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Versions: (draft-sajassi-l2vpn-pbb-evpn) 00 01 02 03 04 05 06 07

Internet Working Group                                  Ali Sajassi, Ed.
Internet Draft                                               Samer Salam
Category: Standards Track                                          Cisco
                                                             Nabil Bitar
                                                                 Verizon
                                                            Aldrin Isaac
                                                               Bloomberg
                                                          Wim Henderickx
                                                          Alcatel-Lucent
                                                             Lizhong Jin
                                                                     ZTE
Expires: December 18, 2014                                 June 18, 2014


                               PBB-EVPN
                      draft-ietf-l2vpn-pbb-evpn-07

Status of this Memo

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

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Copyright and License Notice

   Copyright (c) 2013 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



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   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.

Abstract

   This document discusses how Ethernet Provider Backbone Bridging
   [802.1ah] can be combined with EVPN in order to reduce the number of
   BGP MAC advertisement routes by aggregating Customer/Client MAC (C-
   MAC) addresses via Provider Backbone MAC address (B-MAC), provide
   client MAC address mobility using C-MAC aggregation and B-MAC sub-
   netting, confine the scope of C-MAC learning to only active flows,
   offer per site policies and avoid C-MAC address flushing on topology
   changes. The combined solution is referred to as PBB-EVPN.

Conventions

   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  MAC Advertisement Route Scalability  . . . . . . . . . . .  5
     4.2.  C-MAC Mobility with MAC Summarization  . . . . . . . . . .  5
     4.3.  C-MAC Address Learning and Confinement . . . . . . . . . .  5
     4.4.  Per Site Policy Support  . . . . . . . . . . . . . . . . .  6
     4.5.  Avoiding C-MAC Address Flushing  . . . . . . . . . . . . .  6
   5.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  6
   6.  BGP Encoding . . . . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Ethernet Auto-Discovery Route  . . . . . . . . . . . . . .  7
     6.2.  BGP MAC Advertisement Route  . . . . . . . . . . . . . . .  8
     6.3.  Inclusive Multicast Ethernet Tag Route . . . . . . . . . .  8
     6.4.  Ethernet Segment Route . . . . . . . . . . . . . . . . . .  8
     6.5.  ESI Label Extended Community . . . . . . . . . . . . . . .  9
     6.6.  ES-Import Route Target . . . . . . . . . . . . . . . . . .  9
     6.7.  MAC Mobility Extended Community  . . . . . . . . . . . . .  9
     6.8.  Default Gateway Extended Community . . . . . . . . . . . .  9
   7.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  MAC Address Distribution over Core . . . . . . . . . . . .  9
     7.2.  Device Multi-homing  . . . . . . . . . . . . . . . . . . .  9



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       7.2.1 Flow-based Load-balancing  . . . . . . . . . . . . . . . 10
         7.2.1.1  PE B-MAC Address Assignment . . . . . . . . . . . . 10
         7.2.1.2.  Automating B-MAC Address Assignment  . . . . . . . 12
         7.2.1.3  Split Horizon and Designated Forwarder Election . . 12
       7.2.2 I-SID Based Load-balancing . . . . . . . . . . . . . . . 13
         7.2.2.1 PE B-MAC Address Assignment  . . . . . . . . . . . . 13
         7.2.2.2 Split Horizon and Designated Forwarder Election  . . 13
         7.2.2.3 Handling Failure Scenarios . . . . . . . . . . . . . 13
     7.3.  Network Multi-homing . . . . . . . . . . . . . . . . . . . 14
     7.4.  Frame Forwarding . . . . . . . . . . . . . . . . . . . . . 15
       7.4.1.  Unicast  . . . . . . . . . . . . . . . . . . . . . . . 15
       7.4.2.  Multicast/Broadcast  . . . . . . . . . . . . . . . . . 15
   8.  Minimizing ARP Broadcast . . . . . . . . . . . . . . . . . . . 16
   9. Seamless Interworking with IEEE 802.1aq/802.1Qbp  . . . . . . . 16
     9.1 B-MAC Address Assignment . . . . . . . . . . . . . . . . . . 16
     9.2  IEEE 802.1aq / 802.1Qbp B-MAC Advertisement Route . . . . . 17
     9.3 Operation: . . . . . . . . . . . . . . . . . . . . . . . . . 17
   10.  Solution Advantages . . . . . . . . . . . . . . . . . . . . . 17
     10.1.  MAC Advertisement Route Scalability . . . . . . . . . . . 18
     10.2.  C-MAC Mobility with MAC Sub-netting . . . . . . . . . . . 18
     10.3.  C-MAC Address Learning and Confinement  . . . . . . . . . 18
     10.4.  Seamless Interworking with TRILL and 802.1aq Access
            Networks  . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.5.  Per Site Policy Support . . . . . . . . . . . . . . . . . 19
     10.6.  Avoiding C-MAC Address Flushing . . . . . . . . . . . . . 19
   11.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 20
   12.  Security Considerations . . . . . . . . . . . . . . . . . . . 20
   13.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
   14.  Intellectual Property Considerations  . . . . . . . . . . . . 20
   15.  Normative References  . . . . . . . . . . . . . . . . . . . . 20
   16.  Informative References  . . . . . . . . . . . . . . . . . . . 20
   17.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . 21



















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1.  Introduction

   [EVPN] introduces a solution for multipoint L2VPN services, with
   advanced multi-homing capabilities, using BGP for distributing
   customer/client MAC address reach-ability information over the core
   MPLS/IP network. [PBB] defines an architecture for Ethernet Provider
   Backbone Bridging (PBB), where MAC tunneling is employed to improve
   service instance and MAC address scalability in Ethernet as well as
   VPLS networks [PBB-VPLS].

