Provider Backbone Bridging Combined with Ethernet VPN (PBB-EVPN)

   Internet Working Group                                 Ali Sajassi
   Internet Draft                                         Samer Salam
   Category: Standards Track                             Sami Boutros
   Florin Balus
   Wim Henderickx                                         Nabil Bitar
   Alcatel-Lucent                                             Verizon
   Clarence Filsfils                                     Aldrin Issac
   Dennis Cai                                               Bloomberg
                                                           Lizhong Jin
   Expires: August 27, 2012                         February 27, 2012
                                 PBB E-VPN
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   Sajassi, et. al.                                           [Page 1]

   publication of this document.  Please review these documents
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   This document discusses how Ethernet Provider Backbone Bridging
   [802.1ah] can be combined with E-VPN 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.
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119
   Table of Contents
   1. Introduction.................................................... 3
   2. Contributors.................................................... 4
   3. Terminology..................................................... 4
   4. Requirements.................................................... 4
   4.1. MAC Advertisement Route Scalability........................... 4
   4.2. C-MAC Mobility with MAC Sub-netting........................... 5
   4.3. C-MAC Address Learning and Confinement........................ 5
   4.4. Interworking with TRILL and 802.1aq Access Networks with C-MAC
   Address Transparency............................................... 5
   4.5. Per Site Policy Support....................................... 6
   4.6. Avoiding C-MAC Address Flushing............................... 6
   5. Solution Overview............................................... 6
   6. BGP Encoding.................................................... 7
   6.1. BGP MAC Advertisement Route................................... 7
   6.2. Ethernet Auto-Discovery Route................................. 8
   6.3. Per VPN Route Targets......................................... 8
   6.4. MAC Mobility Extended Community............................... 8
   Sajassi, et al.                                            [Page 2]

   7. Operation....................................................... 8
   7.1. MAC Address Distribution over Core............................ 8
   7.2. Device Multi-homing........................................... 8
   7.2.1. MES MAC Layer Addressing & Multi-homing..................... 8
   7.2.2. Split Horizon and Designated Forwarder Election............ 11
   7.3. Network Multi-homing......................................... 11
   7.3.1. B-MAC Address Advertisement................................ 12
   7.3.2. Failure Handling........................................... 12
   7.4. Frame Forwarding............................................. 13
   7.4.1. Unicast.................................................... 13
   7.4.2. Multicast/Broadcast........................................ 14
   8. Minimizing ARP Broadcast....................................... 14
   9. Seamless Interworking with TRILL and IEEE 802.1aq/802.1Qbp..... 14
   9.1. TRILL Nickname Advertisement Route........................... 15
   9.2. IEEE 802.1aq / 802.1Qbp B-MAC Advertisement Route............ 16
   9.3. Operation.................................................... 16
   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. Interworking with TRILL and 802.1aq Access Networks with C-MAC
   Address Transparency.............................................. 18
   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............................................ 20
   [E-VPN] introduces a solution for multipoint L2VPN services with
   advanced multi-homing capabilities using BGP for distributing
   customer/clinent MAC address reach-ability information over the core
   MPLS/IP network. [802.1ah] defines an architecture for Ethernet
   Provider Backbone Bridging (PBB), where MAC tunneling is employed to
   improve service instance and MAC address scalability in Ethernet
   networks and in VPLS networks [PBB-VPLS].
   Sajassi, et al.                                            [Page 3]

   In this document, we discuss how PBB can be combined with E-VPN 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.
   In addition to the authors listed above, the following individuals
   also contributed to this document.
   Keyur Patel
   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
   MES: MPLS Edge Switch
   MP2MP: Multipoint to Multipoint
   P2MP: Point to Multipoint
   P2P: Point to Point
   PoA: Point of Attachment
   PW: Pseudowire
   E-VPN: Ethernet VPN
   The requirements for PBB-EVPN include all the requirements for E-VPN
   that were described in [EVPN-REQ], in addition to the following:
        MAC Advertisement Route Scalability
   In typical operation, an [E-VPN] MES 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
   virtualized data center environments 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'
   Sajassi, et al.                                            [Page 4]

