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RFC 7734 - Support for Shortest Path Bridging MAC Mode over Ethe

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Internet Engineering Task Force (IETF)                     D. Allan, Ed.
Request for Comments: 7734                                   J. Tantsura
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                                 D. Fedyk
                                                              A. Sajassi
                                                            January 2016

  Support for Shortest Path Bridging MAC Mode over Ethernet VPN (EVPN)


   This document describes how Ethernet Shortest Path Bridging MAC mode
   (SPBM) can be combined with Ethernet VPN (EVPN) to interwork with
   Provider Backbone Bridging Provider Edges (PBB PEs) as described in
   the PBB-EVPN solution (RFC 7623).  This is achieved via operational
   isolation of each Ethernet network attached to an EVPN core while
   supporting full interworking between the different variations of
   Ethernet networks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   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. Requirements Language ......................................3
   2. Conventions Used in This Document ...............................3
      2.1. Terminology ................................................3
   3. Solution Overview ...............................................4
   4. Elements of Procedure ...........................................5
      4.1. PE Configuration ...........................................5
      4.2. DF Election ................................................6
      4.3. Control-Plane Interworking ISIS-SPB to EVPN ................6
      4.4. Control-Plane Interworking EVPN to ISIS-SPB ................7
      4.5. Data-Plane Interworking SPBM Island or PBB PE to EVPN ......8
      4.6. Data-Plane Interworking EVPN to SPBM Island ................8
      4.7. Data-Plane Interworking EVPN to PBB PE .....................8
      4.8. Multicast Support ..........................................8
   5. Other Aspects ...................................................8
      5.1. Transit ....................................................8
   6. Security Considerations .........................................9
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................10
   Acknowledgments ...................................................11
   Authors' Addresses ................................................11

1.  Introduction

   This document describes how Ethernet Shortest Path Bridging MAC mode
   (SPBM) [IEEE.802.1Q] along with Provider Backbone Bridging Provider
   Edges (PBB PEs) and Provider Backbone Bridged Networks (PBBNs) can be
   supported by Ethernet VPNs (EVPNs) such that each SPBM island is
   operationally isolated while providing full L2 connectivity between
   the different types of PBBNs where desired.  Each SPBM island uses
   its own control-plane instance and multipathing design, be it
   multiple equal-cost tree sets or multiple spanning trees.

   The intention is to permit past, current, and emerging future
   versions of Ethernet to be seamlessly interconnected to permit large-
   scale, geographically diverse numbers of Ethernet end systems to be
   fully supported with EVPN as the unifying system.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Conventions Used in This Document

2.1.  Terminology

   Terms used in this document are used as specified in IEEE
   802.1Q-2014, which incorporates earlier IEEE 802.1 projects.

   BEB: Backbone Edge Bridge
   BGP: Border Gateway Protocol
   B-MAC: Backbone MAC
   B-VID: Backbone VLAN ID
   CE: Customer Edge
   DA: Destination Address
   DF: Designated Forwarder
   ESI: Ethernet Segment Identifier
   EVPN: Ethernet VPN
   IB-BEB: A BEB that has both an I-component (customer-layer VLAN-aware
           bridge) and a B-component (backbone-layer VLAN-aware bridge)
   ISIS-SPB: IS-IS as extended for SPB
   I-SID: Backbone Service Instance Identifier
   NLRI: Network Layer Reachability Information
   PBB: Provider Backbone Bridging as in Clauses 25 and 26 of
   PBBN: Provider Backbone Bridged Network
   PBB PE: Co-located BEB and EVPN PE
   PE: Provider Edge

   SPB: Shortest Path Bridging
   SPBM: Shortest Path Bridging MAC mode as in Clauses 27 and 28 of
   SPBM-PE: Co-located SPBM<->EVPN interworking function and EVPN PE

3.  Solution Overview

   The EVPN solution for SPBM, as specified in [IEEE.802.1Q],
   incorporates control-plane interworking in the PE to map ISIS-SPB
   [RFC6329] information elements into the EVPN Next Layer Reachability
   Information (NLRI) and vice versa.  This requires each PE to act both
   as an EVPN BGP speaker and as an ISIS-SPB edge node.  Associated with
   this are procedures for configuring the forwarding operations of the
   PE such that an arbitrary number of EVPN-attached SPBM islands can be
   interconnected without any topological or multipathing dependencies.
   This model also permits PBB PEs as defined in [RFC7623] to seamlessly
   communicate with the SPBM islands.