   In this document, we discuss how PBB can be combined with EVPN in
   order to: reduce the number of BGP MAC advertisement routes by
   aggregating Customer/Client MAC (C-MAC) addresses via Provider
   Backbone MAC address (B-MAC), provide client MAC address mobility
   using C-MAC aggregation and B-MAC sub-netting, confine the scope of
   C-MAC learning to only active flows, offer per site policies and
   avoid C-MAC address flushing on topology changes. The combined
   solution is referred to as PBB-EVPN.

2.  Contributors

   In addition to the authors listed above, the following individuals
   also contributed to this document.

   Sami Boutros, Cisco
   Dennis Cai, Cisco
   Keyur Patel, Cisco
   Clarence Filsfils, Cisco
   Sam Aldrin, Huawei
   Himanshu Shah, Ciena
   Florin Balus, ALU

3.  Terminology

   BEB: Backbone Edge Bridge
   B-MAC: Backbone MAC Address
   CE: Customer Edge
   C-MAC: Customer/Client MAC Address
   DHD: Dual-homed Device
   DHN: Dual-homed Network
   LACP: Link Aggregation Control Protocol
   LSM: Label Switched Multicast
   MDT: Multicast Delivery Tree
   MP2MP: Multipoint to Multipoint
   P2MP: Point to Multipoint
   P2P: Point to Point
   PE: Provider Edge
   PoA: Point of Attachment



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   PW: Pseudowire
   EVPN: Ethernet VPN
   Single-Active Redundancy Mode: When only a single PE, among a group
   of PEs attached to an Ethernet segment, is allowed to forward traffic
   to/from that Ethernet Segment, then the Ethernet segment is defined
   to be operating in Single-Active redundancy mode.

   All-Active Redundancy Mode: When all PEs attached to an Ethernet
   segment are allowed to forward traffic to/from that Ethernet Segment,
   then the Ethernet segment is defined to be operating in All-Active
   redundancy mode.


4.  Requirements

   The requirements for PBB-EVPN include all the requirements for EVPN
   that were described in [EVPN-REQ], in addition to the following:

4.1.  MAC Advertisement Route Scalability

   In typical operation, an [EVPN] PE sends a BGP MAC Advertisement
   Route per customer/client MAC (C-MAC) address. In certain
   applications, this poses scalability challenges, as is the case in
   data center interconnect (DCI) scenarios where the number of virtual
   machines (VMs), and hence the number of C-MAC addresses, can be in
   the millions. In such scenarios, it is required to reduce the number
   of BGP MAC Advertisement routes by relying on a 'MAC summarization'
   scheme, as is provided by PBB.

4.2.  C-MAC Mobility with MAC Summarization

   Certain applications, such as virtual machine mobility, require
   support for fast C-MAC address mobility. For these applications, the
   exact virtual machine MAC address needs to be transmitted in BGP MAC
   Advertisement route. Otherwise, traffic would be forwarded to the
   wrong segment when a virtual machine moves from one Ethernet segment
   to another. This means MAC address prefixes cannot be used in data
   center applications.

   In order to support C-MAC address mobility, while retaining the
   scalability benefits of MAC summarization, PBB technology is used. It
   defines a Backbone MAC (B-MAC) address space that is independent of
   the C-MAC address space, and aggregate C-MAC addresses via a B-MAC
   address and then apply summarization to B-MAC addresses.

4.3.  C-MAC Address Learning and Confinement

   In EVPN, all the PE nodes participating in the same EVPN instance are



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   exposed to all the C-MAC addresses learnt by any one of these PE
   nodes because a C-MAC learned by one of the PE nodes is advertise in
   BGP to other PE nodes in that EVPN instance. This is the case even if
   some of the PE nodes for that EVPN instance are not involved in
   forwarding traffic to, or from, these C-MAC addresses. Even if an
   implementation does not install hardware forwarding entries for C-MAC
   addresses that are not part of active traffic flows on that PE, the
   device memory is still consumed by keeping record of the C-MAC
   addresses in the routing table (RIB). In network applications with
   millions of C-MAC addresses, this introduces a non-trivial waste of
   PE resources. As such, it is required to confine the scope of
   visibility of C-MAC addresses only to those PE nodes that are
   actively involved in forwarding traffic to, or from, these addresses.

4.4.  Per Site Policy Support

   In many applications, it is required to be able to enforce
   connectivity policy rules at the granularity of a site (or segment).
   This includes the ability to control which PE nodes in the network
   can forward traffic to, or from, a given site. PBB-EVPN is capable of
   providing this granularity of policy control. In the case where per
   C-MAC address granularity is required, the EVI can always continue to
   operate in EVPN mode.

