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Versions: 00 01 02

L2VPN Working Group                                         Nabil Bitar
Internet Draft                                                  Verizon
Intended status: Informational
Expires: November 2012                                     Florin Balus
                                                          Marc Lasserre
                                                         Wim Henderickx
                                                         Alcatel-Lucent

                                                            Ali Sajassi
                                                            Luyuan Fang
                                                                  Cisco

                                                         Yuichi Ikejiri
                                                     NTT Communications

                                                          Mircea Pisica
                                                                     BT

                                                           May 18, 2012

             Cloud Networking: Framework and VPN Applicability
              draft-bitar-datacenter-vpn-applicability-02.txt


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 18, 2012.







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Copyright Notice

   Copyright (c) 2012 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.

Abstract

   Cloud Computing has been attracting a lot of attention from the
   networking industry. Some of the most publicized requirements are
   related to the evolution of the Cloud Networking Infrastructure to
   accommodate a large number of tenants, efficient network utilization,
   scalable loop avoidance, and Virtual Machine Mobility.

   This draft describes a framework for cloud networking, highlighting
   the applicability of existing work in various IETF Working Groups
   (e.g., RFCs and drafts developed in IETF L2VPN and L3VPN Working
   Groups) to cloud networking, and the gaps and problems that need to
   be further addressed. That is, the goal is to understand what may be
   re-used from the current protocols and call out requirements specific
   to the Cloud space that need to be addressed by new standardization
   work with proposed solutions in certain cases.

Table of Contents

   1. Introduction...................................................3
   2. General terminology............................................4
      2.1. Conventions used in this document.........................5
   3. Brief overview of Ethernet, L2VPN and L3VPN deployments........5
   4. Cloud Networking Framework.....................................6
   5. DC problem statement...........................................9
      5.1. VLAN Space................................................9
      5.2. MAC, IP, ARP Explosion...................................10
      5.3. Per VLAN flood containment...............................11
      5.4. Convergence and multipath support........................12
      5.5. Optimal traffic forwarding...............................12
      5.6. Efficient multicast support..............................14
      5.7. Connectivity to existing VPN sites.......................14
      5.8. DC Inter-connect requirements............................15
      5.9. L3 virtualization considerations.........................15



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      5.10. VM Mobility requirements................................15
   6. L2VPN Applicability to Cloud Networking.......................16
      6.1. VLANs and L2VPN toolset..................................16
      6.2. PBB and L2VPN toolset....................................17
         6.2.1. Addressing VLAN space exhaustion and MAC explosion..18
         6.2.2. Fast convergence, L2 multi-pathing..................19
         6.2.3. Per ISID flood containment..........................20
         6.2.4. Efficient multicast support.........................20
         6.2.5. Tunneling options for PBB ELAN: Ethernet, IP, MPLS..20
         6.2.6. Use Case examples...................................20
            6.2.6.1. PBBN in DC, L2 VPN in DC GW....................20
            6.2.6.2. PBBN in VSw, L2VPN in the ToR..................22
         6.2.7. Connectivity to existing VPN sites and Internet.....23
         6.2.8. DC Interconnect.....................................25
         6.2.9. Interoperating with existing DC VLANs...............25
      6.3. TRILL and L2VPN toolset..................................27
   7. L3VPN applicability to Cloud Networking.......................28
   8. Solutions for other DC challenges.............................29
      8.1. Addressing IP/ARP explosion..............................29
      8.2. Optimal traffic forwarding...............................29
      8.3. VM Mobility..............................................29
   9. Security Considerations.......................................30
   10. IANA Considerations..........................................30
   11. References...................................................30
      11.1. Normative References....................................30
      11.2. Informative References..................................31
   12. Acknowledgments..............................................32

1. Introduction

   The initial Data Center (DC) networks were built to address the needs
   of individual enterprises and/or individual applications. Ethernet
   VLANs and regular IP routing are used to provide connectivity between
   compute, storage resources and the related customer sites.

   The virtualization of compute resources in a DC environment provides
   the foundation for selling compute and storage resources to multiple
   customers, or selling application services to multiple customers. For
   example, a customer may buy a group of Virtual Machines (VMs) that
   may reside on server blades distributed throughout a DC or across
   DCs. In this latter case, the DCs may be owned and operated by a
   cloud service provider connected to one or more network service
   providers, two or more cloud service providers each connected to one
   or more network service providers, or a hybrid of DCs operated by the
   customer and the cloud service provider(s). In addition, multiple




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   customers may be assigned resources on the same compute and storage
   hardware.

   In order to provide access for multiple customers to the virtualized
   compute and storage resources, the DC network and DC interconnect
   have to evolve from the basic VLAN and IP routing architecture to
   provide equivalent connectivity virtualization at a large scale.

   This document describes in separate sections existing DC networking
   architecture, challenges faced by existing DC network models, and the
   applicability of VPN technologies to address such challenges. In
   addition, challenges not addressed by existing solutions are called
   out to describe the problem or to suggest solutions.

2. General terminology

   Some general terminology is defined here; most of the terminology
   used is from [802.1ah] and [RFC4026]. Terminology specific to this
   memo is introduced as needed in later sections.

   DC: Data Center

   ELAN: MEF ELAN, multipoint to multipoint Ethernet service

   EVPN: Ethernet VPN as defined in [EVPN]

   PBB: Provider Backbone Bridging, new Ethernet encapsulation designed
   to address VLAN exhaustion and MAC explosion issues; specified in
   IEEE 802.1ah [802.1ah]

   PBB-EVPN: defines how EVPN can be used to transport PBB frames

   BMAC: Backbone MACs, the backbone source or destination MAC address
   fields defined in the 802.1ah provider MAC encapsulation header.

   CMAC: Customer MACs, the customer source or destination MAC address
   fields defined in the 802.1ah customer MAC encapsulation header.

   BEB: A backbone edge bridge positioned at the edge of a provider
   backbone bridged network. It is usually the point in the network
   where PBB encapsulation is added or removed from the frame.

   BCB: A backbone core bridge positioned in the core of a provider
   backbone bridged network. It performs regular Ethernet switching
   using the outer Ethernet header.




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2.1. Conventions used in this document

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

   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lower case uses of these words are not to be
   interpreted as carrying RFC-2119 significance.

3. Brief overview of Ethernet, L2VPN and L3VPN deployments

   Initial Ethernet networks have been deployed in LAN environments,
   where the total number of hosts (hence MAC addresses) to manage was
   limited. Physical Ethernet topologies in LANs were pretty simple.
   Hence, a simple loop resolution protocol such as the Spanning Tree
   Protocol was sufficient in the early days. Efficient utilisation of
   physical links was not a major concern in LANs, while at the same
   time leveraging existing and mature technologies.

   As more hosts got connected to a LAN, or the need arose to create
   multiple LANs on the same physical infrastructure, it became
   necessary to partition the physical topology into multiple Virtual
   LANs (VLANs). STP evolved to cope with multiple VLANs with Multiple-
   STP (MSTP). Bridges/Switches evolved to learn behind which VLAN
   specific MACs resided, a process known as qualified learning. As
   Ethernet LANs moved into the provider space, the 12-bit VLAN space
   limitation (i.e. a total of 4k VLANs) led to Q-in-Q and later to
   Provider backbone Bridging (PBB).

   With PBB, not only can over 16M virtual LAN instances (24-bit Service
   I-SID) be supported, but a clean separation between customer and
   provider domains has been defined with separate MAC address spaces
   (Customer-MACs (CMACs) versus Provider Backbone-MACs (BMACs)). CMACs
   are only learned at the edge of the PBB network on PBB Backbone Edge
   Bridges (BEBs) in the context of an I-component while only B-MACs are
   learnt by PBB Backbone Core Bridges (BCBs). This results in BEB
   switches creating MAC-in-MAC tunnels to carry customer traffic,
   thereby hiding C-MACs in the core.