   scheme, as is provided by PBB. Note that the MAC sub-netting
   capability already built into E-VPN is not sufficient in those
   environments, as will be discussed next.
        C-MAC Mobility with MAC Sub-netting
   Certain applications, such as virtual machine mobility, require
   support for fast C-MAC address mobility. For these applications, it
   is not possible to use MAC address sub-netting in E-VPN, i.e.
   advertise reach-ability to a MAC address prefix. Rather, 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 hinders the scalability benefits of sub-netting.
   It is required to support C-MAC address mobility, while retaining
   the scalability benefits of MAC sub-netting. This can be achieved by
   leveraging PBB technology, which 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 sub-
   netting to B-MAC addresses.
        C-MAC Address Learning and Confinement
   In E-VPN, all the MES nodes participating in the same E-VPN instance
   are exposed to all the C-MAC addresses learnt by any one of these
   MES nodes because a C-MAC learned by one of the MES nodes is
   advertise in BGP to other MES nodes in that E-VPN instance. This is
   the case even if some of the MES nodes for that E-VPN 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 MES, 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 MES resources. As such, it is
   required to confine the scope of visibility of C-MAC addresses only
   to those MES nodes that are actively involved in forwarding traffic
   to, or from, these addresses.
        Interworking with TRILL and 802.1aq Access Networks with C-MAC
  Address Transparency
   [TRILL] and [802.1aq] define next generation Ethernet bridging
   technologies that offer optimal forwarding using IS-IS control
   plane, and C-MAC address transparency via Ethernet tunneling
   technologies. When access networks based on TRILL or 802.1aq are
   interconnected over an MPLS/IP network, it is required to guarantee
   C-MAC address transparency on the hand-off point and the edge (i.e.
   MES) of the MPLS network. As such, solutions that require
   termination of the access data-plane encapsulation (i.e. TRILL or
   Sajassi, et al.                                            [Page 5]

   802.1aq) at the hand-off to the MPLS network do not meet this
   transparency requirement, and expose the MPLS edge devices to the
   MAC address scalability problem.
   PBB-EVPN supports seamless interconnect with these next generation
   Ethernet solutions while guaranteeing C-MAC address transparency on
   the MES nodes.
        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 MES 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 E-VPN mode.
        Avoiding C-MAC Address Flushing
   It is required to avoid C-MAC address flushing upon link, port or
   node failure for multi-homed devices and networks. This is in order
   to speed up re-convergence upon failure.
      Solution Overview
   The solution involves incorporating IEEE 802.1ah Backbone Edge
   Bridge (BEB) functionality on the E-VPN MES nodes similar to PBB-
   VPLS PEs (PBB-VPLS) where BEB functionality is incorporated in PE
   nodes. The MES 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 E-VPN
   MES, the PBB header is removed following the MPLS disposition, and
   the original 802.1Q Ethernet frame is delivered to the customer
                   BEB   +--------------+  BEB
                   ||    |              |  ||
                   \/    |              |  \/
       +----+ AC1 +----+ |              | +----+   +----+
       | CE1|-----|    | |              | |    |---| CE2|
       +----+\    |MES1| |   IP/MPLS    | |MES3|   +----+
              \   +----+ |   Network    | +----+
               \         |              |
             AC2\ +----+ |              |
                 \|    | |              |
                  |MES2| |              |
                  +----+ |              |
                    /\   +--------------+
   Sajassi, et al.                                            [Page 6]

         <-802.1Q-> <------PBB over MPLS------> <-802.1Q->
                        Figure 1: PBB-EVPN Network
   The MES 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 from
   traffic ingress from the core per [802.1ah] bridging operation.
   - Advertise local B-MAC address reach-ability information in BGP to
   all other MES nodes in the same set of service instances. Note that
   every MES has a set of local B-MAC addresses that uniquely identify
   the device. More on the MES addressing in section 5.
   - Build a forwarding table from remote BGP advertisements received
   associating remote B-MAC addresses with remote MES IP addresses and
   the associated MPLS label(s).
      BGP Encoding
   PBB-EVPN leverages the same BGP Routes and Attributes defined in [E-
   VPN], adapted as follows:
        BGP MAC Advertisement Route
   The E-VPN MAC Advertisement Route is used to distribute B-MAC
   addresses of the MES 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 RD is set to a Type 1 RD [RFC4364]. The value field encodes
     the IP address of the MES (typically, the loopback address)
     followed by 0.  The reason for such encoding is that the RD cannot
     be that of a single EVI since the same B-MAC address can span
     across multiple EVIs.
   - The MAC address field contains the B-MAC address.
   - The Ethernet Tag field is set to 0.
   The route is tagged with the set of RTs corresponding to all EVIs
   associated with the B-MAC address.
   All other fields are set as defined in [E-VPN].
   Sajassi, et al.                                            [Page 7]