4.5.  Avoiding C-MAC Address Flushing

   It is required to avoid C-MAC address flushing upon link, port or
   node failure for All-Active multi-homed devices and networks. This is
   in order to speed up re-convergence upon failure.

5.  Solution Overview

   The solution involves incorporating IEEE Backbone Edge Bridge (BEB)
   functionality on the EVPN PE nodes similar to PBB-VPLS, where BEB
   functionality is incorporated in the VPLS PE nodes. The PE devices
   would then receive 802.1Q Ethernet frames from their attachment
   circuits, encapsulate them in the PBB header and forward the frames
   over the IP/MPLS core. On the egress EVPN PE, the PBB header is
   removed following the MPLS disposition, and the original 802.1Q
   Ethernet frame is delivered to the customer equipment.











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                   BEB   +--------------+  BEB
                   ||    |              |  ||
                   \/    |              |  \/
       +----+ AC1 +----+ |              | +----+   +----+
       | CE1|-----|    | |              | |    |---| CE2|
       +----+\    | PE1| |   IP/MPLS    | | PE3|   +----+
              \   +----+ |   Network    | +----+
               \         |              |
             AC2\ +----+ |              |
                 \|    | |              |
                  | PE2| |              |
                  +----+ |              |
                    /\   +--------------+
                    ||
                    BEB
         <-802.1Q-> <------PBB over MPLS------> <-802.1Q->

   Figure 1: PBB-EVPN Network

   The PE nodes perform the following functions:- Learn customer/client
   MAC addresses (C-MACs) over the attachment circuits in the data-
   plane, per normal bridge operation.

   - Learn remote C-MAC to B-MAC bindings in the data-plane for traffic
   received from the core per [PBB] bridging operation.

   - Advertise local B-MAC address reach-ability information in BGP to
   all other PE nodes in the same set of service instances. Note that
   every PE has a set of local B-MAC addresses that uniquely identify
   the device. More on the PE addressing in section 5.

   - Build a forwarding table from remote BGP advertisements received
   associating remote B-MAC addresses with remote PE IP addresses and
   the associated MPLS label(s).

6.  BGP Encoding

   PBB-EVPN leverages the same BGP Routes and Attributes defined in
   [EVPN], adapted as follows:


6.1.  Ethernet Auto-Discovery Route

   This route and all of its associated modes are not needed in PBB-
   EVPN.

   The receiving PE knows that it need not wait for the receipt of the
   Ethernet A-D route for route resolution by means of the reserved ESI



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   encoded in the MAC Advertisement route: the ESI values of 0 and MAX-
   ESI indicate that the receiving PE can resolve the path without an
   Ethernet A-D route.


6.2.  BGP MAC Advertisement Route

   The EVPN MAC Advertisement Route is used to distribute B-MAC
   addresses of the PE nodes instead of the C-MAC addresses of end-
   stations/hosts. This is because the C-MAC addresses are learnt in the
   data-plane for traffic arriving from the core. The MAC Advertisement
   Route is encoded as follows:

   - The MAC address field contains the B-MAC address.
   - The Ethernet Tag field is set to 0.
   - The Ethernet Segment Identifier field must be set either to 0 (for
   single-homed Segments or multi-homed Segments with per-ISID load-
   balancing) or to MAX-ESI (for multi-homed Segments with per-flow
   load-balancing). All other values are not permitted.
   - All other fields are set as defined in [EVPN].

   This route is tagged with the RT corresponding to its EVI. This EVI
   is analogous to a B-VID.


6.3.  Inclusive Multicast Ethernet Tag Route

   This route is used for multicast pruning per I-SID. It is used for
   auto-discovery of PEs participating in a given I-SID so that a
   multicast tunnel (MP2P, P2P, P2MP, or MP2MP LSP) can be setup for
   that I-SID . [PBB-VPLS] uses multicast pruning per I-SID based on
   [MMRP] which is a soft-state protocol. The advantages of multicast
   pruning using this BGP route over [MMRP] are that a) it scales very
   well for large number of PEs and b) it works with any type of LSP
   (MP2P, P2P, P2MP, or MP2MP); whereas, [MMRP] only works over P2P PWs.
   The Inclusive Multicast Ethernet Tag Route is encoded as follow:

   - The Ethernet Tag field is set with the appropriate I-SID value.
   - All other fields are set as defined in [EVPN].

   This route is tagged with an RT. This RT SHOULD be set to a value
   corresponding to its EVI (which is analogous to a B-VID). The RT for
   this route MAY also be auto-derived from the corresponding Ethernet
   Tag (I-SID) based on the procedure specified in section 9.4.1.1.1 of
   [EVPN].


6.4.  Ethernet Segment Route



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   This route is used as defined in [EVPN].


6.5.  ESI Label Extended Community

   This extended community is not used in PBB-EVPN. In [EVPN], this
   extended community is used along with the Ethernet AD route to
   advertise an MPLS label for the purpose of split-horizon filtering.
   Since in PBB-EVPN, the split-horizon filtering is performed natively
   using B-MAC SA, there is no need for this extended community.


