on a per L2 flow basis.  The Single-Flow-Active multi-homing
   mechanism also requires new EVPN considerations for aliasing, mass-
   withdraw, fast-switchover and
   [I-D.ietf-bess-evpn-inter-subnet-forwarding] as described in the
   solution section.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2.  Terms and Abbreviations

   AC:       Attachment Circuit

   BUM:      Broadcast, Unknown unicast, Multicast

   DF:       Designated Forwarder

   GW:       Gateway

   L2 Flow:  A given flow of a VLAN, represented by (MAC-SA, MAC-DA)

   L2GW:     Layer-2 Gateway

   MAC-IP:   EVPN Route-Type 2 with non-zero IP field

   G.8032:   Ethernet Ring Protection

   (M)STP:   Multi-Spanning Tree Protocol

   REP:      Resilient Ethernet Protocol

   TCN:      Topology Change Notification

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2.  Requirements

   The EVPN L2GW framework for L2GW protocols in Access-Gateway mode,
   consists of the following rules:

   o  Peering PEs MUST share the same ESI.

   o  The Ethernet-Segment DF election MUST NOT be performed and
      forwarding state MUST be dictated by the L2GW protocol.  In
      gateway mode, both PEs are usually in forwarding state.  In fact,
      the access protocol is responsible for operationally setting the
      forwarding state for each VLAN.

   o  Split-horizon filtering is NOT needed because L2GW protocol
      ensures there will never be a loop in the access network.  The
      forwarding between peering PEs MUST also be preserved.  In
      Figure 1, CE1/CE4 device may need reachability with CE2 device.
      ESI-filtering capability MUST be disabled.  The ESI label extended
      comunity advertised to other peering PEs in the redundancy group
      MUST NOT be applied it if received.

   o  ESI label BGP Extended Community MUST support a new multi-homing
      mode named "Single-Flow-Active" corresponding largely to the
      single-active behaviour of [RFC7432], applied per L2 flow rather
      than per VLAN.

   o  Upon receiving ESI label BGP Extended Community with the single-
      flow-active load-balancing mode, remote PE MUST:

      *  Disable ESI label processing

      *  Disable aliasing (at Layer-2 and Layer-3

   o  The Ethernet-Segment procedures in the EVPN core such as Ethernet
      A-D per ES and per Ethernet A-D per EVI routes advertisement/
      withdraw, as well as MAC and MAC+IP advertisement, remains as
      explained in [RFC7432] and

   o  For fast-convergence, remote PE3 MAY set up two distinct backup
      paths on a per-flow basis:

      *  { PE1 active, PE2 backup }

      *  { PE2 active, PE1 backup }

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      The backup paths so created, operate as in [RFC7432] section 8.4
      where the backup PE of the redundancy group MAY immediately be
      selected for forwarding upon detection of a specific subset of
      failures: Ethernet A-D per ES route withdraw, Active PE loss of
      reachability (via IGP detection).  An Ethernet A-D per EVI
      withdraw MUST NOT result in automatic switching to the backup PE
      as only a subset of the hosts may be changing reachability to the
      Backup PE, and the remote cannot determine which.

   o  MAC mobility procedures SHALL have precedence over backup path
      procedure in Single-Flow-Active for tracking host reachability.

3.  Solution

                +-----| PE3 |-----+
                |     +-----+     |
                |                 |
                |     MPLS/IP     |
                |      CORE       |
                |                 |
             +-----+           +-----+
             | PE1 |-----------| PE2 |
             +-----+           +-----+
             AC1|                 |AC2
                |                 |
              +---+             +---+
              |CE1|             |CE2|
              +---+             +---+
                |                 |
                |    +---+        |
                +----|CE4|---/ /--+

            Figure 1: EVPN network with L2 access GW protocols

   Figure 1 shows a typical EVPN network with an access network running
   a L2GW protocol, typically one of the following: G.8032, (M)STP, REP,
   MPLS- TP, etc.  The L2GW protocol usually starts from AC1 (on PE1) up
   to AC2 (on PE2) in an open "ring" manner.  AC1 and AC2 interfaces of
   PE1 and PE2 are participants in the access protocol.