   In the meantime, interconnecting L2 domains across geographical areas
   has become a necessity. VPN technologies have been defined to carry
   both L2 and L3 traffic across IP/MPLS core networks. The same
   technologies could also be used within the same data center to
   provide for scale or for interconnecting services across L3 domains,
   as needed. Virtual Private LAN Service (VPLS) has been playing a key



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   role to provide transparent LAN services over IP/MPLS WANs while IP
   VPNs, including BGP/MPLS IP VPNs and IPsec VPNs, have been used to
   provide virtual IP routing instances over a common IP/MPLS core
   network.

   All these technologies have been combined to maximize their
   respective benefits. At the edge of the network, such as in access
   networks, VLAN and PBB are commonly used technologies. Aggregation
   networks typically use VPLS or BGP/MPLS IP VPNs to groom traffic on a
   common IP/MPLS core.

   It should be noted that Ethernet has kept evolving because of its
   attractive features, specifically its auto-discovery capabilities and
   the ability of hosts to physically relocate on the same LAN without
   requiring renumbering. In addition, Ethernet switches have become
   commodity, creating a financial incentive for interconnecting hosts
   in the same community with Ethernet switches. The network layer
   (layer3), on the other hand, has become pre-dominantly IP. Thus,
   communication across LANs uses IP routing.

4. Cloud Networking Framework

   A generic architecture for Cloud Networking is depicted in Figure 1:


























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                                ,---------.
                              ,'           `.
                             (    IP/MPLS    )
                              `.           ,'
                                `-+------+'
                             +--+--+   +-+---+
                             | GW  |+-+| GW  |
                             +-+---+   +-----+
                                /         \
                         +----+---+   +---+-----+
                         | Core   |   |  Core   |
                         | SW/Rtr |   | SW/Rtr  |
                         +-+----`.+   +-+---+---+
                            /      \   .'     \
                      +---+--+   +-`.+--+  +--+----+
                      | ToR  |   | ToR  |  |  ToR  |
                      +-+--`.+   +-+-`.-+  +-+--+--+
                       .'     \   .'    \   .'    `.
                    __/_      _i./       i./_      _\__
                   :VSw :    :VSw :     :VSw :    :VSw :
                   '----'    '----'     '----'    '----'

          Figure 1 : A Generic Architecture for Cloud Networking

   A cloud network is composed of intra-Data Center (DC) networks and
   network services, and inter-DC network connectivity. DCs may belong
   to a cloud service provider connected to one or more network service
   providers, different cloud service providers each connected to one or
   more network service providers, or a hybrid of DCs operated by the
   enterprise customers and the cloud service provider(s). It may also
   provide access to the public and/or enterprise customers.

   The following network components are present in a DC:

      -  VSw or virtual switch - software based Ethernet switch running
        inside the server blades. VSw may be single or dual-homed to
        the Top of Rack switches (ToRs). The individual VMs appear to a
        VSw as IP hosts connected via logical interfaces. The VSw may
        evolve to support IP routing functionality.

      -  ToR or Top of Rack - hardware-based Ethernet switch aggregating
        all Ethernet links from the server blades in a rack
        representing the entry point in the physical DC network for the
        hosts. ToRs may also perform routing functionality. ToRs are
        usually dual-homed to the Core SW. Other deployment scenarios




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        may use an EoR (End of Row) switch to provide similar function
        as a ToR.

      -  Core SW (switch) - high capacity core node aggregating multiple
        ToRs. This is usually a cost effective Ethernet switch. Core
        switches can also support routing capabilities.

      -  DC GW - gateway to the outside world providing DC Interconnect
        and connectivity to Internet and VPN customers. In the current
        DC network model, this may be a Router with Virtual Routing
        capabilities and/or an IPVPN/L2VPN PE.

   A DC network also contains other network services, such as firewalls,
   load-balancers, IPsec gateways, and SSL acceleration gateways. These
   network services are not currently discussed in this draft as the
   focus is on the routing and switching services. The usual DC
   deployment employs VLANs to isolate different VM groups throughout
   the Ethernet switching network within a DC. The VM Groups are mapped
   to VLANs in the VSws. The ToRs and Core SWs may employ VLAN trunking
   to eliminate provisioning touches in the DC network. In some
   scenarios, IP routing is extended down to the ToRs, and may be
   further extended to the hypervisor.

   Any new DC and cloud networking technology needs to be able to fit as
   seamlessly as possible with this existing DC model, at least in a
   non-greenfield environment. In particular, it should be possible to
   introduce enhancements to various tiers in this model in a phased
   approach without disrupting the other elements.

   Depending upon the scale, DC distribution, operations model, Capex
   and Opex aspects, DC switching elements can act as strict L2 switches
   and/or provide IP routing capabilities, including VPN routing and/or
   MPLS support. In smaller DCs, it is likely that some tier layers will
   be collapsed, and that Internet connectivity, inter-DC connectivity
   and VPN support will be handled by Core Nodes which perform the DC GW
   role.

   The DC network architecture described in this section can be used to
   provide generic L2-L3 service connectivity to each tenant as depicted
   in Figure 2:

                      ,--+-'.                      ;-`.--.
                 .....  VRF1 )......              .  VRF2 )
                 |    '-----'      |               '-----'
                 |     Tenant1     |ELAN12     Tenant1|
                 |ELAN11       ....|........          |ELAN13



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             '':'''''''':'       |        |     '':'''''''':'
              ,'.      ,'.      ,+.      ,+.     ,'.      ,'.
             (VM )....(VM )    (VM )... (VM )   (VM )....(VM )
              `-'      `-'      `-'      `-'     `-'      `-'

          Figure 2 : Logical Service connectivity for one tenant

   In this example one or more virtual routing contexts distributed on
   multiple DC GWs and one or more ELANs (e.g., one per Application)
   running on DC switches are assigned for DC tenant 1. ELAN is a
   generic term for Ethernet multipoint service, which in the current DC
   environment is implemented using 12-bit VLAN tags. Other possible
   ELAN technologies are discussed in section 6.

   For a multi-tenant DC, this type of service connectivity or a
   variation could be used for each tenant. In some cases only L2
   connectivity is required, i.e., only an ELAN may be used to
   interconnect VMs and customer sites.

5. DC problem statement

   This section summarizes the challenges faced with the present mode of
   operation described in the previous section and implicitly describes
   the requirements for next generation DC network.

   With the introduction of Compute virtualization, the DC network must
   support multiple customers or tenants that need access to their
   respective computing and storage resources in addition to making some
   aspect of the service available to other businesses in a B-to-B model
   or to the public. Every tenant requires service connectivity to its
   own resources with secure separation from other tenant domains.
   Connectivity needs to support various deployment models, including
   interconnecting customer-hosted data center resources to cloud
   service provider hosted resources (Virtualized DC for the customer).
   This connectivity may be at layer2 or layer3.

   Currently, large DCs are often built on a service architecture where
   VLANs configured in Ethernet edge and core switches are
   interconnected by IP routing running in a few centralized routers.
   There may be some cases though where IP routing might be used in the
   core nodes or even in the TORs inside a DC.

5.1. VLAN Space

   Existing DC deployments provide customer separation and flood
   containment, including support for DC infrastructure



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   interconnectivity, using Ethernet VLANs. A 12-bit VLAN tag provides
   support for a maximum of 4K VLANs.

   4K VLANs are inadequate for a Cloud Provider looking to expand its
   customer base. For example, there are a number of VPN deployments
   (VPLS and IP VPN) which serve more than 20K customers, each requiring
   multiple VLANs. Thus, 4K VLANs will likely support less than 4K
   customers.

   The cloud networking infrastructure needs to provide support for a
   much bigger number of virtual L2 domains.