6.6.  ES-Import Route Target

   This RT is used as defined in [EVPN].


6.7.  MAC Mobility Extended Community

   This extended community is defined in [EVPN] and it is used with a
   MAC route (B-MAC route in case of PBB-EVPN). The B-MAC route is
   tagged with the RT corresponding to its EVI (which is analogous to a
   B-VID). When this extended community is used along with a B-MAC route
   in PBB-EVPN, it indicates that all C-MAC addresses associated with
   that B-MAC address across all corresponding I-SIDs must be flushed.


6.8.  Default Gateway Extended Community

   This extended community is not used in PBB-EVPN.


7.  Operation

   This section discusses the operation of PBB-EVPN, specifically in
   areas where it differs from [EVPN].

7.1.  MAC Address Distribution over Core

   In PBB-EVPN, host MAC addresses (i.e. C-MAC addresses) need not be
   distributed in BGP. Rather, every PE independently learns the C-MAC
   addresses in the data-plane via normal bridging operation. Every PE
   has a set of one or more unicast B-MAC addresses associated with it,
   and those are the addresses distributed over the core in MAC
   Advertisement routes.

7.2.  Device Multi-homing




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7.2.1 Flow-based Load-balancing

   This section describes the procedures for supporting device multi-
   homing in an All-Active redundancy mode (i.e., flow-based load-
   balancing).

7.2.1.1  PE B-MAC Address Assignment

   In [PBB] every BEB is uniquely identified by one or more B-MAC
   addresses. These addresses are usually locally administered by the
   Service Provider. For PBB-EVPN, the choice of B-MAC address(es) for
   the PE nodes must be examined carefully as it has implications on the
   proper operation of multi-homing. In particular, for the scenario
   where a CE is multi-homed to a number of PE nodes with All-Active
   redundancy mode, a given C-MAC address would be reachable via
   multiple PE nodes concurrently. Given that any given remote PE will
   bind the C-MAC address to a single B-MAC address, then the various PE
   nodes connected to the same CE must share the same B-MAC address.
   Otherwise, the MAC address table of the remote PE nodes will keep
   oscillating between the B-MAC addresses of the various PE devices.
   For example, consider the network of Figure 1, and assume that PE1
   has B-MAC BM1 and PE2 has B-MAC BM2. Also, assume that both links
   from CE1 to the PE nodes are part of the same Ethernet link
   aggregation group. If BM1 is not equal to BM2, the consequence is
   that the MAC address table on PE3 will keep oscillating such that the
   C-MAC address M1 of CE1 would flip-flop between BM1 or BM2, depending
   on the load-balancing decision on CE1 for traffic destined to the
   core.

   Considering that there could be multiple sites (e.g. CEs) that are
   multi-homed to the same set of PE nodes, then it is required for all
   the PE devices in a Redundancy Group to have a unique B-MAC address
   per site. This way, it is possible to achieve fast convergence in the
   case where a link or port failure impacts the attachment circuit
   connecting a single site to a given PE.
















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                               +---------+
                +-------+  PE1 | IP/MPLS |
               /               |         |
            CE1                | Network |     PEr
           M1  \               |         |
                +-------+  PE2 |         |
                /-------+      |         |
               /               |         |
            CE2                |         |
          M2   \               |         |
                \              |         |
                 +------+  PE3 +---------+

   Figure 2: B-MAC Address Assignment

   In the example network shown in Figure 2 above, two sites
   corresponding to CE1 and CE2 are dual-homed to PE1/PE2 and PE2/PE3,
   respectively. Assume that BM1 is the B-MAC used for the site
   corresponding to CE1. Similarly, BM2 is the B-MAC used for the site
   corresponding to CE2. On PE1, a single B-MAC address (BM1) is
   required for the site corresponding to CE1. On PE2, two B-MAC
   addresses (BM1 and BM2) are required, one per site. Whereas on PE3, a
   single B-MAC address (BM2) is required for the site corresponding to
   CE2. All three PE nodes would advertise their respective B-MAC
   addresses in BGP using the MAC Advertisement routes defined in
   [EVPN]. The remote PE, PEr, would learn via BGP that BM1 is reachable
   via PE1 and PE2, whereas BM2 is reachable via both PE2 and PE3.
   Furthermore, PEr establishes, via the PBB bridge learning procedure,
   that C-MAC M1 is reachable via BM1, and C-MAC M2 is reachable via
   BM2. As a result, PEr can load-balance traffic destined to M1 between
   PE1 and PE2, as well as traffic destined to M2 between both PE2 and
   PE3. In the case of a failure that causes, for example, CE1 to be
   isolated from PE1, the latter can withdraw the route it has
   advertised for BM1. This way, PEr would update its path list for BM1,
   and will send all traffic destined to M1 over to PE2 only.

   For single-homed sites, it is possible to assign a unique B-MAC
   address per site, or have all the single-homed sites connected to a
   given PE share a single B-MAC address. The advantage of the first
   model over the second model is the ability to avoid C-MAC destination
   address lookup on the disposition PE (even though source C-MAC
   learning is still required in the data-plane). Also, by assigning the
   B-MAC addresses from a contiguous range, it is possible to advertise
   a single B-MAC subnet for all single-homed sites, thereby rendering
   the number of MAC advertisement routes required at par with the
   second model.

