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Versions: 00 01 02 03 04 05 06 07 08 09 10 11                           
Network working group                                             X. Xu
Internet Draft                                      Huawei Technologies
Category: Informational
                                                               S. Hares

                                                                 Y. Fan
                                                          China Telecom

                                                           C. Jacquenet
                                                         France Telecom

Expires: January 2014                                     July 15, 2013


          Virtual Subnet: A L3VPN-based Subnet Extension Solution

                      draft-xu-virtual-subnet-11


Abstract

   This document describes a Layer3 Virtual Private Network (L3VPN)-
   based subnet extension solution referred to as Virtual Subnet, which
   can be used as a kind of Layer3 network virtualization overlay
   approach for data center interconnect.

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
   Task Force (IETF), its areas, and its working groups. Note that
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   Drafts.

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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.





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   This Internet-Draft will expire on January 15, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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].

Table of Contents

   1. Introduction ................................................ 4
   2. Terminology ................................................. 6
   3. Solution Description......................................... 6
      3.1. Unicast ................................................ 6
         3.1.1. Intra-subnet Unicast .............................. 6
         3.1.2. Inter-subnet Unicast .............................. 7
      3.2. Multicast .............................................. 9
      3.3. CE Host Discovery ...................................... 9
      3.4. ARP/ND Proxy .......................................... 10
      3.5. CE Host Mobility ...................................... 10
      3.6. Forwarding Table Scalability .......................... 10
         3.6.1. MAC Table Reduction on Data Center Switches ...... 10
         3.6.2. PE Router FIB Reduction .......................... 11
         3.6.3. PE Router RIB Reduction .......................... 12
      3.7. ARP/ND Cache Table Scalability on Default Gateways .... 14
      3.8. ARP/ND and Unknown Uncast Flood Avoidance ............. 14
      3.9. Path Optimization ..................................... 14
   4. Considerations for Non-IP traffic .......................... 15
   5. Security Considerations .................................... 15
   6. IANA Considerations ........................................ 15
   7. Acknowledgements ........................................... 15
   8. References ................................................. 15



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      8.1. Normative References .................................. 15
      8.2. Informative References ................................ 15
   Authors' Addresses ............................................ 16













































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1. Introduction

   For business continuity purposes, Virtual Machine (VM) migration
   across data centers is commonly used in those situations such as data
   center maintenance, data center migration, data center consolidation,
   data center expansion, and data center disaster avoidance. It's
   generally admitted that IP renumbering of servers (i.e., VMs) after
   the migration is usually complex and costly at the risk of extending
   the business downtime during the process of migration. To allow the
   migration of a VM from one data center to another without IP
   renumbering, the subnet on which the VM resides needs to be extended
   across these data centers.

   In Infrastructure-as-a-Service (IaaS) cloud data center environments,
   to achieve subnet extension across multiple data centers in a
   scalable way, the following requirements SHOULD be considered for any
   data center interconnect solution:

    1) VPN Instance Space Scalability

      In a modern cloud data center environment, thousands or even tens
      of thousands of tenants could be hosted over a shared network
      infrastructure. For security and performance isolation purposes,
      these tenants need to be isolated from one another. Hence, the
      data center interconnect solution SHOULD be capable of providing a
      large enough Virtual Private Network (VPN) instance space for
      tenant isolation.

   2) Forwarding Table Scalability

      With the development of server virtualization technologies, a
      single cloud data center containing millions of VMs is not
      uncommon. This number already implies a big challenge for data
      center switches, especially for core/aggregation switches, from
      the perspective of forwarding table scalability. Provided that
      multiple data centers of such scale were interconnected at layer2,
      this challenge would be even worse. Hence an ideal data center
      interconnect solution SHOULD prevent the forwarding table size of
      data center switches from growing by folds as the number of data
      centers to be interconnected increases. Furthermore, if any kind
      of L2VPN or L3VPN technologies is used for interconnecting data
      centers, the scale of forwarding tables on PE routers SHOULD be
      taken into consideration as well.

   3) ARP/ND Cache Table Scalability on Default Gateways




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      [RFC6820] notes that the Address Resolution Protocol
      (ARP)/Neighbor Discovery (ND) cache tables maintained by data
      center default gateways in cloud data centers can raise both
      scalability and security issues. Therefore, an ideal data center
      interconnect solution SHOULD prevent the ARP/ND cache table size
      from growing by multiples as the number of data centers to be
      connected increases.

   4) ARP/ND and Unknown Unicast Flood Suppression or Avoidance

      It's well-known that the flooding of Address Resolution Protocol
      (ARP)/Neighbor Discovery (ND) broadcast/multicast and unknown
      unicast traffic within a large Layer2 network are likely to affect
      performances of networks and hosts. As multiple data centers each
      containing millions of VMs are interconnected together across the
      Wide Area Network (WAN) at layer2, the impact of flooding as
      mentioned above will become even worse. As such, it becomes
      increasingly desirable for data center operators to suppress or
      even avoid the flooding of ARP/ND broadcast/multicast and unknown
      unicast traffic across data centers.