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   The L2GW protocol is used for loop avoidance.  In above example, the
   loop is broken on the right side of CE4.

3.1.  Single-Flow-Active redundancy mode

   PE1 and PE2 are peering PEs in a redundancy group, and sharing a same
   ESI.  In the proposed Single-Flow-Active mode, load-balancing at PE1
   and PE2 shares similarities with singular aspects of both Single-
   Active and All-Active.  Designated Forwarder election must not
   compete with the L2GW protocol and must not result in blocked ports
   or portions of the access may become isolated.  Additionally, the
   reachability between CE1/CE4 and CE2 is achieved with the forwarding
   path through the EVPN MPLS/IP core side.  Thus, the ESI-Label
   filtering of [RFC7432] is disabled for Single-Flow-Active Ethernet

   Finally, PE3 behaves according to EVPN [RFC7432] rules for traffic
   to/from PE1/PE2.  Peering PE, selected per L2 flow, is chosen by the
   L2GW protocol in the access, and is out of EVPN control.

   From PE3 point of view, the L2 flows from PE3 destined to CE1/CE4
   transit via edge node PE1 and the L2 flows destined to CE2 transit
   via edge node PE2.  A specific unicast L2 flow never goes to both
   peering PEs.  Therefore, aliasing of [RFC7432] Section 8.4 cannot be
   performed by PE3.  That node operates in a single-active fashion for
   each of the unicast L2 flows.

   The backup path of [RFC7432] Section 8.4 which is also setup for
   single-active rapid convergence on a per-VLAN basis, is not
   applicable here.  For example, in Figure 1, if a failure happens
   between CE1 and CE4 the loop-prevention at the right of CE4 is
   released and:

   o  L2 flows coming from CE3 behind PE3 destined to CE1 still transit
      through edge device PE1, and shall not switch to PE2 as a backup

   o  L2 flows destined to CE4 on the other hand, may be backup switched
      to PE2 transit node.

   On PE3, there is no way to know which L2 flow specifically is
   affected.  During the transition time, PE3 may flood until unicast
   traffic recovers properly.

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3.2.  Fast-Convergence

3.2.1.  Handling of Topology Change Notification (TCN)

   In order to address rapid Layer-2 convergence requirement, topology
   change notification received from the L2GW protocols must be sent
   across the EVPN network to perform the equivalent of legacy L2VPN
   remote MAC flush.

   The generation of TCN is done differently based on the access
   protocol.  In the case of REP and G.8032, TCN gets generated in both
   directions and thus both of the dual-homing PEs receive it.  However,
   with (M)STP, TCN gets generated only in one direction and thus only a
   single PE can receive it.  That TCN is propagated to the other
   peering PE for local MAC flushing, and relaying back into the access.

   In fact, PEs have no direct visibility on failures happening in the
   access network nor on the impact of those failures over the
   connectivity between CE devices.  Hence, both peering PEs require to
   perform a local MAC flush on corresponding interfaces.

   There are two options to relay the access protocol's TCN to the
   peering PE: in-band or out-of-band messaging.  The first method is
   better for rapid convergence, and requires a dedicated channel
   between peering PEs.  An EVPN-VPWS connection MAY be dedicated for
   that purpose, connecting the Untagged ACs of both PEs.  The latter
   choice relies on the MAC Mobility BGP Extended Community applied to
   the Ethernet A-D per EVI route, detailed below.  It is a slower
   method but has the advantage of avoiding a dedicated channel between
   peering PEs.