        Ethernet Auto-Discovery Route
   This route and any of its associated modes is not needed in PBB-
        Per VPN Route Targets
   PBB-EVPN uses the same set of route targets defined in [E-VPN]. More
   specifically, the RT associated with a VPN is set to the value of
   the I-SID associated with the service instance. This eliminates the
   need for manually configuring the VPN-RT.
        MAC Mobility Extended Community
   This extended community is a new transitive extended community. It
   may be advertised along with MAC Advertisement routes. When used in
   PBB-EVPN, it indicates that the C-MAC forwarding tables for the I-
   SIDs associated with the RTs tagging the MAC Advertisement routes
   must be flushed. This extended community is encoded in 8-bytes as
   - Type (1 byte) = Pending IANA assignment.
   - Sub-Type (1 byte) = Pending IANA assignment.
   - Reserved (2 bytes)
   - Counter (4 bytes)
   Note that all other BGP messages and/or attributes are used as
   defined in [E-VPN].
   This section discusses the operation of PBB-EVPN, specifically in
   areas where it differs from [E-VPN].
        MAC Address Distribution over Core
   In PBB-EVPN, host MAC addresses (i.e. C-MAC addresses) need not be
   distributed in BGP. Rather, every MES independently learns the C-MAC
   addresses in the data-plane via normal bridging operation. Every MES
   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. Given that these B-MAC addresses are global
   within the provider's network, there's no need to advertise them on
   a per service instance basis.
        Device Multi-homing
          MES MAC Layer Addressing & Multi-homing
   In [802.1ah] every BEB is uniquely identified by one or more B-MAC
   addresses. These addresses are usually locally administered by the
   Sajassi, et al.                                            [Page 8]

   Service Provider. For PBB-EVPN, the choice of B-MAC address(es) for
   the MES 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 MES nodes with
   all-active redundancy and flow-based load-balancing, a given C-MAC
   address would be reachable via multiple MES nodes concurrently.
   Given that any given remote MES will bind the C-MAC address to a
   single B-MAC address, then the various MES nodes connected to the
   same CE must share the same B-MAC address. Otherwise, the MAC
   address table of the remote MES nodes will keep flip-flopping
   between the B-MAC addresses of the various MES devices. For example,
   consider the network of Figure 1, and assume that MES1 has B-MAC BM1
   and MES2 has B-MAC BM2. Also, assume that both links from CE1 to the
   MES nodes are part of an all-active multi-chassis Ethernet link
   aggregation group. If BM1 is not equal to BM2, the consequence is
   that the MAC address table on MES3 will keep oscillating such that
   the C-MAC address CM 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 MES nodes, then it is required for
   all the MES 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 MES.
                +-------+ MES1 | IP/MPLS |
               /               |         |
            CE1                | Network |    MESr
           M1  \               |         |
                +-------+ MES2 |         |
                /-------+      |         |
               /               |         |
            CE2                |         |
          M2   \               |         |
                \              |         |
                 +------+ MES3 +---------+
   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 MES1/MES2 and
   MES2/MES3, 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 MES1, a single B-MAC address (BM1) is
   required for the site corresponding to CE1. On MES2, two B-MAC
   addresses (BM1 and BM2) are required, one per site. Whereas on MES3,
   Sajassi, et al.                                            [Page 9]

