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BESS Working Group                                     P. Brissette, Ed.
Internet-Draft                                                A. Sajassi
Intended status: Standards Track                               L. Burdet
Expires: May 7, 2020                                       Cisco Systems
                                                                D. Voyer
                                                             Bell Canada
                                                        November 4, 2019

       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 are appropriate to support access networks with Layer-2
   Gateway protocols such as G.8032, MPLS-TP, STP, etc.  These Layer-2
   Gateway protocols require a new multi-homing mechanism defined in
   this draft.

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
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   This Internet-Draft will expire on May 7, 2020.

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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.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Single-Flow-Active redundancy mode  . . . . . . . . . . .   4
     2.2.  Backwards compatibility . . . . . . . . . . . . . . . . .   5
       2.2.1.  The two-ESI solution  . . . . . . . . . . . . . . . .   5
       2.2.2.  RFC7432 Remote PE . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Handling of Topology Change Notification (TCN)  . . . . . . .   7
   5.  ESI-label Extended Community Extension  . . . . . . . . . . .   9
   6.  EVPN MAC-Flush Extended Community . . . . . . . . . . . . . .   9
   7.  EVPN Inter-subnet Forwarding  . . . . . . . . . . . . . . . .  10
   8.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Existing EVPN multi-homing mechanisms of Single-Active and All-Active
   are not sufficient to support access Layer-2 Gateway protocols such
   as G.8032, MPLS-TP, STP, etc.

   These Layer-2 Gateway protocols require that a given flow of a VLAN
   (represented by {MAC-SA, MAC-DA}) to be only active on one of the PEs
   in the multi-homing group.  This is in contrast with Single-Active
   redundancy mode where all flows of a VLAN are active on one of the
   multi-homing PEs and it is also in contrast with All-Active
   redundancy mode where all L2 flows of a VLAN are active on all PEs in
   the redundancy group.

   This draft defines a new multi-homing mechanism "Single-Flow-Active"
   which defines that a VLAN can be active on all PEs in the redundancy
   group but a single given flow of that VLAN can be active on only one
   of the PEs in the redundancy group.  In fact, the carving scheme,
   performed by the DF(Designated Forwarder) election algorithm for

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   these L2 Gateway protocols, is not per VLAN but rather for a given
   VLAN.  A selected PE in the redundancy group can 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.

   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.  This new multi-homing mechanism also
   requires new EVPN considerations for aliasing, mass-withdraw, fast-
   switchover and [EVPN-IRB] 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

   G.8032:   Ethernet Ring Protection

   MST-AG:   Multi-Spanning Tree Access Gateway

   REP-AG:   Resilient Ethernet Protocol Access Gateway

   TCN:      Topology Change Notification

2.  Solution

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                      | PE3 |
                |                 |
                |     MPLS/IP     |
                |      CORE       |
                |                 |
             +-----+           +-----+
             | PE1 |           | PE2 |
             +-----+           +-----+
             AC1|                 |AC2
                |                 |
              +---+             +---+
              |CE1|             |CE3|
              +---+             +---+
                |                 |
                |    +---+       |

            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, STP, 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.

   The L2GW protocol is used for loop avoidance.  In above example, the
   loop is broken on the right side of CE2.

2.1.  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, PE1 and PE2 'Access
   Gateway' load-balancing mode shares similarities with both Single-
   Active and All-Active.  DF election must not result in blocked ports
   or portions of the access may become isolated.  Additionally, the
   reachability between CE1/CE2 and CE3 is achieved with the forwarding
   path through the EVPN MPLS/IP core side.  Thus, the ESI-Label

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   filtering of [RFC7432] is disabled for Single-Flow-Active Ethernet

   Finally, PE3 behaves according to EVPN 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, some of the L2 flows coming from PE3 may
   reach CE3 via PE2 and some of the L2 flows may reach CE1/CE2 via PE1.
   A specific L2 flow never goes to both peering PEs.  Therefore,
   aliasing cannot be performed by PE3.  That node operates in a single-
   active fashion for each of these L2 flows.

   The backup path which is also setup for rapid convergence, is not
   applicable here.  For example, in Figure 1, if a failure happens
   between CE1 and CE2, L2 flows coming from CE4 behind PE3 destined to
   CE1 still goes through PE1 and shall not switch to PE2 as a backup
   path.  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.

2.2.  Backwards compatibility

2.2.1.  The two-ESI solution

   As background, an alternative solution which achieves some, but not
   all, of the requirements exists and is backwards compatible with

   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 AD 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 AD per-EVI route with the MAC Mobility extended
       community defined in Section 6 and a higher sequence number.