3.2.2.  Propagating L2GW Protocol Events

   Peering PE in Single Flow Active mode, upon receiving notification of
   a protocol convergence-event from access (such as TCN), MUST:

   o  As per legacy VPLS, perform a local MAC flush on the access-facing
      interfaces.  An ARP probe is also sent for all hosts previously

   o  Advertise Ethernet A-D per EVI route along with the MAC Mobility
      BGP Extended Community, with incremented sequence number if
      previously advertised, in order to perform a remote MAC flush and
      steer L2 traffic to proper peering PE.  The sequence number is
      incremented by one as a flushing indication to remote PEs.

   o  Ensure MAC and MAC+IP route re-advertisement, with incremented
      sequence number when host reachability is NOT moving to peering

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      PE.  This is to ensure a re-advertisement of current MAC and MAC-
      IP which may have been flushed remotely upon MAC Mobility Extended
      Community reception.  In theory, it should happen automatically
      since peering PE, receiving TCN from the access, performs local
      MAC flush on corresponding interface and will re-learn that local
      MAC or MAC+IP.

   o  Where an access protocol relies on TCN BPDU propagation to all
      participant nodes, a dedicated EVPN-VPWS connection MAY be used as
      an in-band channel to relay TCN between peering PEs.  That
      connection may be auto-generated or can simply be configured by

3.2.3.  MAC Flush and Invalidation Procedure

   The MAC-Flush procedure described in [RFC7623] is borrowed, and the
   MAC mobility BGP Extended community is signaled along with the
   Ethernet A-D per EVI route from a PE in Single-Flow-Active mode.

   When MAC Mobility BGP Extended Community is received on the Ethernet
   A-D per EVI route, it indicates to all remote PEs that all MAC
   addresses associated with that EVI/ESI are "flushed" i.e. must be

   Remote PEs, having previously received Ethernet A-D per ES with
   Single Flow Active indication from an originating PE, treat the MAC
   Mobility indication to simply invalidate the MAC entries for that
   originating PE on an EVI/ESI basis, similar to [RFC7432]'s mass-
   withdraw mechanism.

   They remain unresolved until the remote PE receives a route update
   (or withdraw) for those MAC addresses.  Note: the MAC may be re-
   advertised by the same PE, but also some are expected to have moved
   to a multi-homing peer, within the same ESI, due to the L2 protocol's

   The sequence number of the MAC Mobility extended community is of
   local significance from the originating PE, and is not used for
   comparison between peering PEs.  Rather, it is used to signal via BGP
   successive MAC Flush requests from a given PE per EVI/ESI.

3.3.  Backwards compatibility

3.3.1.  The two-ESI solution

   As a reference, an alternative solution which achieves some, but not
   all, of the requirements exists:

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   On the PE1 and PE2,

   a.  A single-homed (different) non-zero ESI, or zero-ESI, is used for
       each PE;

   b.  With no remote Ethernet-Segment routes received matching local
       ESI, each PE will be designated forwarder for all the local

   c.  Each L2GW PE will send Ethernet A-D per ES and per EVI routes for
       its ESI if non-zero; and

   d.  When the L2GW PEs receive a MAC-Flush notification (STP TCN,
       G.8032 mac-flush, LDP MAC withdrawal etc.), they send an update
       of the Ethernet A-D per EVI route with the MAC Mobility extended
       community and a higher sequence number, using the procedure
       outlined in Section 3.2.3.

   While this solution is feasible, it is considered to fall short of
   the requirements listed in Section 2, namely for all aspects meant to
   achieve fast-convergence.

3.3.2.  RFC7432 Remote PE

   A PE which receives an Ethernet A-D per ES route with the Single-
   Flow-Active bit set in the ESI-flags, and which does not support/
   understand this bit, SHALL discard the bit and continue operating per
   [RFC7432] (All-Active).  The operator should understand the usage of
   single-flow-active load-balancing mode else it is highly recommended
   to use the two-ESI approach as described in Section 3.3.1

   The remote PE3 which does not support Single-Flow-Active redundancy
   mode as described, will ECMP traffic to peering PE1 and PE2 in the
   example topology above (Figure 1), per [RFC7432], Section 8.4
   aliasing and load-balancing rules.  PE1 and PE2, which support the
   Single-Flow-Active redundancy mode MUST setup redirections towards
   the PE at which the flow is currently active (sub-optimal Layer-2
   forwarding and sub-optimal Layer-3 routing).