   In summary, every PE may use a unicast B-MAC address shared by all



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   single-homed CEs or a unicast B-MAC address per single-homed CE and,
   in addition, a unicast B-MAC address per All-Active multi-homed CE.
   In the latter case, the B-MAC address MUST be the same for all PE
   nodes in a Redundancy Group connected to the same CE.

7.2.1.2.  Automating B-MAC Address Assignment

   The PE B-MAC address used for single-homed sites can be automatically
   derived from the hardware (using for e.g. the backplane's address).
   However, the B-MAC address used for multi-homed sites must be
   coordinated among the RG members. To automate the assignment of this
   latter address, the PE can derive this B-MAC address from the MAC
   Address portion of the CE's LACP System Identifier by flipping the
   'Locally Administered' bit of the CE's address. This guarantees the
   uniqueness of the B-MAC address within the network, and ensures that
   all PE nodes connected to the same multi-homed CE use the same value
   for the B-MAC address.

   Note that with this automatic provisioning of the B-MAC address
   associated with multi-homed CEs, it is not possible to support the
   uncommon scenario where a CE has multiple bundles towards the PE
   nodes, and the service involves hair-pinning traffic from one bundle
   to another. This is because the split-horizon filtering relies on B-
   MAC addresses rather than Site-ID Labels (as will be described in the
   next section). The operator must explicitly configure the B-MAC
   address for this fairly uncommon service scenario.

   Whenever a B-MAC address is provisioned on the PE, either manually or
   automatically (as an outcome of CE auto-discovery), the PE MUST
   transmit an MAC Advertisement Route for the B-MAC address with a
   downstream assigned MPLS label that uniquely identifies that address
   on the advertising PE. The route is tagged with the RTs of the
   associated EVIs as described above.

7.2.1.3  Split Horizon and Designated Forwarder Election

   [EVPN] relies on access split horizon, where the Ethernet Segment
   Label is used for egress filtering on the attachment circuit in order
   to prevent forwarding loops. In PBB-EVPN, the B-MAC source address
   can be used for the same purpose, as it uniquely identifies the
   originating site of a given frame. As such, Ethernet Segment (ES)
   Labels are not used in PBB-EVPN, and the egress split-horizon
   filtering is done based on the B-MAC source address. It is worth
   noting here that [PBB] defines this B-MAC address based filtering
   function as part of the I-Component options, hence no new functions
   are required to support split-horizon beyond what is already defined
   in [PBB]. Given that the ES label is not used in PBB-EVPN, the PE
   sets the Label field in the Ethernet Segment Route to 0.



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   The Designated Forwarder election procedures are defined in [EVPN].

7.2.2 I-SID Based Load-balancing

   This section describes the procedures for supporting device multi-
   homing in a Single-Active redundancy mode with per-ISID load-
   balancing.

7.2.2.1 PE B-MAC Address Assignment

   In the case where per-ISID load-balancing is desired among the PE
   nodes in a given redundancy group, multiple unicast B-MAC addresses
   are allocated per multi-homed Ethernet Segment: Each PE connected to
   the multi-homed segment is assigned a unique B-MAC. Every PE then
   advertises its B-MAC address using the BGP MAC advertisement route.
   In this mode of operation, two B-MAC address assignment models are
   possible:

   - The PE may use a shared B-MAC address for multiple Ethernet
   Segments. This includes the single-homed segments as well as the
   multi-homed segments operating with per-ISID load-balancing mode.

   - The PE may use a dedicated B-MAC address for each Ethernet Segment
   operating with per-ISID load-balancing mode.

   All PE implementations MUST support the shared B-MAC address model
   and MAY support the dedicated B-MAC address model.

   A remote PE initially floods traffic to a destination C-MAC address,
   located in a given multi-homed Ethernet Segment, to all the PE nodes
   configured with that I-SID. Then, when reply traffic arrives at the
   remote PE, it learns (in the data-path) the B-MAC address and
   associated next-hop PE to use for said C-MAC address.

7.2.2.2 Split Horizon and Designated Forwarder Election The procedures
   are similar to the flow-based load-balancing case, with the only
   difference being that the DF filtering must be applied to unicast as
   well as multicast traffic, and in both core-to-segment as well as
   segment-to-core directions.

7.2.2.3 Handling Failure Scenarios

   When a PE connected to a multi-homed Ethernet Segment loses
   connectivity to the segment, due to link or port failure, it needs to
   notify the remote PEs to trigger C-MAC address flushing. This can be
   achieved in one of two ways, depending on the B-MAC assignment model:

   - If the PE uses a shared B-MAC address for multiple Ethernet



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   Segments, then the C-MAC flushing is signaled by means of having the
   failed PE re-advertise the MAC Advertisement route for the associated
   B-MAC, tagged with the MAC Mobility Extended Community attribute. The
   value of the Counter field in that attribute must be incremented
   prior to advertisement. This causes the remote PE nodes to flush all
   C-MAC addresses associated with the B-MAC in question. This is done
   across all I-SIDs that are mapped to the EVI of the withdrawn MAC
   route.