   a single B-MAC address (BM2) is required for the site corresponding
   to CE2. All three MES nodes would advertise their respective B-MAC
   addresses in BGP using the MAC Advertisement routes defined in [E-
   VPN]. The remote MES, MESr, would learn via BGP that BM1 is
   reachable via MES1 and MES2, whereas BM2 is reachable via both MES2
   and MES3. Furthermore, MESr establishes via the normal bridge
   learning that C-MAC M1 is reachable via BM1, and C-MAC M2 is
   reachable via BM2. As a result, MESr can load-balance traffic
   destined to M1 between MES1 and MES2, as well as traffic destined to
   M2 between both MES2 and MES3. In the case of a failure that causes,
   for example, CE1 to be isolated from MES1, the latter can withdraw
   the route it has advertised for BM1. This way, MESr would update its
   path list for BM1, and will send all traffic destined to M1 over to
   MES2 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 MES 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 MES may use a unicast B-MAC address shared by all
   single-homed CEs or a unicast B-MAC address per single-homed CE, and
   in addition a unicast B-MAC address per dual-homed CE. In the latter
   case, the B-MAC address MUST be the same for all MES nodes in a
   Redundancy Group connected to the same CE.
            Automating B-MAC Address Assignment
   The MES 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 MES 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 MES 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 mult-homed CEs, it is not possible to support the
   uncommon scenario where a CE has multiple bundles towards the MES
   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
   Sajassi, et al.                                           [Page 10]

   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 MES, either manually
   or automatically (as an outcome of CE auto-discovery), the MES 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 MES. The route is tagged with the RTs of the
   associated EVIs as described above.
          Split Horizon and Designated Forwarder Election
   [E-VPN] 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, Segment Labels are
   not used in PBB-EVPN, and the egress filtering is done based on the
   B-MAC source address. It is worth noting here that [802.1ah] 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 [802.1ah].
   Given that the Segment label is not used in PBB-EVPN, the MES sets
   the Label field in the Ethernet Segment Route to 0.
   The Designated Forwarder election procedures remain unchanged from
        Network Multi-homing
   When an Ethernet network is multi-homed to a set of MES nodes
   running PBB-EVPN, an all-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 MES node in
   the redundancy group is responsible for forwarding traffic
   associated 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, a unique B-MAC address
   is used on the MES per multi-homed network (Segment). This helps
   eliminate the need for C-MAC address flushing in all but one failure
   scenario (more details on this in the Failure Handling section
   below). The B-MAC address may be auto-provisioned by snooping on the
   BPDUs of the multi-homed network: the B-MAC address is set to the
   root bridge ID of the CIST albeit with the 'Locally Administered'
   bit set.
   Sajassi, et al.                                           [Page 11]

          B-MAC Address Advertisement
   For every multi-homed network, the MES advertises two MAC
   Advertisement routes with different RDs and identical MAC addresses
   and ESIs. One of these routes will be tagged with a lower Local Pref
   attribute than the other. The route with the higher Local Pref will
   be tagged with the RTs corresponding to the I-SIDs for which the
   advertising MES is the DF. Whereas, the route with the lower Local
   Pref will be tagged with the RTs corresponding to the I-SIDs for
   which the advertising MES is the backup DF. Consider the example
   network of the figure below, where a multi-homed network (MHN1) is
   connected to two MES nodes (MES1 and MES2).
                +-------+ MES1 | IP/MPLS |
       +------+         BM1    |         |
       |      |                | Network |    MESr
       | MHN1 |         BM1    |         |
       +------+ +-------+ MES2 |         |
   Figure 3: Multi-homed Network
   Both MES nodes use the same B-MAC address (BM1) for the Ethernet
   Segment (ESI1) associated with MHN1. Assume, for instance, that MES1
   is the DF for the even I-SIDs whereas MES2 is the DF for the odd I-
   SIDs. In this example, the routes advertised by MES1 and MES2 would
   be as follows:
   Route 1: RD11, BM1, ESI1, Local Pref = 120, RT2, RT4, RT6...
   Route 2: RD12, BM1, ESI1, Local Pref = 80, RT1, RT3, RT5...
   Route 1: RD21, BM1, ESI1, Local Pref = 120, RT1, RT3, RT5...
   Route 2: RD22, BM1, ESI1, Local Pref = 80, RT2, RT4, RT6
   Upon receiving the above MAC Advertisement routes, the remote MES
   nodes (e.g. MESr) would install forwarding entries for BM1 towards
   MES1 for the even I-SIDs, and towards MES2 for the odd I-SIDs.
   It is worth noting that the procedures of this section can also be
   used for a multi-homed device in order to support all-active
   redundancy with per I-SID load-balancing.
          Failure Handling
   In the case of an MES node failure, or when the MES is isolated from
   the multi-homed network due to a port or link failure, the affected
   Sajassi, et al.                                           [Page 12]