5.2. MAC, IP, ARP Explosion

   Virtual Machines are the basic compute blocks being sold to Cloud
   customers. Every server blade supports today 16-40 VMs with 100 or
   more VMs per server blade coming in the near future. Every VM may
   have multiple interfaces for provider and enterprise management, VM
   mobility and tenant access, each with its own MAC and IP addresses.
   For a sizable DC, this may translate into millions of VM IP and MAC
   addresses. From a cloud network viewpoint, this scale number will be
   an order of magnitude higher.

   Supporting this amount of IP and MAC addresses, including the
   associated dynamic behavior (e.g., ARP), throughout the DC Ethernet
   switches and routers is very challenging in an Ethernet VLAN and
   regular routing environment. Core Ethernet switches running Ethernet
   VLANs learn the MAC addresses for every single VM interface that
   sends traffic through that switch. Throwing memory to increase the
   MAC Forwarding DataBase (FDB) size affects the cost of these
   switches. In addition, as the number of of MACs that switches need to
   learn increases, convergence time could increase, and flooding
   activity will increase upon a topology change as the core switches
   flush and re-learn the MAC addresses. Simple operational mistakes may
   lead to duplicate MAC entries within the same VLAN domain and
   security issues due to administrative MAC assignment used today for
   VM interfaces. Similar concerns about memory requirements and related
   cost apply to DC Edge switches (ToRs/EoRs) and DC GWs.

   From a router perspective, it is important to maximize the
   utilization of available resources in both control and data planes
   through flexible mapping of VMs and related VLANs to routing
   interfaces. This is not easily done in the current VLAN based
   deployment environment where the use of VLAN trunking limits the
   allocation of VMs to only local routers.




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   The amount of ARP traffic grows linearly with the number of hosts on
   a LAN. For 1 million VM hosts, it can be expected that the amount of
   ARP traffic will be in the range of half million ARPs per second at
   the peak, which corresponds to over 200 Mbps of ARP traffic [MYERS].
   Similarly, on a server, the amount of ARP traffic, grows linearly
   with the number of virtual L2 domains/ELANs instantiated on that
   server and the number of VMs in that domain. Besides the link
   capacity wasted, which may be small compared to the link capacities
   deployed in DCs, the computational burden may be prohibitive. In a
   large-DC environment, the large number of hosts and the distribution
   of ARP traffic may lead to a number of challenges:

     .  Processing overload and overload of ARP entries on the
        Server/Hypervisor. This is caused by the increased number of VMs
        per server blade and the size of related ELAN domains. For
        example, a server blade with 100 VMs, each in a separate L2
        domain with 100 VMs each would need to support 10K ARP entries
        and the associated ARP processing while performing the other
        compute tasks.

     .  Processing overload and exhaustion of ARP entries on the
        Routers/PEs and any other L3 Service Appliances (Firewall (FW),
        Load-Balancer (LB) etc). This issue is magnified by the L3
        virtualization at the service gateways. For example, a gateway
        PE handling 10K ELANs each with 10 VMs will result in 100K hosts
        sending/receiving traffic to/from the PE, thus requiring the PE
        to learn 100K ARP entries. It should be noted that if the PE
        supports Integrated Routing and Bridging (IRB), it must support
        the associated virtual IP RIBs/FIBs and MAC FDBs for these hosts
        in addition to the ARP entries.

     .  Flood explosion throughout Ethernet switching network. This is
        caused by the use of VLAN trunking and implicitly by the lack of
        per VPN flood containment.

     DC and DC-interconnect technologies that minimize the negative
     impact of ARP, MAC and IP entry explosion on individual network
     elements in a DC or cloud network hierarchy are needed.

5.3. Per VLAN flood containment

   From an operational perspective, DC operators try to minimize the
   provisioning touches required for configuring a VLAN domain by
   employing VLAN trunks on the L2 switches. This comes at the cost of
   flooding broadcast, multicast and unknown unicast frames outside of
   the boundaries of the actual VLAN domain.



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   The cloud networking infrastructure needs to prevent unnecessary
   traffic from being sent/leaked to undesired locations.

5.4. Convergence and multipath support

   Spanning Tree is used in the current DC environment for loop
   avoidance in the Ethernet switching domain.

   STP can take 30 to 50 seconds to repair a topology. Practical
   experience shows that Rapid STP (RSTP) can also take multiple seconds
   to converge, such as when the root bridge fails.

   STP eliminates loops by disabling ports. The result is that only one
   path is used to carry traffic. The capacity of disabled links cannot
   be utilized, leading to inefficient use of resources.

   In a small DC deployment, multi-chassis LAG (MC-LAG) support may be
   sufficient initially to provide for loop-free redundancy as an STP
   alternative. However, in medium or large DCs it is challenging to use
   MC-LAGs solely across the network to provide for resiliency and loop-
   free paths without introducing a layer2 routing protocol: i.e. for
   multi-homing of server blades to ToRs, ToRs to Core SWs, Core SWs to
   DC GWs. MC-LAG may work as a local mechanism but it has no knowledge
   of the end-to-end paths so it does not provide any degree of traffic
   steering across the network.

   Efficient and mature link-state protocols, such as IS-IS, provide
   rapid failover times, can compute optimal paths and can fully utilize
   multiple parallel paths to forward traffic between 2 nodes in the
   network.

   Unlike OSPF, IS-IS runs directly at L2 (i.e. no reliance on IP) and
   does not require any configuration. Therefore, IS-IS based DC
   networks are to be favored over STP-based networks. IEEE Shortest
   Path Bridging (SPB) based on IEEE 802.1aq and IEEE 802.1Qbp, and IETF
   TRILL [RFC6325] are technologies that enable Layer2 networks using
   IS-IS for Layer2 routing.

5.5. Optimal traffic forwarding

   Optimal traffic forwarding requires (1) efficient utilization of all
   available link capacity in a DC and DC-interconnect, and (2) traffic
   forwarding on the shortest path between any two communicating VMs
   within the DC or across DCs.





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   Optimizing traffic forwarding between any VM pair in the same virtual
   domain is dependent on (1) the placement of these VMs and their
   relative proximity from a network viewpoint, and (2) the technology
   used for computing the routing/switching path between these VMs. The
   latter is especially important in the context of VMotion, moving a VM
   from one network location to another, while maintaining its layer2
   and Layer3 addresses.

   Ethernet-based forwarding between two VMs relies on the MAC-
   destination Address that is unique per VM interface in the context of
   a virtual domain. In traditional IEEE technologies (e.g., 802.1ad,
   802.1ah) and IETF L2VPN (i.e., VPLS), Ethernet MAC reachability is
   always learnt in the data plane. That applies to both B-MACs and C-
   MACs. IETF EVPN [EVPN] supports C-MAC learning in the control plane
   via BGP.  In addition, with newer IEEE technologies (802.1aq and
   802.1Qbp) and IETF PBB-EVPN [PBB-EVPN], B-MAC reachability is learnt
   in the control plane while C-MACs are learnt in the data plane at
   BEBs, and tunneled in PBB frames. In all these cases, it is important
   that as a VM is moved from one location to another: (1) VM MAC
   reachability convergence happens fast to minimize traffic black-
   holing, and (2) forwarding takes the shortest path.

   IP-based forwarding relies on the destination IP address. ECMP load
   balancing relies on flow-based criteria. An IP host address is unique
   per VM interface. However, hosts on a LAN share a subnet mask, and IP
   routing entries are based on that subnet address. Thus, when VMs are
   on the same LAN and traditional forwarding takes place, these VMs
   forward traffic to each other by relying on ARP or IPv6 Neighbor
   discovery to identify the MAC address of the destination and on the
   underlying layer2 network to deliver the resulting MAC frame to is
   destination. However, when VMs, as IP hosts across layer2 virtual
   domains, need to communicate they rely on the underlying IP routing
   infrastructure.