   - If the PE uses a dedicated B-MAC address for each Ethernet Segment
   operating under per-ISID load-balancing mode, the the failed PE
   simply withdraws the B-MAC route previously advertised for that
   segment. This causes the remote PE nodes to flush all C-MAC addresses
   associated with the B-MAC in question. This is done across all I-SIDs
   that are mapped to the EVI of the withdrawn MAC route.

   When a PE connected to a multi-homed Ethernet Segment fails (i.e.
   node failure) or when the PE becomes completely isolated from the
   EVPN network, the remote PEs will start purging the MAC Advertisement
   routes that were advertised by the failed PE. This is done either as
   an outcome of the remote PEs detecting that the BGP session to the
   failed PE has gone down, or by having a Route Reflector withdrawing
   all the routes that were advertised by the failed PE. The remote PEs,
   in this case, will perform C-MAC address flushing as an outcome of
   the MAC Advertisement route withdrawals.

   For all failure scenarios (link/port failure, node failure and PE
   node isolation), when the fault condition clears, the recovered PE
   re-advertises the associated Ethernet Segment route to other members
   of its Redundancy Group. This triggers the backup PE(s) in the
   Redundancy Group to block the I-SIDs for which the recovered PE is a
   DF. When a backup PE blocks the I-SIDs, it triggers a C-MAC address
   flush notification to the remote PEs by re-advertising the MAC
   Advertisement route for the associated B-MAC, with the MAC Mobility
   Extended Community attribute. The value of the Counter field in that
   attribute must be incremented prior to advertisement. This causes the
   remote PE nodes to flush all C-MAC addresses associated with the B-
   MAC in question. This is done across all I-SIDs that are mapped to
   the EVI of the withdrawn MAC route.

7.3.  Network Multi-homing

   When an Ethernet network is multi-homed to a set of PE nodes running
   PBB-EVPN, a single-active redundancy model can be supported with per
   service instance (i.e. I-SID) load-balancing. In this model, DF
   election is performed to ensure that a single PE node in the
   redundancy group is responsible for forwarding traffic associated



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   with a given I-SID. This guarantees that no forwarding loops are
   created. Filtering based on DF state applies to both unicast and
   multicast traffic, and in both access-to-core as well as core-to-
   access directions (unlike the multi-homed device scenario where DF
   filtering is limited to multi-destination frames in the core-to-
   access direction). Similar to the multi-homed device scenario, with
   I-SID based load-balancing, a unique B-MAC address is assigned to
   each of the PE nodes connected to the multi-homed network (Segment).

7.4.  Frame Forwarding

   The frame forwarding functions are divided in between the Bridge
   Module, which hosts the [PBB] Backbone Edge Bridge (BEB)
   functionality, and the MPLS Forwarder which handles the MPLS
   imposition/disposition. The details of frame forwarding for unicast
   and multi-destination frames are discussed next.

7.4.1.  Unicast

   Known unicast traffic received from the AC will be PBB-encapsulated
   by the PE using the B-MAC source address corresponding to the
   originating site. The unicast B-MAC destination address is determined
   based on a lookup of the C-MAC destination address (the binding of
   the two is done via transparent learning of reverse traffic). The
   resulting frame is then encapsulated with an LSP tunnel label and the
   MPLS label which uniquely identifies the B-MAC destination address on
   the egress PE. If per flow load-balancing over ECMPs in the MPLS core
   is required, then a flow label is added as the end of stack label.

   For unknown unicast traffic, the PE forwards these frames over MPLS
   core. When these frames are to be forwarded, then the same set of
   options used for forwarding multicast/broadcast frames (as described
   in next section) are used.

7.4.2.  Multicast/Broadcast

   Multi-destination frames received from the AC will be PBB-
   encapsulated by the PE using the B-MAC source address corresponding
   to the originating site. The multicast B-MAC destination address is
   selected based on the value of the I-SID as defined in [PBB]. The
   resulting frame is then forwarded over the MPLS core using one out of
   the following two options:

   Option 1: the MPLS Forwarder can perform ingress replication over a
   set of MP2P tunnel LSPs. The frame is encapsulated with a tunnel LSP
   label and the EVPN ingress replication label advertised in the
   Inclusive Multicast Route.




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   Option 2: the MPLS Forwarder can use P2MP tunnel LSP per the
   procedures defined in [EVPN]. This includes either the use of
   Inclusive or Aggregate Inclusive trees.

   Note that the same procedures for advertising and handling the
   Inclusive Multicast Route defined in [EVPN] apply here.

8.  Minimizing ARP Broadcast

   The PE nodes implement an ARP-proxy function in order to minimize the
   volume of ARP traffic that is broadcasted over the MPLS network. This
   is achieved by having each PE node snoop on ARP request and response
   messages received over the access interfaces or the MPLS core. The PE
   builds a cache of IP / MAC address bindings from these snooped
   messages. The PE then uses this cache to respond to ARP requests
   ingress on access ports and targeting hosts that are in remote sites.
   If the PE finds a match for the IP address in its ARP cache, it
   responds back to the requesting host and drops the request.
   Otherwise, if it does not find a match, then the request is flooded
   over the MPLS network using either ingress replication or LSM.