   MES withdraws its MAC Advertisement routes for the associated B-MAC.
   This serves as a trigger for the remote MES nodes to adjust their
   forwarding entries to point to the backup DF. Because the same B-MAC
   address is used on both the DF and backup DF nodes, then there is no
   need to flush the C-MAC address table upon the occurrence of these
   In the case where the multi-homed network is partitioned, the MES
   nodes can detect this condition by snooping on the network's BPDUs.
   When a MES detects that the root bridge ID has changed, it must
   change the value of the B-MAC address associated with the Ethernet
   Segment. This is done by the MES withdrawing the previous MAC
   Advertisement route, and advertising a new route for the updated B-
   MAC. The MES, which detects the failure, must inform the remote MES
   nodes to flush their C-MAC address tables for the affected I-SIDs.
   This is required because when the multi-homed network is
   partitioned, certain C-MAC addresses will move from being associated
   with the old B-MAC address to the new B-MAC addresses. Other C-MAC
   addresses will have their reachability remaining intact. Given that
   the MES node has no means of identifying which C-MACs have moved and
   which have not, the entire C-MAC forwarding table for the affected
   I-SIDs must be flushed. The affected MES signals the need for the C-
   MAC flushing by sending the MAC Mobility Extended Community in the
   MP_UNREACH_NLRI attribute containing the E-VPN NLRI for the
   withdrawn MAC Advertisement route.
        Frame Forwarding
   The frame forwarding functions are divided in between the Bridge
   Module, which hosts the [802.1ah] 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.
   Known unicast traffic received from the AC will be PBB-encapsulated
   by the MES 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 MES. 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 MES forwards these frames over MPLS
   core. When these frames are to be forwarded, then the same set of
   Sajassi, et al.                                           [Page 13]

   options used for forwarding multicast/broadcast frames (as described
   in next section) are used.
   Multi-destination frames received from the AC will be PBB-
   encapsulated by the MES 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 [802.1ah].
   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 E-VPN ingress replication label advertised in the
   Inclusive Multicast Route.
   Option 2: the MPLS Forwarder can use P2MP tunnel LSP per the
   procedures defined in [E-VPN]. 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 [E-VPN] apply here.
      Minimizing ARP Broadcast
   The MES 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 MES node snoop on ARP request and
   response messages received over the access interfaces or the MPLS
   core. The MES builds a cache of IP / MAC address bindings from these
   snooped messages. The MES then uses this cache to respond to ARP
   requests ingress on access ports and targeting hosts that are in
   remote sites. If the MES 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
      Seamless Interworking with TRILL and IEEE 802.1aq/802.1Qbp
   PBB-EVPN enables seamless connectivity of TRILL or 802.1aq/802.1Qbp
   networks over an MPLS/IP core while maintaining control-plane
   separation among these networks. We will refer to one or any of
   TRILL, 802.1aq or 802.1Qbp networks collectively as 'NG-Ethernet
   networks' thereafter.
   Every NG-Ethernet network that is connected to the MPLS core runs an
   independent instance of the corresponding IS-IS control-plane. Each
   MES participates in the NG-Ethernet network control plane of its
   local site. The MES peers, in IS-IS protocol, with the switches
   internal to the site, but does not terminate the TRILL / PBB data-
   Sajassi, et al.                                           [Page 14]