                            +--------------+ +----+   +---+
                            |              | |PBB |---|CE2|
                            |              | |PE3 |   +---+
         +-----+     +----+ |              | +----+
         |     |-----|SPBM| |              |
         |SPBM |     |PE1 | |   IP/MPLS    |
   +---+ |NTWK1|     +----+ |   Network    |
   |CE1|-|     |            |              |
   +---+ |     |     +----+ |              |
         |     |-----|SPBM| |              | +----+   +-----+
         +-----+     |PE2 | |              | |SPBM|   |SPBM | +---+
                     +----+ |              | |PE5 |---|NTWK2|-|CE3|
                            +--------------+ +----+   +-----+ +---+

               Figure 1: PBB and SPBM EVPN Network

   Figure 1 illustrates the generalized space addressed by this memo.
   SPBM networks may be multihomed onto an IP/MPLS network that
   implements EVPN for the purpose of interconnecting with other SPBM
   networks and/or PBB PEs.  The multipathing configuration of each SPBM
   network can be unique as the backbone VLAN ID (B-VID) configuration
   (how multipathing is performed in SPBM) is not propagated across the
   IP/MPLS network implementing EVPN.  As with PBB networking, the B-VID
   is local to the SPBM network, so in SPBM a B-MAC associated with the
   B-VID is advertised with the supported I-SIDs at the PBB gateway.

   Each EVPN is identified by a route target.  I-SID-based load-
   balancing as specified in [RFC7623] allows multiple gateways per
   B-VID (each with different I-SIDs) across the EVPN; it is supported
   by the interworking between PBBNs and SPBM networks.  However, SPBM

   only allows a single active designated forwarder (DF) per B-VID as
   described below.  The route target identifies the set of SPBM islands
   and PBB PEs that are allowed to communicate.  Each SPBM island is
   administered to have an Ethernet Segment ID (ESI) Label extended
   community associated with it.

   BGP acts as a common repository of the I-Component Service ID (I-SID)
   attachment points for the set of attached PEs / SPBM islands.  This
   is in the form of {B-MAC address, I-SID, Tx-Rx-attribute} tuples.
   BGP distributes I-SID information into each SPBM island on the basis
   of locally registered interest.  If an SPBM island has no Backbone
   Edge Bridges (BEBs) registering interest in a particular I-SID,
   information about that I-SID from other SPBM islands, PBB PEs, or
   PBBNs MUST NOT be leaked into the local ISIS-SPB routing system.  For
   each B-VID in an SPBM island, a single SPBM-PE MUST be elected the DF
   for the B-VID.  An SPBM-PE can be a DF for more than one B-VID.  This
   is described further in Section 4.2.  The SPBM-PE originates IS-IS
   advertisements as if it were an IB-BEB that proxies for the other
   SPBM islands and PBB PEs in the EVPN defined by the route target, but
   the PE typically will not actually host any I-components.

   An SPBM-PE that is a DF for a B-VID MUST strip the B-VID tag
   information from frames relayed towards the EVPN.  The DF MUST also
   insert the appropriate B-VID tag information into frames relayed
   towards the SPBM island on the basis of the local I-SID/B-VID
   bindings advertised in ISIS-SPB.

4.  Elements of Procedure

   A PE MUST implement and perform the following procedures.

4.1.  PE Configuration

   At SPBM island commissioning a PE is configured with:

   1) The route target for the service instance.  Where a route target
      is defined as identifying the set of SPBM islands, PBBNs and
      PBB PEs are to be interconnected by the EVPN.

   2) The unique ESI for the SPBM island.  Mechanisms for deriving a
      unique ESI for the SPBM island are out of scope.

   The following is configured as part of commissioning an ISIS-SPB

   1) A Shortest Path Source ID (SPSourceID) used for algorithmic
      construction of multicast addresses.  Note this is required for
      SPBM BEB operation independent of the EVPN operation.

   2) The set of B-VIDs used in the SPBM island and multipathing
      algorithm IDs to use for each.  The set of B-VIDs and multipathing
      algorithms used can be different in different domains.  Therefore,
      the B-VID is local to an SPBM domain and is removed for frames
      carried over the IP/MPLS network.