   In addition, when a DC is an all-IP DC, VMs are assigned a host
   address with /32 subnet in the IPv4 case, or /64 or /128 host address
   in the IPv6 case, and rely on the IP routing infrastructure to route
   the IP packets among VMs. In this latter case, there is really no
   need for layer2 awareness potentially beyond the hypervisor switch at
   the server hosting the VM. In either case, when a VM moves location
   from one physical router to another while maintaining its IP identity
   (address), the underlying IP network must be able to route the
   traffic to the destination and must be able to do that on the
   shortest path.





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   Thus, in the case of IP address aggregation as in a subnet,
   optimality in traffic forwarding to a VM will require reachability to
   the VM host address rather than only the subnet. That is what is
   often referred to as punching a hole in the aggregate at the expense
   of routing and forwarding table size increase.

   As in layer2, layer3 may capitalize on hierarchical tunneling to
   optimize the routing/FIB resource utilization at different places in
   the network. If a hybrid of subnet-based routing and host-based
   routing (host-based routing here is used to refer to hole-punching in
   the aggregate) is used, then during VMotion, routing transition can
   take place, and traffic may be routed to a location based on subnet
   reachability or to a location where the VM used to be attached. In
   either of these cases, traffic must not be black-holed. It must be
   directed potentially via tunneling to the location where the VM is.
   This requires that the old routing gateway knows where the VM is
   currently attached. How to obtain that information can be based on
   different techniques with tradeoffs. However, this traffic
   triangulation is not optimal and must only exist in the transition
   until the network converges to a shortest path to the destination.

5.6. Efficient multicast support

   STP bridges typically perform IGMP and/or PIM snooping in order to
   optimize multicast data delivery. However, this snooping is performed
   locally by each bridge following the STP topology where all the
   traffic goes through the root bridge. This may result in sub-optimal
   multicast traffic delivery. In addition, each customer multicast
   group is associated with a forwarding tree throughout the Ethernet
   switching network. Solutions must provide for efficient Layer2
   multicast. In an all-IP network, explicit multicast trees in the DC
   network can be built via multicast signaling protocols (e.g., PIM-
   SSM) that follows the shortest path between the destinations and
   source(s). In an IPVPN context, Multicast IPVPN based on [MVPN] can
   be used to build multicast trees shared among IPVPNs, specific to
   VPNs, and/or shared among multicast groups across IPVPNs.

5.7. Connectivity to existing VPN sites

   It is expected that cloud services will have to span larger
   geographical areas in the near future and that existing VPN customers
   will require access to VM and storage facilities for virtualized data
   center applications. Hence, the DC network virtualization must
   interoperate with deployed and evolving VPN solutions - e.g. IP VPN,
   VPLS, VPWS, PBB-VPLS, E-VPN and PBB-EVPN.




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5.8. DC Inter-connect requirements

   Cloud computing requirements such as VM Mobility across DCs,
   Management connectivity, and support for East-West traffic between
   customer applications located in different DCs imply that inter-DC
   connectivity must be supported. These DCs can be part of a hybrid
   cloud operated by the cloud service provider(s) and/or the end-
   customers.

   Mature VPN technologies can be used to provide L2/L3 DC interconnect
   among VLANs/virtual domains located in different DCs.

5.9. L3 virtualization considerations

   In order to provide customer L3 separation while supporting
   overlapping IP addressing and privacy, a number of schemes were
   implemented in the DC environment. Some of these schemes, such as
   double NATing are operationally complex and prone to operator errors.
   Virtual Routing contexts (or Virtual Device contexts) or dedicated
   hardware-routers are positioned in the DC environment as an
   alternative to these mechanisms. Every customer is assigned a
   dedicated routing context with associated control plane protocols.
   For instance, every customer gets an IP Forwarding instance
   controlled by its own BGP and/or IGP routing. Assigning virtual or
   hardware routers to each customer while supporting thousands of
   customers in a DC is neither scalable nor cost-efficient.

5.10. VM Mobility requirements

   The ability to move VMs within a resource pool, whether it is a local
   move within the same DC to another server or to a distant DC, offers
   multiple advantages for a number of scenarios, for example:

   - In the event of a possible natural disaster, moving VMs to a safe
     DC location decreases downtime and allows for meeting SLA
     requirements.

   - Optimized resource location: VMs can be moved to locations that
     offer significant cost reduction (e.g. power savings), or
     locations close to the application users. They can also be moved
     to simply load-balance across different locations.

   When VMs change location, it is often important to maintain the
   existing client sessions. The VM MAC and IP addresses must be
   preserved, and the state of the VM sessions must be copied to the new
   location.



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   Current VM mobility tools like VMware VMotion require L2 connectivity
   among the hypervisors on the servers participating in a VMotion pool.
   This is in addition to "tenant ELAN" connectivity which provides for
   communication between the VM and the client(s).

   A VMotion ELAN might need to cross multiple DC networks to provide
   the required protection or load-balancing. In addition, in the
   current VMotion procedure, the new VM location must be part of the
   tenant ELAN domain. When the new VM is activated, a Gratuitous ARP is
   sent so that the MAC FIB entries in the "tenant ELAN" are updated to
   direct traffic destined to that VM to the new VM location. In
   addition, if a portion of the path requires IP forwarding, the VM
   reachability information must be updated to direct the traffic on the
   shortest path to the VM.

   VM mobility requirements may be addressed through the use of Inter-DC
   VLANs to address VMotion and tenant ELANs. However expanding "tenant
   VLANs" across two or more DCs will accelerate VLAN exhaustion and MAC
   explosion issues. In addition, STP needs to run across DCs leading to
   increased convergence times and the blocking of expensive WAN
   bandwidth. VLAN trunking used throughout the network creates
   indiscriminate flooding across DCs.

   L2 VPN solutions over IP/MPLS are designed to interconnect sites
   located across the WAN.


6. L2VPN Applicability to Cloud Networking

   The following sections will discuss different solution alternatives,
   re-using IEEE and IETF technologies to provide a gradual migration
   path from the current Ethernet switching VLAN-based model to more
   advanced Ethernet switching and IP/MPLS based models. This evolution
   is targeted to address inter-DC requirements, cost considerations and
   the efficient use of processing/memory resources on DC networking
   components.

6.1. VLANs and L2VPN toolset

   One approach to address some of the DC challenges discussed in the
   previous section is to gradually deploy additional technologies
   within existing DC networks. For example, an operator may start by
   breaking its DC VLAN domains into different VLAN islands so that each
   island can support up to 4K VLANs. VLAN Domains can then be
   interconnected via VPLS using the DC GW as a VPLS PE. An ELAN service
   can be identified with one VLAN ID in one island and another VLAN ID
   in another island with the appropriate VLAN ID processed at the GW.


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   As the number of tenants in individual VLAN islands surpasses 4K, the
   operator could push VPLS deployment deeper in the DC network. It is
   possible in the end to retain existing VLAN-based solution only in
   VSw and to provide L2VPN support starting at the ToRs. The ToR and DC
   core elements need to be MPLS enabled with existing VPLS solutions.

   However, this model does not solve the MAC explosion issue as ToRs
   still need to learn VM MAC addresses. In addition, it requires
   management of both VLAN and L2VPN addressing and mapping of service
   profiles. Per VLAN, per port and per VPLS configurations are required
   at the ToR, increasing the time it takes to bring up service
   connectivity and complicating the operational model.

6.2. PBB and L2VPN toolset

   As highlighted in the problem statement section, the expected large
   number of VM MAC addresses in the DC calls out for a VM MAC hiding
   solution so that the ToRs and the Core Switches only need to handle a
   limited number of MAC addresses.

   PBB IEEE 802.1ah encapsulation is a standard L2 technique developed
   by IEEE to achieve this goal. It was designed also to address other
   limitations of VLAN-based encapsulations while maintaining the native
   Ethernet operational model deployed in the DC network.