   plane encapsulation. So, from a control-plane viewpoint, the MES
   appears as an edge switch; whereas, from a data-plane viewpoint, the
   MES appears as a core switch to the NG-Ethernet network.
   The MES nodes encapsulate TRILL / PBB frames with MPLS in the
   imposition path, and de-capsulate them in the disposition path.
        TRILL Nickname Advertisement Route
   A new BGP route is defined to support the interconnection of TRILL
   networks over PBB-EVPN: the TRILL Nickname Advertisement' route,
   encoded as follows:
   | RD (8 octets)                         |
   |Ethernet Segment Identifier (10 octets)|
   | Ethernet Tag ID (4 octets)            |
   | Nickname Length (1 octet)             |
   | RBridge Nickname (2 octets)           |
   | MPLS Label (n * 3 octets)             |
   Figure 4: TRILL Nickname Advertisement Route
   The MES uses this route to advertise the reachability of TRILL
   RBridge nicknames to other MES nodes in the VPN instance. The MPLS
   label advertised in this route can be allocated on a per VPN basis
   and serves the purpose of identifying to the disposition MES that
   the MPLS-encapsulated packet holds an MPLS encapsulated TRILL frame.
   The encapsulation for the transport of TRILL frames over MPLS is
   encoded as shown in the figure below:
   | IP/MPLS Header   |
   | TRILL Header     |
   | Ethernet Header  |
   | Ethernet Payload |
   | Ethernet FCS     |
   Figure 5: TRILL over MPLS Encapsulation
   Sajassi, et al.                                           [Page 15]

   It is worth noting here that while it is possible to transport
   Ethernet encapsulated TRILL frames over MPLS, that approach
   unnecessarily wastes 16 bytes per packet. That approach further
   requires either the use of well-known MAC addresses or having the
   MES nodes advertise in BGP their device MAC addresses, in order to
   resolve the TRILL next-hop L2 adjacency. To that end, it is simpler
   and more efficient to transport TRILL natively over MPLS and that is
   why we are defining the above BGP route for TRILL Nickname
        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
   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 6: PBB over MPLS Encapsulation
   For correct connectivity, the TRILL Nicknames or 802.1aq/802.1Qbp B-
   MACs must be globally unique in the network. This can be achieved,
   for instance, by using a hierarchical Nickname (or B-MAC) assignment
   paradigm, and encoding a Site ID in the high-order bits of the
   Nickname (or B-MAC):
   Nickname (or B-MAC) = [Site ID : Rbridge ID (or MAC)]
   The only practical difference between TRILL Nicknames and B-MACs, in
   this regards, is with respect to the size of the address space:
   Nicknames are 16-bits wide whereas B-MACs are 48-bits wide.
   Sajassi, et al.                                           [Page 16]

   Every MES then advertises (in BGP) the Nicknames (or B-MACs) of all
   switches local to its site in the TRILL Nickname Advertisement
   routes (or MAC Advertisement routes).
   Furthermore, the MES advertises in IS-IS (to the local island) the
   Rbridge nicknames (or B-MACs) of all remote switches in all the
   other TRILL (or IEEE 802.1aq/802.1Qbp) islands that the MES has
   learned via BGP.
   Note that by having multiple MES nodes (connected to the same TRILL
   or 802.1aq /802.1Qbp island) advertise routes to the same RBridge
   nickname (or B-MAC), with equal BGP Local_Pref attribute, it is
   possible to perform active/active load-balancing to/from the MPLS
   When a MES receives an Ethernet-encapsulated TRILL frame from the
   access side, it removes the Ethernet encapsulation (i.e. outer MAC
   header), and performs a lookup on the egress RBridge nickname in the
   TRILL header to identify the next-hop. If the lookup yields that the
   next hop is a remote MES, the local MES would then encapsulate the
   TRILL frame in MPLS. The label stack comprises of the VPN label
   (advertised by the remote MES), followed by an LSP/IGP label. From
   that point onwards, regular MPLS forwarding is applied.
   On the disposition MES, assuming penultimate-hop-popping is
   employed, the MES receives the MPLS-encapsulated TRILL frame with a
   single label: the VPN label. The value of the label indicates to the
   disposition MES that this is a TRILL packet, so the label is popped,
   the TTL field (in the TRILL header) is reinitialized and normal
   TRILL processing is employed from this point onwards.
   By the same token, when a MES 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 MES, the local MES 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 MES, assuming penultimate-hop-popping is
   employed, the MES receives the MPLS-encapsulated PBB frame with a
   single label: the VPN label. The value of the label indicates to the
   disposition MES 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.
       Solution Advantages
   In this section, we discuss the advantages of the PBB-EVPN solution
   in the context of the requirements set forth in section 3 above.
   Sajassi, et al.                                           [Page 17]