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   While this solution is feasible, it is considered to fall short of
   the requirements listed in Section 3, namely for all aspects meant to
   achieve fast-convergence.

2.2.2.  RFC7432 Remote PE

   A PE which receives an Ethernet AD 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 2.2.1.

   The remote PE3 which does not support Single-Flow-Active redundancy
   mode as described, will ECMP traffic to peering PEs 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 sub-optimal Layer-2
   forwarding and sub-optimal Layer-3 routing towards the PE at which
   the flow is currently active.

   Thus, while PE3 is ECMP (on average) 50% of the traffic to the
   incorrect PE in [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 [EVPN-IRB] route advertisements are sufficient.

3.  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 Access
      Gateway mode, both PEs are usually in forwarding state.  In fact,
      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 loop in the access network.  The
      forwarding between peering PEs MUST also be preserved.  In figure
      1, CE1/CE2 device may need reachability with CE3 device.  ESI-
      filtering capability MUST be disabled.  PE MUST NOT advertise

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      corresponding ESI-label to other PEs in the redundancy group, or
      apply it if it is received.

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

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

      *  Disable ESI-Label processing

      *  Disable aliasing (at Layer-2 and Layer-3 [EVPN-IRB])

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

   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 }

      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 AD per-ES route withdraw, Active PE loss of
      reachability (via IGP detection).  An Ethernet AD 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 in Single-Flow-
      Active for tracking host reachability over backup path procedure.

4.  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 STP (REP-AG) and G.8032, TCN gets generated
   in both directions and thus both of the dual-homing PEs receive it.

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   However, with STP (MST-AG), 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 neither 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 a new MAC flush extended community in the Ethernet
   Auto-discovery per EVI route, defined below.  It is a slower method
   but has the advantage of avoid the usage of a dedicated channel
   between peering PEs.

   Peering PE, upon receiving TCN from access, 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 per EVI/EAD route along with a new MAC-flush BGP
      Extended Community 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
      PE.  This is to ensure a re-advertisement of current MAC and MAC/
      IP which may have been flushed remotely upon MAC Flush extcomm
      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 at ARP probe reply.

   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 directly
      configured by user.

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5.  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 extended community

6.  EVPN MAC-Flush Extended Community

   The MAC mobility BGP Extended community, is required for the TCN
   procedures and MAC-Flushing.  The well-known MAC-Flush procedure from
   [RFC7623] is borrowed, only for Ethernet AD per-EVI routes.

   In this Single-Flow-Active mode, the MAC-Flush Extended Community is
   advertised along with Ethernet AD per EVI routes upon reception of
   TCN from the access.  When this extended community is used, it
   indicates, to all remote PEs that all MAC addresses associated with
   that EVI/ESI are "flushed" i.e. unresolved.  They remain unresolved
   until remote PE receives a route update / withdraw for those MAC
   addresses; the MAC may be re-advertised by the same PE, or by
   another, in the same ESI.

   The sequence number used 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.

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                            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=0x?? |        Reserved = 0           |
       |                 Sequence Number                               |

                  Figure 3: MAC-Flush Extended Community

7.  EVPN Inter-subnet Forwarding

   EVPN Inter-subnet forwarding procedures in [EVPN-IRB] 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

   However, leveraging the same-ESI on both L2GW PEs enables ARP/ND
   synchronization procedures which are defined for All-Active
   redundancy in [EVPN-IRB].  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 hosts 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.

8.  Conclusion

   EVPN style="symbols"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 [EVPN-IRB].

9.  Security Considerations

   The same Security Considerations described in [RFC7432] and
   [EVPN-IRB] remain valid for this document.

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10.  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.

11.  IANA Considerations

   A new allocation of Extended Community Sub-Type for EVPN is required
   to support the new EVPN MAC flush mechanism..

12.  References

12.1.  Normative References

              Sajassi, A., "Integrated Routing and Bridging in EVPN",

   [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, <https://www.rfc-editor.org/info/rfc7432>.

   [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, <https://www.rfc-editor.org/info/rfc7623>.

12.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

Authors' Addresses

   Patrice Brissette (editor)
   Cisco Systems
   Ottawa, ON

   Email: pbrisset@cisco.com

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   Ali Sajassi
   Cisco Systems

   Email: sajassi@cisco.com

   Luc Andre Burdet
   Cisco Systems
   Ottawa, ON

   Email: lburdet@cisco.com

   Daniel Voyer
   Bell Canada
   Montreal, QC

   Email: daniel.voyer@bell.ca

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