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INTERNET-DRAFT                                         Patrice Brissette
Intended Status: Proposed Standard                           Ali Sajassi
                                                        Luc Andre Burdet
                                                           Cisco Systems

                                                            Daniel Voyer
                                                             Bell Canada

Expires: May 3, 2018                                    October 30, 2017


       EVPN Multi-Homing Mechanism for Layer-2 Gateway Protocols
                draft-brissette-bess-evpn-l2gw-proto-00


Abstract

   Existing EVPN multi-homing schemes are limited to Single-Active and
   All-Active as per RFC-7432. Neither of these multi-homing mechanisms
   are sufficient to support access networks with Layer-2 Gateway
   protocols such as MST-AG, REP-AG, and G.8032. These Layer-2 Gateway
   protocols require a new multi-homing mechanism defined in this draft.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at
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Copyright and License Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the



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   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Solution . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3. Requirements  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4. Handling of Topology Change Notification (TCN)  . . . . . . . .  5
   5. ESI-label Extended Community Extension  . . . . . . . . . . . .  7
   6. EVPN MAC Flush Extcomm  . . . . . . . . . . . . . . . . . . . .  7
   7. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     10.1  Normative References . . . . . . . . . . . . . . . . . . .  9
     10.2  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9






















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1. Introduction

   EVPN existing multi-homing mechanisms of Single-Active and All-Active
   is not sufficient to support access Layer-2 Gateway protocols such as
   MST-AG, REP-AG, and G.8032.

   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 means a given VLAN can be active on all PEs in the redundancy
   group but a single flow of that VLAN can only be active on only one
   of the PEs in the redundancy group. In fact, the carving scheme
   performed by the DF election algorithm for these L2GW protocols is
   not per VLAN but rather for a given VLAN. A given PE in the
   redundancy group can be the only Designated Forwarder for a specific
   L2 flow. The loop-prevention blocking scheme occurs somewhere 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 and
   fast-switchover as detailed in the solution section.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2  Acronyms

   AC      : Attachment Circuit
   DF      : Designated Forwarder
   EVLAG   : EVPN LAG (equivalent to EVPN MC-LAG)
   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



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2.  Solution

                    +---+
                    |CE4|
                    +---+
                      |
                      |
                   +-----+
                   | PE3 |
                   +-----+
               +-------------+
               |             |
               |   MPLS/IP   |
               |    CORE     |
               |             |
               +-------------+
            +-----+       +-----+
            | PE1 |       | PE2 |
            +-----+       +-----+
            AC1|             |AC2
               |             |
             +---+         +---+
             |CE1|         |CE3|
             +---+         +---+
               |             |
               |    +---+    |
               +----|CE2|-/--+
                    +---+

         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: MST-AG, REP-AG or
   G.8032. 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 participant in the access protocols. PE1 and PE2 are peering
   PEs in a redundancy group and EVLAG capable. The L2GW protocol is
   used for loop avoidance. In above example, the loop is broken on the
   right side of CE2. Due to them running in 'Access Gateway' mode, PE1
   and PE2 EVLAG load-balancing runs in a way very-similar to all-
   active. Additionally, the reachability between CE1/CE2 and CE3 that
   is required is achieved with the forwarding path through the EVPN
   MPLS/IP core side.

   Finally, PE3 behaves according to EVPN ruleset for traffic to/from
   PE1/PE2, and which PE is selected per L2 flow is decided by the L2GW
   protocol in the access, and is out of EVPN control. From PE3 point of
   view, some of the L2 flow coming from PE3 may reach CE3 via PE2 and



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   some of the L2 flow may reach CE1/CE2 via PE1. A specific L2 flow
   never goes to both PEs. Therefore, aliasing cannot be performed by
   PE3. That node operates in a single-active fashion for these L2 flow.
   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 flow coming from CE4 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 will flood until unicast traffic
   recovers properly.