   A conceptual PBB encapsulation is described in Figure 3 (for detailed
   encapsulation see [802.1ah]):

                             +-------------+
                    Backbone | BMAC DA,SA  |12B
                    Ethernet |-------------|
                     Header  |BVID optional| 4B
                             |-------------|
                   Service ID|  PBB I-tag  | 6B
                             |-------------|
                     Regular |VM MAC DA,SA |
                     Payload |-------------|
                             |             |
                             |VM IP Payload|
                             |             |
                             +-------------+

                        Figure 3 PBB encapsulation

   The original Ethernet packet used in this example for Inter-VM
   communication is encapsulated in the following PBB header:

      -  I-tag field - organized similarly with the 802.1q VLAN tag; it
        includes the Ethertype, PCP and DEI bits and a 24 bit ISID tag


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        which replaces the 12 bit VLAN tag, extending the number of
        virtual L2 domain support to 16 Million. It should be noted
        that the PBB I-Tag includes also some reserved bits, and most
        importantly the C-MAC DA and SA. What is designated as 6 bytes
        in the figure is the I-tag information excluding the C-MAC DA
        and SA.

      -  An optional Backbone VLAN field (BVLAN) may be used if grouping
        of tenant domains is desired.

      -  An outer Backbone MAC header contains the source and
        destination MAC addresses for the related server blades,
        assuming the PBB encapsulation is done at the hypervisor
        virtual switch on the server blade.

      -  The total resulting PBB overhead added to the VM-originated
        Ethernet frame is 18 or 22 Bytes (depending on whether the BVID
        is excluded or not)

      -  Note that the original PBB encapsulation allows the use of
        CVLAN and SVLAN in between the VM MACs and IP Payload. These
        fields were removed from Figure 3 since in a VM environment
        these fields do not need to be used on the VSw, their function
        is relegated to the I-SID tag.

6.2.1. Addressing VLAN space exhaustion and MAC explosion

   In a DC environment, PBB maintains traditional Ethernet forwarding
   plane and operational model. For example, a VSw implementation of PBB
   can make use of the 24 bit ISID tag instead of the 12 bit VLAN tag to
   identify the virtual bridging domains associated with different VM
   groups. The VSw uplink towards the ToR in Figure 1 can still be
   treated as an Ethernet backbone interface. A frame originated by a VM
   can be encapsulated with the ISID assigned to the VM VSw interface
   and with the outer DA and SA MACs associated with the respective
   destination and source server blades, and then sent to the ToR
   switch. Performing this encapsulation at the VSw distributes the VM
   MAC learning to server blades with instances in the corresponding
   layer2 domain, and therefore alleviates this load from ToRs that
   aggregate multiple server blades. Alternatively, the PBB
   encapsulation can be done at the ToR.

   With PBB encapsulation, ToRs and Core SWs do not have to handle VM
   MAC addresses so the size of their MAC FIB tables may decrease by two
   or more orders of magnitude, depending on the number of VMs




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   configured in each server blade and the number of VM virtual
   interfaces and associated MACs.

   The original PBB specification [802.1ah] did not introduce any new
   control plane or new forwarding concepts for the PBB core. Spanning
   Tree and regular Ethernet switching based on MAC Learning and
   Flooding were maintained to provide a smooth technology introduction
   in existing Ethernet networks.

6.2.2. Fast convergence and L2 multi-pathing

   Additional specification work for PBB control plane has been done
   since then in both IEEE and IETF L2VPN.

   As stated earlier, STP-based layer2 networks underutilize the
   available network capacity as links are put in an idle state to
   prevent loops. Similarly, existing VPLS technology for
   interconnecting Layer2 network-islands over an IP/MPLS core does not
   support active-active dual homing scenarios.

   IS-IS controlled layer2 networks allow traffic to flow on multiple
   parallel paths between any two servers, spreading traffic among
   available links on the path. IEEE 802.1aq Shortest Path Bridging
   (SPB) [802.1aq] and emerging IEEE 802.1Qbp [802.1Qbp] are PBB control
   plane technologies that utilize different methods to compute parallel
   paths and forward traffic in order to maximize the utilization of
   available links in a DC. In addition, a BGP based solution [PBB-EVPN]
   was submitted and discussed in IETF L2VPN WG.

   One or both mechanisms may be employed as required. IS-IS could be
   used inside the same administrative domain (e.g., a DC), while BGP
   may be employed to provide reachability among interconnected
   Autonomous Systems. Similar architectural models have been widely
   deployed in the Internet and for large VPN deployments.

   IS-IS and/or BGP are also used to advertise Backbone MAC addresses
   and to eliminate B-MAC learning and unknown unicast flooding in the
   forwarding plane, albeit with tradeoffs. The BMAC FIB entries are
   populated as required from the resulting IS-IS or BGP RIBs.

   Legacy loop avoidance schemes using Spanning Tree and local
   Active/Active MC-LAG are no longer required as their function (layer2
   routing) is replaced by the indicated routing protocols (IS-IS and
   BGP).





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6.2.3. Per ISID flood containment

   Service auto-discovery provided by 802.1aq SPB [802.1aq] and BGP
   [PBB-EVPN] is used to distribute ISID related information among DC
   nodes, eliminating any provisioning touches throughout the PBB
   infrastructure. This implicitly creates backbone distribution trees
   that provide per ISID automatic flood and multicast containment.

6.2.4. Efficient multicast support

   IS-IS [802.1aq] and BGP [PBB-EVPN] could be used to build optimal
   multicast distribution trees. In addition, PBB and IP/MPLS tunnel
   hierarchy may be used to aggregate multiple customer multicast trees
   sharing the same nodes by associating them with the same backbone
   forwarding tree that may be represented by a common Group BMAC and
   optionally a P2MP LSP. More details will be discussed in a further
   version of the draft.

6.2.5. Tunneling options for PBB ELAN: Ethernet, IP and MPLS

   The previous section introduces a solution for DC ELAN domains based
   on PBB ISIDs, PBB encapsulation and IS-IS and/or BGP control plane.

   IETF L2 VPN specifications [PBB-VPLS] or [PBB-EVPN] enable the
   transport of PBB frames using PW/MPLS or simply MPLS, and implicitly
   allow the use of MPLS Traffic Engineering and Resiliency toolset to
   provide for advanced traffic steering and faster convergence.

   Transport over IP/L2TPv3 [RFC 4719] or IP/GRE is also possible as an
   alternative to MPLS tunneling. Additional header optimization for PBB
   over IP/GRE encapsulated packets may also be feasible. These
   specifications would allow for ISID based L2 overlay using a regular
   IP backbone.

6.2.6. Use Case examples

6.2.6.1. PBBN in DC, L2VPN in DC GW

   DC environments based on VLANs and native Ethernet operational model
   may want to consider using the native PBB option to provide L2 multi-
   tenancy, in effect the DC ELAN from Figure 2. An example of a network
   architecture that addresses this scenario is depicted in Figure 4:

                                ,---------.
                             ,'  Inter-DC  `.
                             (L2VPN (PBB-VPLS)



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                             `.or PBB-EVPN),'
                               `|-------|-'
                             +--+--+   +-+---+
                             |PE GW|+-+|PE GW|
                            .+-----+   +-----+.
                         .'                    `-.
                      .-'                         `\
                    ,'                              `.
                    +          Intra-DC PBBN          \
                   |                                  +
                   :                                  ;
                    `\+------+   +------+  +--+----+-'
                      | ToR  |.. | ToR  |..|  ToR  |
                      +-+--+-+   +-+--+-+  +-+--+--+
                       .'PBB `.   .'PBB `.  .'PBB `.
                   +--+-+    +-+-++     +-++-+    +-+--+
                   |VSw |    :VSw :     :VSw :    :VSw :
                   +----+    +----+     +----+    +----+

       Figure 4 PBB in DC, PBB-VPLS or PBB-EVPN for DC Interconnect

   PBB inside the DC core interoperates seamlessly with VPLS used for L2
   DC-Interconnect to extend ELAN domains across DCs. This expansion may
   be required to address VM Mobility requirements or to balance the
   load on DC PE gateways. Note than in PBB-VPLS case, just one or a
   handful of infrastructure B-VPLS instances are required, providing
   Backbone VLAN equivalent function.