   A Type 1 Route Distinguisher for the node can be auto-derived.  The
   actual procedure is out of scope for this document.

4.2.  DF Election

   PEs self-appoint themselves for the role of DF for a B-VID for a
   given SPBM island.  The procedure used is as per Section 8.5
   (Designated Forwarder Election) of [RFC7432].

   A PE that assumes the role of DF for a given B-VID is responsible for
   originating specific information into BGP from ISIS-SPB and vice
   versa.  A PE that ceases to perform the role of DF for a given B-VID
   is responsible for withdrawing the associated information from BGP
   and ISIS-SPB, respectively.  The actual information exchanged is
   outlined in the following sections.

4.3.  Control-Plane Interworking ISIS-SPB to EVPN

   When a PE receives an SPBM service identifier and unicast address
   sub-TLV as part of an ISIS-SPB MT capability TLV, the PE checks if it
   is the DF for the B-VID in the sub-TLV.

   If it is the DF, and there is new or changed information, then a
   MAC/IP advertisement route NLRI is created for each new I-SID in the
   sub-TLV.  Changed information that results in modification to
   existing NLRI is processed accordingly.  NLRI creation/modification
   will ensure:

   -  the Route Distinguisher is set to that of the PE.

   -  the ESI is that of the SPBM island.

   -  the Ethernet Tag ID contains the I-SID (including the Tx/Rx
      attributes) copied from the SPBM service identifier and unicast
      address sub-TLV.  The encoding of I-SID information is as per
      Figure 2.  (See [RFC6329] for details on the T bit and R bit.)

       0                   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
      |T|R| Reserved  |                 I-SID                         |

          Figure 2: I-SID Encoding in the Ethernet Tag ID Field

   -  the MAC address is copied from the SPBM service identifier and
      unicast address sub-TLV

   -  a locally assigned MPLS label (which may be common with other NLRI
      originated by the PE and is used to map EVPN traffic to the SPBM

   Similarly, in the scenario where a PE became elected DF for a B-VID
   in an operating network, the IS-IS database would be processed in
   order to construct the NLRI associated with the new role of the PE.

   If the BGP database has NLRI for the I-SID, and this is the first
   instance of registration of interest in the I-SID from the SPBM
   island, the NLRI for the I-SID is processed to construct an updated
   set of SPBM service identifier and unicast address sub-TLVs to be
   advertised by the PE.

   The ISIS-SPB information is also used to keep current a local table
   indexed by I-SID to indicate the associated B-VID for processing of
   frames received from the EVPN.  When an I-SID is associated with more
   than one B-VID, only one entry is allowed in the table.  Rules for
   preventing this are out of scope for this memo.

4.4.  Control-Plane Interworking EVPN to ISIS-SPB

   When a PE receives a BGP NLRI that has new information, the PE checks
   if it is the elected DF to communicate this information into ISIS-SPB
   by checking if the I-SID in the Ethernet Tag ID locally maps to the
   B-VID for which it is an elected DF.  Note that if no BEBs in the SPB
   island have advertised any interest in the I-SID, it will not be
   associated with any B-VID locally, and therefore will not be of
   interest.  If the I-SID is of local interest to the SPBM island and
   the PE is the DF for the B-VID to which the I-SID is locally mapped,
   a SPBM service identifier and unicast address sub-TLV are
   constructed/updated for advertisement into ISIS-SPB.

   The NLRI advertised into ISIS-SPB is also used to locally populate a
   forwarding table indexed by B-MAC + I-SID that points to the label
   stack to impose on the SPBM frame.  The bottom label in the stack is
   that obtained from the NLRI.

4.5.   Data-Plane Interworking SPBM Island or PBB PE to EVPN

   When a PE receives a frame from the SPBM island in a B-VID for which
   it is a DF, it looks up the B-MAC/I-SID information to determine the
   label stack to be added to the frame for forwarding in the EVPN.  The
   PE strips the B-VID information from the frame, adds the label
   information to the frame, and forwards the resulting MPLS packet.

4.6.  Data-Plane Interworking EVPN to SPBM Island

   When a PE receives a packet from the EVPN, it can infer the B-VID to
   overwrite in the SPBM frame from the I-SID or by other means (such as
   via the bottom label in the MPLS stack).