9. Seamless Interworking with IEEE 802.1aq/802.1Qbp

                           +--------------+
                           |              |
           +---------+     |     MPLS     |    +---------+
   +----+  |         |   +----+        +----+  |         |  +----+
   |SW1 |--|         |   | PE1|        | PE2|  |         |--| SW3|
   +----+  | 802.1aq |---|    |        |    |--| 802.1aq |  +----+
   +----+  |  .1Qbp  |   +----+        +----+  |  .1Qbp  |  +----+
   |SW2 |--|         |     |   Backbone   |    |         |--| SW4|
   +----+  +---------+     +--------------+    +---------+  +----+

   |<------ IS-IS -------->|<-----BGP----->|<------ IS-IS ------>|  CP


   |<-------------------------   PBB  -------------------------->|  DP
                           |<----MPLS----->|

   Legend: CP = Control Plane View
           DP = Data Plane View

   Figure 7: Interconnecting 802.1aq/802.1Qbp Networks with PBB-EVPN

9.1 B-MAC Address Assignment

   For the same reasons cited in the TRILL section, the B-MAC addresses



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   need to be globally unique across all the IEEE 802.1aq / 802.1Qbp
   networks. The same hierarchical address assignment scheme depicted
   above is proposed for B-MAC addresses as well.

9.2  IEEE 802.1aq / 802.1Qbp B-MAC Advertisement Route

   B-MAC addresses associated with 802.1aq / 802.1Qbp switches are
   advertised using the BGP MAC Advertisement route already defined in
   [EVPN].

   The encapsulation for the transport of PBB frames over MPLS is
   similar to that of classical Ethernet, albeit with the additional PBB
   header, as shown in the figure below:

   +------------------+
   | IP/MPLS Header   |
   +------------------+
   | PBB Header       |
   +------------------+
   | Ethernet Header  |
   +------------------+
   | Ethernet Payload |
   +------------------+
   | Ethernet FCS     |
   +------------------+

   Figure 8: PBB over MPLS Encapsulation

9.3 Operation:

   When a PE receives a PBB-encapsulated Ethernet frame from the access
   side, it performs a lookup on the B-MAC destination address to
   identify the next hop. If the lookup yields that the next hop is a
   remote PE, the local PE would then encapsulate the PBB frame in MPLS.
   The label stack comprises of the VPN label (advertised by the remote
   PE), followed by an LSP/IGP label. From that point onwards, regular
   MPLS forwarding is applied.

   On the disposition PE, assuming penultimate-hop-popping is employed,
   the PE receives the MPLS-encapsulated PBB frame with a single label:
   the VPN label. The value of the label indicates to the disposition PE
   that this is a PBB frame, so the label is popped, the TTL field (in
   the 802.1Qbp F-Tag) is reinitialized and normal PBB processing is
   employed from this point onwards.

10.  Solution Advantages

   In this section, we discuss the advantages of the PBB-EVPN solution



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   in the context of the requirements set forth in section 3 above.

10.1.  MAC Advertisement Route Scalability

   In PBB-EVPN the number of MAC Advertisement Routes is a function of
   the number of segments (sites), rather than the number of
   hosts/servers. This is because the B-MAC addresses of the PEs, rather
   than C-MAC addresses (of hosts/servers) are being advertised in BGP.
   And, as discussed above, there's a one-to-one mapping between multi-
   homed segments and B-MAC addresses, whereas there's a one-to-one or
   many-to-one mapping between single-homed segments and B-MAC addresses
   for a given PE. As a result, the volume of MAC Advertisement Routes
   in PBB-EVPN is multiple orders of magnitude less than EVPN.

10.2.  C-MAC Mobility with MAC Sub-netting

   In PBB-EVPN, if a PE allocates its B-MAC addresses from a contiguous
   range, then it can advertise a MAC prefix rather than individual 48-
   bit addresses. It should be noted that B-MAC addresses can easily be
   assigned from a contiguous range because PE nodes are within the
   provider administrative domain; however, CE devices and hosts are
   typically not within the provider administrative domain. The
   advantage of such MAC address sub-netting can be maintained even as
   C-MAC addresses move from one Ethernet segment to another. This is
   because the C-MAC address to B-MAC address association is learnt in
   the data-plane and C-MAC addresses are not advertised in BGP. To
   illustrate how this compares to EVPN, consider the following example:

   If a PE running EVPN advertises reachability for a MAC subnet that
   spans N addresses via a particular segment, and then 50% of the MAC
   addresses in that subnet move to other segments (e.g. due to virtual
   machine mobility), then in the worst case, N/2 additional MAC
   Advertisement routes need to be sent for the MAC addresses that have
   moved. This defeats the purpose of the sub-netting. With PBB-EVPN, on
   the other hand, the sub-netting applies to the B-MAC addresses which
   are statically associated with PE nodes and are not subject to
   mobility. As C-MAC addresses move from one segment to another, the
   binding of C-MAC to B-MAC addresses is updated via data-plane
   learning.