   PBB encapsulation addresses the expansion of the ELAN service
   identification space with 16M ISIDs and solves MAC explosion through
   VM MAC hiding from the Ethernet core.

   PBB SPB [802.1aq] is used for core routing in the ToRs, Core SWs and
   PEs. If the DCs that need to be interconnected at L2 are part of the
   same administrative domain, and scaling is not an issue, SPB/IS-IS
   may be extended across the VPLS infrastructure. If different AS
   domains are present, better load balancing is required between the
   DCs and the WAN, or IS-IS extension across DCs causes scaling issues,
   then BGP extensions described in [PBB-EVPN] must be employed.

   The forwarding plane, MAC FIB requirements and the Layer2 operational
   model in the ToR and Core SW are maintained. The VSw sends PBB
   encapsulated frames to the ToR as described in the previous section.
   ToRs and Core SWs still perform standard Ethernet switching using the
   outer Ethernet header.




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   From a control plane perspective, VSw uses a default gateway
   configuration to send traffic to the ToR, as in regular IP routing
   case. VSw BMAC learning on the ToR is done through either LLDP or VM
   Discovery Protocol (VDP) described in [802.1Qbg]. Identical
   mechanisms may be used for the ISID. Once this information is learned
   on the ToR it is automatically advertised through SPB. If PBB-EVPN is
   used in the DC GWs, MultiProtcol (MP)-BGP will be used to advertise
   the ISID and BMAC over the WAN as described in [PBB-EVPN].

6.2.6.2. PBBN in VSw, L2VPN in the ToR

   A variation of the use case example from the previous section is
   depicted in Figure 5:

                                ,---------.
                             ,'  Inter-DC  `.
                             (L2VPN (PBB-VPLS)
                             `.or PBB-EVPN),'
                               `|-------|-'
                             +--+--+   +-+---+
                             |PE GW|+-+|PE GW|
                            .+-----+   +-----+.
                         .'                    `-.
                      .-'                         `\
                    ,'                              `.
                   +        Intra-DC L2VPN over       \
                   |        IP or MPLS tunneling       +
                   :                                  ;
                    `\+------+   +------+  +--+----+-'
                      | ToR  |.. | ToR  |..|  ToR  |
                      +-+--+-+   +-+--+-+  +-+--+--+
                       .'PBB `.   .'PBB `.  .'PBB `.
                   +--+-+    +-+-++     +-++-+    +-+--+
                   |VSw |    :VSw :     :VSw :    :VSw :
                   +----+    +----+     +----+    +----+

                   Figure 5 PBB in VSw, L2VPN at the ToR

   The procedures from the previous section are used at the VSw: PBB
   encapsulation and Ethernet BVLANs can be used on the VSw uplink.
   L2VPN infrastructure is replacing the BVLAN at the ToR enabling the
   use of IP (GRE or L2TP) or MPLS tunneling.

   L2 networking still has the same control plane choices: IS-IS
   [802.1aq] and/or BGP [PBB-EVPN], independently from the tunneling
   choice.



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6.2.7. Connectivity to existing VPN sites and Internet

   The main reason for extending the ELAN space beyond the 4K VLANs is
   to be able to serve multiple DC tenants whereby the total number of
   service domains needed exceeds 4K. Figure 6 represents the logical
   service view where PBB ELANs are used inside one or multiple DCs to
   connect to existing IP VPN sites. It should be noted that the PE GW
   should be able to perform integrated routing in a VPN context and
   bridging in VSI context:

                   Tenant 1 sites connected over IP VPN

                          ,--+-'.              ;-`.--.
                         (  PE  ) VRFs on PEs .  PE  )
                          '-----'              '-----'
                             |                    |
                     ,-------------------------------.
                    (     IP VPN over IP/MPLS WAN     )
                     `---.'-----------------------`.-'
                      +--+--+ IP VPN VRF on PE GWs +-+---+
                 .....|PE GW|......                |PE GW|
   DC with PBB   |    +-----+      |               +--+--+
   Tenant 1      |                 |PBB ELAN12        |
   view       PBB|ELAN11     ......|......         PBB|ELAN13
             '':'''''''':'       |        |     '':'''''''':'
              ,'.      ,'.      ,+.      ,+.     ,'.      ,'.
             (VM )....(VM )    (VM )... (VM )   (VM )....(VM )
              `-'      `-'      `-'      `-'     `-'      `-'
                        Compute Resources inside DC

                 Figure 6 Logical Service View with IP VPN

   DC ELANs are identified with 24-bit ISIDs instead of VLANs. At the PE
   GWs, an IP VPN VRF is configured for every DC tenant. Each "ISID
   ELAN" for Tenant 1 is seen as a logical Ethernet endpoint and is
   assigned an IP interface on the Tenant 1 VRF. Tenant 1 enterprise
   sites are connected to IP VPN PEs distributed across the WAN. IP VPN
   instances on PE GWs can be automatically discovered and connected to
   the WAN IP VPN using standard procedures - see [RFC4364].

   In certain cases, the DC GW PEs are part of the IPVPN service
   provider network providing IPVPN services to the enterprise
   customers. In other cases, DC PEs are operated and managed by the
   DC/cloud provider and interconnect to multiple IPVPN service
   providers using inter-AS BGP/MPLS models A, B, or C [RFC4364]. The




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   same discussion applies to the case of IPSec VPNs from a PBB ELAN
   termination perspective.

   If tenant sites are connected to the DC using WAN VPLS, the PE GWs
   need to implement the BEB function described in the PBB-VPLS PE model
   [PBB-VPLS] and the procedures from [PBB-Interop] to perform the
   required translation. Figure 7 describes the VPLS WAN scenario:

                    Customer sites connected over VPLS

                          ,--+-'.              ;-`.--.
                         (  PE  ) VPLS on PEs .  PE  )
                          '-----'              '-----'
                             |                    |
                     ,-------------------------------.
                    (      VPLS over IP/MPLS WAN     )
                     `---.'-----------------------`.-'
                      +--+--+                      +-+---+
                      |PE GW| <-- PBB-VPLS/BEB --> |PE GW|
        DC with PBB   +--+--+                      +--+--+
           Tenant 1      |                            |
             view     PBB|ELAN11                   PBB|ELAN13
                    '':'''''''':'               '':'''''''':'
                     ,'.      ,'.                ,'.      ,'.
                    (VM ) .. (VM )              (VM ) .. (VM )
                     `-'      `-'                `-'      `-'
                        Compute Resources inside DC

                Figure 7 Logical Service View with VPLS WAN

   One VSI is required at the PE GW for every DC ELAN domain. Same as in
   the IP VPN case, DC PE GWs may be fully integrated as part of the WAN
   provider network or using Inter-AS/Inter-provider models A,B or C.

   The VPN connectivity may be provided by one or multiple PE GWs,
   depending on capacity need and/or the operational model used by the
   DC/cloud operator.

   If a VM group is serving Internet connected customers, the related
   ISID ELAN will be terminated into a routing context (global public
   instance or another VRF) connected to the Internet. Same as in the IP
   VPN case, the 24bit ISID will be represented as a logical Ethernet
   endpoint on the Internet routing context and an IP interface will be
   allocated to it. Same PE GW may be used to provide both VPN and
   Internet connectivity with the routing contexts separated internally
   using the IP VPN models.