   If the frame has a local multicast destination address (DA), it
   overwrites the SPSourceID in the frame with the local SPSourceID.

4.7.  Data-Plane Interworking EVPN to PBB PE

   A PBB PE actually has no attached PBBN nor concept of B-VID, so no
   frame processing is required.

   A PBB PE is required to accept SPBM-encoded multicast DAs as if they
   were PBB-encoded (i.e., using the Backbone Service Instance Group
   address) for multicast DAs.  The only information of interest is that
   it is a multicast frame and the I-SID encoded in the lower 24 bits.

4.8.  Multicast Support

   Within a PBBN domain, Ethernet unicast and multicast end services are
   supported.  PBB can tunnel multicast traffic in unicast PBB frames
   when using head-end replication.  This is the only form of multicast
   traffic interworking supported by this document.  Native PBB
   multicast forwarding over EVPN, PE replication, or optimizing PBB
   multicast across the EVPN is not addressed by this memo.

5.  Other Aspects

5.1.  Transit

   Any PE that does not need to participate in the tandem calculations
   at the B-MAC layer can use the IS-IS overload bit to exclude SPBM
   tandem paths and behave as a pure interworking platform.

6.  Security Considerations

   Security issues associated with incorrect interconnection of customer
   LANs cannot be directly addressed by implementations of this
   document, as it requires misconfiguration in the Shortest Path
   Bridging domains.  The identifiers so administered have global
   significance to the larger system.  They are relayed transparently by
   EVPN and only policed in the SPBM domains.  Therefore, care is
   required in synchronization of identifiers that need to be common for
   inter-domain operation.

   There are only two identifiers unique to this solution provisioned at
   an SPBM-PE at service turn-up: the route target and the ESI.  The ESI
   needs to be unique and common to all SPBM-PEs connected to a common
   SPBM network or PBBN, else portions of the overall network will not
   share reachability.  (The EVPN will assume that separate networks are
   interconnected by SPBM.)  Security issues exist when SPBM domains are
   incorrectly cross-connected together via EVPN; this will result in
   black-holing or incorrect delivery of data with associated privacy
   issues.  This error may occur by provisioning the incorrect RT value
   at an SPBM-PE or associating the RT with the wrong interface.  This
   error can be avoided by consistency-checking the route target
   provisioning at SPBM-PEs when performing service additions and/or

   The behavior that is potentially most destructive to the overall
   system is frequent changes to the DF elections for a given ESI.  This
   would occur if the SPBM-PEs continuously had their links go up and
   down in a such a way that the SPBM-PE was being severed from and
   reconnected to either the IP/MPLS network or the attached SPBM
   network.  Either of these scenarios would result in significant
   control-plane traffic as DF associated information was advertised and
   withdrawn from both the SPBM and BGP control planes.  Dual-homing of
   SPBM-PEs on both networks would minimize the likelihood of this
   scenario occurring.

   The issues associated with securing the BGP control plane
   (independent of this particular memo) are reflected in the Security
   Considerations section of [RFC4761].

7.  References

7.1.  Normative References

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

   [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,

   [RFC6329]  Fedyk, D., Ed., Ashwood-Smith, P., Ed., Allan, D., Bragg,
              A., and P. Unbehagen, "IS-IS Extensions Supporting IEEE
              802.1aq Shortest Path Bridging", RFC 6329,
              DOI 10.17487/RFC6329, April 2012,

   [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>.

7.2. Informative References

              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Bridges and Bridged Networks", IEEE 802.1Q-2014,
              DOI 10.1109/ieeestd.2014.6991462, December 2014.

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


   The authors would like to thank Peter Ashwood-Smith, Martin Julien,
   and Janos Farkas for their detailed reviews of this document.

Authors' Addresses

   Dave Allan (editor)
   300 Holger Way
   San Jose, CA  95134
   United States

   Email: david.i.allan@ericsson.com

   Jeff Tantsura
   300 Holger Way
   San Jose, CA  95134
   United States

   Email: jeff.tantsura@ericsson.com

   Don Fedyk
   Hewlett-Packard Enterprise
   153 Taylor Street
   Littleton, MA  01460
   United States

   Email: don.fedyk@hpe.com

   Ali Sajassi
   170 West Tasman Drive
   San Jose, CA  95134
   United States

   Email: sajassi@cisco.com


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