   Thus, while PE3 will ECMP (on average) 50% of the traffic to the
   incorrect PE using [RFC7432] operation, PE1 and PE2 will handle this
   gracefully in Single-Flow-Active mode and redirect across peering
   pair of PEs appropriately.

   No extra route or information is required for this.  The [RFC7432]
   and [I-D.ietf-bess-evpn-inter-subnet-forwarding] route advertisements
   are sufficient.

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4.  ESI-label Extended Community Extension

   In order to support the new EVPN load-balancing mode (single-flow-
   active), the ESI label Extended Community is updated.

   The 1 octet flag field, part of the ESI Label Extended Community, is
   modified as follows:

                            1                   2                   3
        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=0x01 | Flags(1 octet)|  Reserved=0   |
       |  Reserved=0   |          ESI Label                            |

   Low-order bit: [7:0]
   [2:0]- 000 = all-active,
          001 = single-active,
          010 = single-flow-active,
          others = unassigned
   [7:3]- Reserved

                Figure 2: ESI Label BGP Extended Community

5.  EVPN Inter-subnet Forwarding

   EVPN Inter-subnet forwarding procedures in
   [I-D.ietf-bess-evpn-inter-subnet-forwarding] works with the current
   proposal and does not require any extension.  Host routes continue to
   be installed at PE3 with a single remote nexthop, no aliasing.

   However, leveraging the same-ESI on both L2GW PEs enables ARP/ND
   synchronization procedures which are defined for All-Active
   redundancy in [I-D.ietf-bess-evpn-inter-subnet-forwarding].  In
   steady-state, on PE2 where a host is not locally-reachable the
   routing table will reflect PE1 as the destination.  However, with
   ARP/ND synchronization based on a common ESI, the ARP/ND cache may be
   pre-populated with the local AC as destination for the host, should
   an AC failure occur on PE1.  This achieves fast-convergence.

   When a host moves to PE2 from the PE1 L2GW peer, the MAC mobility
   sequence number is incremented to signal to remote peers that a
   'move' has occurred and the routing tables must be updated to PE2.
   This is required when an Access Protocol is running where the loop is
   broken between two CEs in the access and the L2GWs, and the host is
   no longer reachable from the PE1-side but now from the PE2-side of
   the access network.

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6.  Conclusion

   EVPN Multi-Homing Mechanism for Layer-2 Gateway Protocols solves a
   true problem due to the wide legacy deployment of these access L2GW
   protocols in Service Provider networks.  The current draft has the
   main advantage to be fully compliant with [RFC7432] and

7.  Security Considerations

   The same Security Considerations described in [RFC7432] and
   [I-D.ietf-bess-evpn-inter-subnet-forwarding] remain valid for this

8.  Acknowledgements

   Authors would like to thank Thierry Couture for valuable review and
   inputs with respect to access protocol deployments related to
   procedures proposed in this document.

9.  IANA Considerations

   There are no IANA considerations.

10.  References

10.1.  Normative References

              Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", draft-
              ietf-bess-evpn-inter-subnet-forwarding-11 (work in
              progress), October 2020.

   [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, <>.

   [RFC7623]  Sajassi, A., Ed., Salam, S., Bitar, N., Isaac, A., and W.
              Henderickx, "Provider Backbone Bridging Combined with
              Ethernet VPN (PBB-EVPN)", RFC 7623, DOI 10.17487/RFC7623,
              September 2015, <>.

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10.2.  Informative References

   [RFC6378]  Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
              N., and A. Fulignoli, Ed., "MPLS Transport Profile (MPLS-
              TP) Linear Protection", RFC 6378, DOI 10.17487/RFC6378,
              October 2011, <>.

Authors' Addresses

   Patrice Brissette
   Cisco Systems
   Ottawa, ON


   Ali Sajassi
   Cisco Systems


   Luc Andre Burdet (editor)
   Cisco Systems
   Ottawa, ON


   Daniel Voyer
   Bell Canada
   Montreal, QC


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