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6.2.8. DC Interconnect

   L2 DC interconnect may be required to expand the ELAN domains for
   Management, VM Mobility or when a VM Group needs to be distributed
   across DCs.

   PBB may be used to provide ELAN extension across multiple DCs as
   depicted in Figure 8:

                     ,-------------------------------.
                    (           IP/MPLS WAN          )
                     `---.'------------------------`.'
                      +--+--+                      +-+---+
                      |PE GW| <----- PBB BCB ----> |PE GW|
        DC with PBB   +--+--+                      +--+--+
           Tenant 1      |                            |
             view     PBB|ELAN11                   PBB|ELAN11
                    '':'''''''':'               '':'''''''':'
                     ,'.      ,'.                ,'.      ,'.
                    (Hvz) .. (Hvz)              (Hvz) .. (Hvz)
                     `-'      `-'                `-'      `-'
                        Compute Resources inside DC

                  Figure 8 PBB BCB providing VMotion ELAN

   ELAN11 is expanded across DC to provide interconnect for the pool of
   server blades assigned to the same VMotion domain. This time
   Hypervisors are connected directly to ELAN11. The PE GW operates in
   this case as a PBB Backbone Core Bridge (BCB) [PBB-VPLS] combined
   with PBB-EVPN capabilities [PBB-EVPN]. The ISID ELANs do not require
   any additional provisioning touches and do not consume additional
   MPLS resources on the PE GWs. Per ISID auto-discovery and flood
   containment is provided by IS-IS/SPB [802.1aq] and BGP [PBB-EVPN].

6.2.9. Interoperating with existing DC VLANs

   While green field deployments will definitely benefit from all the
   advantages described in the previous sections, in many other
   scenarios, existing DC VLAN environments will have to be gradually
   migrated to the new architecture. Figure 9 depicts an example of a
   possible migration scenario where both PBB and VLAN technologies are
   present:

                                ,---------.
                             ,'  Inter-DC  `.
                             (L2VPN (PBB-VPLS)



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                             `.or PBB-EVPN),'
                               `-/------\-'
                             +---+-+   +-+---+
                             |PE GW|+-+|PE GW|
                           .-+-----+   +-----+:-.
                       .-'                      `-.
                     ,'                            `-:.
                    +          PBBN/SPB DC             \
                    |                                  +
                    :                                  ;
                     `-+------+   +------+  +--+----+-'
                       | ToR  |.. | ToR  |..|  ToR  |
                       +-+--+-+   +-+--+-+  +-+--+--+
                       .'PBB `.   .'    `.  .'VLAN`.
                   +--+-+    +-+-++     +-++-+    +-+--+
                   |VSw |    :VSw :     :VSw :    :VSw :
                   +----+    +----+     +----+    +----+
                      Figure 9 DC with PBB and VLANs

   This example assumes that the two VSWs on the right do not support
   PBB but the ToRs do. The VSw on the left side are running PBB while
   the ones on the right side are still using VLANs. The left ToR is
   performing only Ethernet switching whereas the one on the right is
   translating from VLANs to ISIDs and performing PBB encapsulation
   using the BEB function [802.1ah] and [PBB-VPLS]. The ToR in the
   middle is performing both functions: core Ethernet tunneling for the
   PBB VSw and BEB function for the VLAN VSw.

   The SPB control plane is still used between the ToRs, providing the
   benefits described in the previous section. The VLAN VSw must use
   regular multi-homing functions to the ToRs: for example STP or Multi-
   chassis-LAG.

   DC VLANs may be also present initially on some of the legacy ToRs or
   Core SWs. PBB interoperability will be performed as follows:

     .  If VLANs are used in the ToRs, PBB BEB function may be
        performed by the Core SW(s) where the ToR uplink is connected

     .  If VLANs are used in the Core SW, PBB BEB function may be
        performed by the PE GWs where the Core SW uplink is connected

   It is possible that some DCs may run PBB or PBB-VLAN combination
   while others may still be running VLANs. An example of this
   interoperability scenario is described in Figure 10:




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                     ,-------------------------------.
                    (          IP/MPLS WAN            )
                    `------/-----------------\-------'
                        +--/--+             +--\--+
                        |PE GW|PBB-VPLS     |PE GW|VPLS
                      .'+-----+-'         .'+-----+'.
                     /           \       /           \
                    |            |      |            |
                    |   PBB DC   |      |  VLAN DC   |
                     \           /       \           /
                      +---+ +---+         +---+ +---+
                      |VSw|.|VSw|         |VSw|.|VSw|
                      +---+ +---+         +---+ +---+
               Figure 10 Interoperability to a VLAN-based DC

   Interoperability with existing VLAN DC is required for DC
   interconnect. The PE-GW in the PBB DC or the PE GW in the VLAN DC
   must implement PBB-VPLS PE model described in [PBB-VPLS]. This
   interoperability scenario is addressed in detail in [PBB-Interop].

   Connectivity to existing VPN customer sites (IP VPN, VPLS, IPSec) or
   Internet does not require any additional procedures beyond the ones
   described in the VPN connectivity section. The PE GW in the DC VLAN
   will aggregate DC ELANs through IP interfaces assigned to VLAN
   logical endpoints whereas the PE GW in the PBB DC will assign IP
   interfaces to ISID logical endpoints.

   If EVPN is used to interconnect the two DCs, PBB-EVPN functions
   described in [PBB-EVPN] must be implemented in one of the PE-GWs.

6.3. TRILL and L2VPN toolset

   TRILL and SPB control planes provide similar functions. IS-IS is the
   base protocol used in both specifications to provide multi-pathing
   and fast convergence for core networking. [PBB-EVPN] describes how
   seamless Inter-DC connectivity can be provided over an MPLS/IP
   network for both TRILL [RFC6325] and SPB [802.1aq]/[802.1Qbp]
   networks.

   The main differences exist in the encapsulation and data plane
   forwarding. TRILL encapsulation [RFC6325] was designed initially for
   large enterprise and campus networks where 4k VLANs are sufficient.
   As a consequence the ELAN space in [RFC6325] is limited to 4K VLANs;
   however, this VLAN scale issue is being addressed in [Fine-Grained].




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7. L3VPN applicability to Cloud Networking

   This section discusses the role of IP VPN technology in addressing
   the L3 Virtualization challenges described in section 5.

   IP VPN technology defined in L3VPN working group may be used to
   provide L3 virtualization in support of multi-tenancy in the DC
   network as depicted in Figure 11.

                     ,-------------------------------.
                    (     IP VPNs over IP/MPLS WAN    )
                    `----.'------------------------`.'
                      ,--+-'.                      ;-`.--.
                 .....  VRF1 )......              .  VRF2 )
                 |    '-----'      |               '-----'
                 |     Tenant1     |ELAN12     Tenant1|
                 |ELAN11       ....|........          |ELAN13
             '':'''''''':'       |        |     '':'''''''':'
              ,'.      ,'.      ,+.      ,+.     ,'.      ,'.
             (VM )....(VM )    (VM )... (VM )   (VM )....(VM )
              `-'      `-'      `-'      `-'     `-'      `-'
                Figure 11 Logical Service View with IP VPN

   Tenant 1 might buy Cloud Services in different DC locations and
   choose to associate the VMs in 3 different groups, each mapped to a
   different ELAN: ELAN11, ELAN12 and ELAN13. L3 interconnect between
   the ELANs belonging to tenant1 is provided using an IP/MPLS VPN and
   associated VRF1 and VRF2, possibly located in different DCs. Each
   tenant that requires L3 virtualization will be allocated a different
   IP VPN instance. Using full fledge IP VPN for L3 Virtualization
   inside a DC presents the following advantages compared with existing
   DC technologies like Virtual Routing:

      -  Interoperates with existing WAN VPN technology

      -  Deployment tested, provides a full networking toolset

      -  Scalable core routing - only one BGP-MP routing instance is
        required compared with one per customer/tenant in the Virtual
        Routing case

      -  Service Auto-discovery - automatic discovery and route
        distribution between related service instances

      -  Well defined and deployed Inter-Provider/Inter-AS models




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      -  Supports a variety of VRF-to-VRF tunneling options
        accommodating different operational models: MPLS [RFC4364], IP
        or GRE [RFC4797]

   To provide Cloud services to related customer IP VPN instances
   located in the WAN the following connectivity models may be employed:

      -  DC IP VPN instance may participate directly in the WAN IP VPN

      -  Inter-AS Options A, B or C models may be employed with
        applicability to both Intra and Inter-Provider use cases
        [RFC4364]

8. Solutions for other DC challenges

   This section touches on some of the DC challenges that may be
   addressed by a combination of IP VPN, L2VPN and IP toolset.
   Additional details will be provided in a future revision.