         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 MESes,
   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 MES. As a result, the volume of MAC
   Advertisement Routes in PBB-EVPN is multiple orders of magnitude
   less than E-VPN.
         C-MAC Mobility with MAC Sub-netting
   In PBB-EVPN, if a MES 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 MES 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 E-VPN, consider the following
   If a MES running E-VPN 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 MES 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
         C-MAC Address Learning and Confinement
   In PBB-EVPN, C-MAC address reachability information is built via
   data-plane learning. As such, MES 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, MES nodes will not maintain
   any record of them in control-plane routing table.
         Seamless Interworking with TRILL and 802.1aq Access Networks
   Sajassi, et al.                                           [Page 18]

   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 |--|         |   |MES1|        |MES2|  |         |--| CE |
       +----+  | 802.1aq |---|    |        |    |--|  MPLS   |  +----+
       +----+  |         |   +----+        +----+  |         |  +----+
       | CE |--|         |     |   Backbone   |    |         |--| CE |
       +----+  +---------+     +--------------+    +---------+  +----+
   Figure 7: Interoperability with 802.1aq
   If the MPLS backbone network employs E-VPN, then the 802.1aq data-
   plane encapsulation must be terminated on MES1 or the edge device
   connecting to MES1. Either way, all the MES 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 MES1. If PBB-EVPN
   is also extended over the MPLS access network on the right, then C-
   MAC addresses would be transparent to MES2 as well.
   Interoperability with TRILL access network will be described in
   future revision of this draft.
         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.
         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 MES1 and MES2
   advertize the same B-MAC address (BM1) to MES3. MES3 then learns the
   C-MAC addresses of the servers/hosts behind CE1 via data-plane
   learning. If AC1 fails, then MES3 does not need to flush any of the
   C-MAC addresses learnt and associated with BM1. This is because MES1
   will withdraw the MAC Advertisement routes associated with BM1,
   Sajassi, et al.                                           [Page 19]

   thereby leading MES3 to have a single adjacency (to MES2) for this
   B-MAC address. Therefore, the topology change is communicated to
   MES3 and no C-MAC address flushing is required.
       Security Considerations
   There are no additional security aspects beyond those of VPLS/H-VPLS
   that need to be discussed here.
       IANA Considerations
   This document requires IANA to assign a new SAFI value for L2VPN_MAC
       Intellectual Property Considerations
   This document is being submitted for use in IETF standards
       Normative References
   [802.1ah] "Virtual Bridged Local Area Networks Amendment 7: Provider
   Backbone Bridges", IEEE Std. 802.1ah-2008, August 2008.
       Informative References
   [PBB-VPLS] Sajassi et al., "VPLS Interoperability with Provider
   Backbone Bridges", draft-ietf-l2vpn-vpls-pbb-interop-00.txt, work in
   progress, September, 2011.
    [EVPN-REQ] Sajassi et al., "Requirements for Ethernet VPN (E-VPN)",
   draft-sajassi-raggarwa-l2vpn-evpn-req-00.txt, work in progress,
   October, 2010.
   [E-VPN] Aggarwal et al., "BGP MPLS Based Ethernet VPN", draft-
   raggarwa-sajassi-l2vpn-evpn-01.txt, November, 2010.
   , work in progress, June, 2010.
       Authors' Addresses
   Ali Sajassi
   170 West Tasman Drive
   San Jose, CA  95134, US
   Samer Salam
   Sajassi, et al.                                           [Page 20]

   595 Burrard Street, Suite 2123
   Vancouver, BC V7X 1J1, Canada
   Sami Boutros
   170 West Tasman Drive
   San Jose, CA  95134, US
   Nabil Bitar
   Verizon Communications
   Email :
   Aldrin Isaac
   Florin Balus
   701 E. Middlefield Road
   Mountain View, CA, USA 94043
   Wim Henderickx
   Clarence Filsfils
   Dennis Cai
   Lizhong Jin
   ZTE Corporation
   889, Bibo Road
   Shanghai, 201203, China
   Sajassi, et al.                                           [Page 21]