10.3.  C-MAC Address Learning and Confinement

   In PBB-EVPN, C-MAC address reachability information is built via
   data-plane learning. As such, PE nodes not participating in active
   conversations involving a particular C-MAC address will purge that
   address from their forwarding tables. Furthermore, since C-MAC
   addresses are not distributed in BGP, PE nodes will not maintain any
   record of them in control-plane routing table.



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10.4.  Seamless Interworking with TRILL and 802.1aq Access Networks

   Consider the scenario where two access networks, one running MPLS and
   the other running 802.1aq, are interconnected via an MPLS backbone
   network. The figure below shows such an example network.


                               +--------------+
                               |              |
               +---------+     |     MPLS     |    +---------+
       +----+  |         |   +----+        +----+  |         |  +----+
       | CE |--|         |   | PE1|        | PE2|  |         |--| CE |
       +----+  | 802.1aq |---|    |        |    |--|  MPLS   |  +----+
       +----+  |         |   +----+        +----+  |         |  +----+
       | CE |--|         |     |   Backbone   |    |         |--| CE |
       +----+  +---------+     +--------------+    +---------+  +----+

   Figure 9: Interoperability with 802.1aq

   If the MPLS backbone network employs EVPN, then the 802.1aq data-
   plane encapsulation must be terminated on PE1 or the edge device
   connecting to PE1. Either way, all the PE nodes that are part of the
   associated service instances will be exposed to all the C-MAC
   addresses of all hosts/servers connected to the access networks.
   However, if the MPLS backbone network employs PBB-EVPN, then the
   802.1aq encapsulation can be extended over the MPLS backbone, thereby
   maintaining C-MAC address transparency on PE1. If PBB-EVPN is also
   extended over the MPLS access network on the right, then C-MAC
   addresses would be transparent to PE2 as well.

   Interoperability with TRILL access network will be described in
   future revision of this draft.

10.5.  Per Site Policy Support

   In PBB-EVPN, a unique B-MAC address can be associated with every site
   (single-homed or multi-homed). Given that the B-MAC addresses are
   sent in BGP MAC Advertisement routes, it is possible to define per
   site (i.e. B-MAC) forwarding policies including policies for E-TREE
   service.

10.6.  Avoiding C-MAC Address Flushing

   With PBB-EVPN, it is possible to avoid C-MAC address flushing upon
   topology change affecting a multi-homed device. To illustrate this,
   consider the example network of Figure 1. Both PE1 and PE2 advertize
   the same B-MAC address (BM1) to PE3. PE3 then learns the C-MAC
   addresses of the servers/hosts behind CE1 via data-plane learning. If



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   AC1 fails, then PE3 does not need to flush any of the C-MAC addresses
   learnt and associated with BM1. This is because PE1 will withdraw the
   MAC Advertisement routes associated with BM1, thereby leading PE3 to
   have a single adjacency (to PE2) for this B-MAC address. Therefore,
   the topology change is communicated to PE3 and no C-MAC address
   flushing is required.

11.  Acknowledgements

   The authors would like to thank Jose Liste and Patrice Brissette for
   their reviews and comments of this document.

12.  Security Considerations

   There are no additional security aspects beyond those of VPLS/H-VPLS
   that need to be discussed here.

13.  IANA Considerations

   This document requires IANA to assign a new SAFI value for L2VPN_MAC
   SAFI.

14.  Intellectual Property Considerations

   This document is being submitted for use in IETF standards
   discussions.

15.  Normative References

   [PBB] Clauses 25 and 26 of "IEEE Standard for Local and metropolitan
              area networks - Media Access Control (MAC) Bridges and
              Virtual Bridged Local Area Networks", IEEE Std 802.1Q,
              2013.

16.  Informative References

   [PBB-VPLS] Sajassi, et al., "VPLS Interoperability with Provider
              Backbone Bridges", draft-ietf-l2vpn-pbb-vpls-interop-
              05.txt, work in progress, October, 2013.

   [EVPN-REQ] Sajassi, et al., "Requirements for Ethernet VPN (EVPN)",
              draft-ietf-l2vpn-evpn-req-05.txt, work in progress,
              October, 2013.

   [EVPN] Sajassi, et al., "BGP MPLS Based Ethernet VPN", draft-ietf-
              l2vpn-evpn-04.txt, work in progress, July, 2013.

   [MMRP] Clause 10 of "IEEE Standard for Local and metropolitan area



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              networks - Media Access Control (MAC) Bridges and Virtual
              Bridged Local Area Networks", IEEE Std 802.1Q, 2013.





17.  Authors' Addresses

   Ali Sajassi
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134, US
   Email: sajassi@cisco.com


   Samer Salam
   Cisco
   595 Burrard Street, Suite # 2123
   Vancouver, BC V7X 1J1, Canada
   Email: ssalam@cisco.com


   Nabil Bitar
   Verizon Communications
   Email : nabil.n.bitar@verizon.com


   Aldrin Isaac
   Bloomberg
   Email: aisaac71@bloomberg.net


   Wim Henderickx
   Alcatel-Lucent
   Email: wim.henderickx@alcatel-lucent.be


   Lizhong Jin
   ZTE Corporation
   889, Bibo Road
   Shanghai, 201203, China
   Email: lizhong.jin@zte.com.cn








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