8.1. Addressing IP/ARP explosion

   Possible solutions for IP/ARP explosion are discussed in [EVPN],
   [PBB-EVPN], [ARPproxy] and in ARMD WG that address certain aspects.
   More discussion is required to clarify the requirements in this
   space, taking into account the different network elements potentially
   impacted by ARP.

8.2. Optimal traffic forwarding

   IP networks, built using links-state protocols such as OSPF or ISIS
   and BGP provide optimal traffic forwarding through the use of equal
   cost multiple path (ECMP) and ECMP traffic load-balancing, and the
   use of traffic engineering tools based on BGP and/or MPLS-TE as
   applicable. In the Layer2 case, SPB or TRILL based protocols provide
   for load-balancing across parallel paths or equal cost paths between
   two nodes. Traffic follows the shortest path. For multicast, data
   plane replication at layer2 or layer3 happens in the data plane
   albeit with different attributes after multicast trees are built via
   a control plane and/or snooping. In the presence of VM mobility,
   optimal forwarding relates to avoiding triangulation and providing
   for optimum forwarding between any two VMs.

8.3. VM Mobility

   IP VPN technology may be used to support DC Interconnect for
   different functions like VM Mobility and Cloud Management. A



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   description of VM Mobility between server blades located in different
   IP subnets using extensions to existing BGP-MP and IP VPN procedure
   is described in [VM-Mobility]. Other solutions can exist as well.
   What is needed is a solution that provides for fast convergence
   toward the steady state whereby communication among any two VMs can
   take place on the shortest path or most optimum path, transit
   triangulation time is minimized, traffic black-holing is avoided, and
   impact on routing scale for both IPv4 and IPv6 is controllable or
   minimized.

9. Security Considerations

   No new security issues are introduced beyond those described already
   in the related L2VPN drafts.

10. IANA Considerations

   IANA does not need to take any action for this draft.

11. References

11.1. Normative References

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

   [RFC4761] Kompella, K. and Rekhter, Y. (Editors), "Virtual Private
             LAN Service (VPLS) Using BGP for Auto-Discovery and
             Signaling", RFC 4761, January 2007.

   [RFC4762] Lasserre, M. and Kompella, V. (Editors), "Virtual Private
             LAN Service (VPLS) Using Label Distribution Protocol (LDP)
             Signaling", RFC 4762, January 2007.

   [PBB-VPLS] Balus, F. et al. "Extensions to VPLS PE model for Provider
             Backbone Bridging", draft-ietf-l2vpn-pbb-vpls-pe-model-
             04.txt (work in progress), October 2011.

   [PBB-Interop] Sajassi, A. et al. "VPLS Interoperability with Provider
             Backbone Bridging", draft-ietf-l2vpn-pbb-vpls-interop-
             02.txt (work in progress), July 2011.

   [802.1ah] IEEE 802.1ah "Virtual Bridged Local Area Networks,
             Amendment 6: Provider Backbone Bridges", Approved Standard
             June 12th, 2008




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   [802.1aq] IEEE Draft P802.1aq/D4.3 "Virtual Bridged Local Area
             Networks, Amendment: Shortest Path Bridging", Work in
             Progress, September 21, 2011

   [RFC6325] Perlman, et al., "Routing Bridges (Rbridges): Base Protocol
             Specification", RFC 6325, July 2011.

   [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
             Networks (VPNs)", RFC 4364, February 2006.

   [RFC4797] Rosen, E. and Y. Rekhter, " Use of Provider Edge to
             Provider Edge (PE-PE) Generic Routing encapsulation (GRE)
             or IP in BGP/MPLS IP Virtual Private Networks ", RFC 4797,
             January 2007.



11.2. Informative References

   [RFC4026] Andersson, L. et Al., "Provider Provisioned Virtual Private
             Network (VPN) Terminology", RFC 4026, May 2005.

   [802.1Qbp] IEEE Draft P802.1Qbp/D0.1 "Virtual Bridged Local Area
             Networks, Amendment: Equal Cost Multiple Paths (ECMP)",
             Work in Progress, October 13, 2011

   [802.1Qbg] IEEE Draft P802.1Qbg/D1.8 "Virtual Bridged Local Area
             Networks, Amendment: Edge Virtual Bridging", Work in
             Progress, October 17, 2011

   [EVPN] Raggarwa, R. et al. "BGP MPLS based Ethernet VPN", draft-
             raggarwa-sajassi-l2vpn-evpn-04.txt (work in progress),
             September 2011.

   [PBB-EVPN] Sajassi, A. et al. "PBB-EVPN", draft-sajassi-l2vpn-pbb-
             evpn-02.txt (work in progress), July 2011.

   [VM-Mobility] Raggarwa, R. et al. "Data Center Mobility based on
             BGP/MPLS, IP Routing and NHRP", draft-raggarwa-data-center-
             mobility-01.txt (work in progress), September 2011.

   [RFC4719] Aggarwal, R. et al., "Transport of Ethernet over Layer 2
             Tunneling Protocol Version 3 (L2TPv3)", RFC 4719, November
             2006.





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   [MVPN] Rosen, E. and Raggarwa, R. "Multicast in MPLS/BGP IP VPN",
             draft-ietf-l3vpn-2547bis-mcast-10.txt (work in progress),
             January 2010.

   [ARPproxy] Carl-Mitchell, S. and Quarterman, S., "Using ARP to
             implement transparent subnet gateways", RFC 1027, October
             1987.

   [MYERS] Myers, A., Ng, E. and Zhang, H., "Rethinking the Service
             Model: Scaling Ethernet to a Million Nodes"
             http://www.cs.cmu.edu/~acm/papers/myers-hotnetsIII.pdf

   [Fine-Grained] Eastlake, D. et Al., "RBridges: Fine-Grained
             Labeling", draft-eastlake-trill-rbridge-fine-labeling-
             01.txt (work in progress), October 2011.

12. Acknowledgments

   In addition to the authors the following people have contributed to
   this document:

   Javier Benitez, Colt

   Dimitrios Stiliadis, Alcatel-Lucent

   Samer Salam, Cisco

   This document was prepared using 2-Word-v2.0.template.dot.





















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Authors' Addresses

   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   Email: nabil.bitar@verizon.com

   Florin Balus
   Alcatel-Lucent
   777 E. Middlefield Road
   Mountain View, CA, USA 94043
   Email: florin.balus@alcatel-lucent.com

   Marc Lasserre
   Alcatel-Lucent
   Email: marc.lasserre@alcatel-lucent.com
































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

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

   Luyuan Fang
   Cisco
   111 Wood Avenue South
   Iselin, NJ 08830
   Email: lufang@cisco.com

   Yuichi Ikejiri
   NTT Communications
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo, 100-8019 Japan
   Email: y.ikejiri@ntt.com


   Mircea Pisica
   BT
   Telecomlaan 9
   Brussels 1831, Belgium
   Email: mircea.pisica@bt.com





















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