3. Requirements

   The EVPN L2GW framework for L2 access protocols consists of the
   following rules:

   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 forwarding state is dictated by the L2GW
   protocol.

   o Split-horizon filtering capability 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 disable. PE MUST NOT advertise corresponding the
   ESI label to other PEs in the redundancy group.

   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 aliasing (at Layer-2 and Layer-3)
      - Disable ESI-label processing
      - Disable backup path programming

   The Ethernet-Segment back end peering procedure such as per ES/EAD
   and per EVI/EAD routes advertisement remains as explained in
   [RFC7432].

4. Handling of Topology Change Notification (TCN)

   In order to address rapid Layer-2 convergence requirements, topology



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   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 topology change notification is done differently
   based on the access protocol. In the case of REP-AG and G.8032, TCN
   gets generated in both directions and thus both of the dual-homing
   PEs receive it. However, with MST-AG, TCN gets generated only in one
   direction and thus only a single PE can receive it. When only one PE
   receives the TCN, it needs to be 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 interface.

   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 is dedicated for that
   purpose. The latter choice relies on the newly defined MAC flush
   extended community in the Ethernet Auto-discovery per EVI route. 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 corresponding
   interface.

   o Advertises per EVI/EAD route along with a new MAC-flush BGP
   Extended Community, with incremented sequence number, in order to
   perform a remote MAC flush and steer L2 traffic to proper peering PE.

   o Ensure MAC/IP route re-advertisement with sequence number is bump
   up when host reachability is NOT moving to peering PE. This is to
   ensure a re-advertisement of current MAC which may have been flushed
   remotely upon MAC Flush extcomm reception. In theory, it should
   happen automatically since PE, receiving TCN from the access,
   performs local MAC flush on corresponding interface and will re-learn
   that local MAC.

   o When MST-AG runs in the access, 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 extcomm is extended. The 1 octet flag field,
   as part of the ESI-label Extcomm, is updated as follow:

   Each ESI Label extended community is encoded as an 8-octet value, as
      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=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 = reserved
   [7:3]- Reserved

6. EVPN MAC Flush Extcomm

   A new BGP Extended community, similar to MAC mobility BGP-extcomm, is
   required by the TCN procedure. It may get advertised along with
   Ethernet Auto-discovery routes (per EVI/EAD) 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 readvertised 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.

       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 = ??    | Sub-Type = ?? |        Reserved = 0          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Sequence Number                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7. Conclusion




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   EVPN Multi-Homing Mechanism for Layer-2 GW 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 [draft-ietf-
   bess-evpn-inter-subnet-forwarding]. EVPN-IRB works with the current
   proposal and does not require any extension.













































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8.  Security Considerations

   The same Security Considerations described in [RFC7432] are valid for
   this document.

9.  IANA Considerations

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

10.  References

10.1  Normative References

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <http://www.rfc-editor.org/info/rfc4684>.

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

   [RFC7209]  Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
              Henderickx, W., and A. Isaac, "Requirements for Ethernet
              VPN (EVPN)", RFC 7209, DOI 10.17487/RFC7209, May 2014,
              <http://www.rfc-editor.org/info/rfc7209>.


10.2  Informative References

   [RFC6246]  Sajassi, A., Ed., Brockners, F., Mohan, D., Ed., and Y.
              Serbest, "Virtual Private LAN Service (VPLS)
              Interoperability with Customer Edge (CE) Bridges",
              RFC 6246, DOI 10.17487/RFC6246, June 2011,
              <http://www.rfc-editor.org/info/rfc6246>.

   [RFC4761]  Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <http://www.rfc-editor.org/info/rfc4761>.



Authors' Addresses



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   Patrice Brissette
   Cisco Systems
   EMail: pbrisset@cisco.com

   Ali Sajassi
   Cisco Systems
   EMail: sajassi@cisco.com

   Luc Andre Burdet
   Cisco Systems
   EMail: lburdet@cisco.com

   Daniel Voyer
   Bell Canada
   EMail: daniel.voyer@bell.ca




































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