   5) Path Optimization

      A subnet usually indicates a location in the network. However,
      when a subnet has been extended across multiple geographically
      dispersed data center locations, the location semantics of such
      subnet is not retained any longer. As a result, the traffic from a
      cloud user (i.e., a VPN user) which is destined for a given server
      located at one data center location of such extended subnet may
      arrive at another data center location firstly according to the
      subnet route, and then be forwarded to the location where the
      service is actually located. This suboptimal routing would
      obviously result in the unnecessary consumption of the bandwidth
      resources which are intended for data center interconnection.
      Furthermore, in the case where the traditional VPLS technology
      [RFC4761, RFC4762] is used for data center interconnect and
      default gateways of different data center locations are configured
      within the same virtual router redundancy group, the returning
      traffic from that server to the cloud user may be forwarded at
      layer2 to a default gateway located at one of the remote data
      center premises, rather than the one placed at the local data
      center location. This suboptimal routing would also unnecessarily
      consume the bandwidth resources which are intended for data center
      interconnect.

   This document describes a L3VPN-based subnet extension solution
   referred to as Virtual Subnet (VS), which can meet all of the



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   requirements of cloud data center interconnect as described above.
   Since VS mainly reuses existing technologies including BGP/MPLS IP
   VPN [RFC4364] and ARP/ND proxy [RFC925][RFC1027][RFC4389], it allows
   those service providers offering IaaS public cloud services to
   interconnect their geographically dispersed data centers in a much
   scalable way, and more importantly, data center interconnection
   design can rely upon their existing MPLS/BGP IP VPN infrastructures
   and their experiences in the delivery and the operation of MPLS/BGP
   IP VPN services.

   Although Virtual Subnet is described as a data center interconnection
   solution in this document, there is no reason to assume that this
   technology couldn't be used within data centers.

2. Terminology

   This memo makes use of the terms defined in [RFC4364], [RFC2338]
   [MVPN] and [VA-AUTO].

3. Solution Description

3.1. Unicast

   3.1.1. Intra-subnet Unicast
                                  +--------------------+
            +-----------------+   |                    |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |    +------+   \++---+-+                +-+---++/   +------+    |
            |    |Host A+----+ PE-1 |                | PE-2 +----+Host B|    |
            |    +------+\   ++-+-+-+                +-+-+-++   /+------+    |
            |     1.1.1.2/24  | | |                    | | |  1.1.1.3/24     |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |     DC East     |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |
        VRF_A :                 V                VRF_A : V
         +------------+---------+--------+        +------------+---------+--------+
         |   Prefix   | Nexthop |Protocol|        |   Prefix   | Nexthop |Protocol|
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.1/32 |127.0.0.1| Direct |        | 1.1.1.1/32 |127.0.0.1| Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.2/32 | 1.1.1.2 | Direct |        | 1.1.1.2/32 |   PE-1  |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.3/32 |   PE-2  |  IBGP  |        | 1.1.1.3/32 | 1.1.1.3 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/24 | 1.1.1.1 | Direct |        | 1.1.1.0/24 | 1.1.1.1 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
                   Figure 1: Intra-subnet Unicast Example


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   As shown in Figure 1, two CE hosts (i.e., Hosts A and B) belonging to
   the same subnet (i.e., 1.1.1.0/24) are located at different data
   centers (i.e., DC West and DC East) respectively. PE routers (i.e.,
   PE-1 and PE-2) which are used for interconnecting these two data
   centers create host routes for their local CE hosts respectively and
   then advertise them via L3VPN signaling. Meanwhile, ARP proxy is
   enabled on VRF attachment circuits of these PE routers.

   Now assume host A sends an ARP request for host B before
   communicating with host B. Upon receiving the ARP request, PE-1
   acting as an ARP proxy returns its own MAC address as a response.
   Host A then sends IP packets for host B to PE-1. PE-1 tunnels such
   packets towards PE-2 which in turn forwards them to host B. Thus,
   hosts A and B can communicate with each other as if they were located
   within the same subnet.

   3.1.2. Inter-subnet Unicast
                                  +--------------------+
            +-----------------+   |                    |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |  +------+     \++---+-+                +-+---++/     +------+  |
            |  |Host A+------+ PE-1 |                | PE-2 +-+----+Host B|  |
            |  +------+\     ++-+-+-+                +-+-+-++ |   /+------+  |
            |   1.1.1.2/24    | | |                    | | |  | 1.1.1.3/24   |
            |   GW=1.1.1.4    | | |                    | | |  | GW=1.1.1.4   |
            |                 | | |                    | | |  |    +------+  |
            |                 | | |                    | | |  +----+  GW  +--|
            |                 | | |                    | | |      /+------+  |
            |                 | | |                    | | |    1.1.1.4/24   |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |      DC East    |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |
        VRF_A :                 V                VRF_A : V
         +------------+---------+--------+        +------------+---------+--------+
         |   Prefix   | Nexthop |Protocol|        |   Prefix   | Nexthop |Protocol|
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.1/32 |127.0.0.1| Direct |        | 1.1.1.1/32 |127.0.0.1| Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.2/32 | 1.1.1.2 | Direct |        | 1.1.1.2/32 |  PE-1   |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.3/32 |   PE-2  |  IBGP  |        | 1.1.1.3/32 | 1.1.1.3 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.4/32 |   PE-2  |  IBGP  |        | 1.1.1.4/32 | 1.1.1.4 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/24 | 1.1.1.1 | Direct |        | 1.1.1.0/24 | 1.1.1.1 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 0.0.0.0/0  |   PE-2  |  IBGP  |        | 0.0.0.0/0  | 1.1.1.4 | Static |
         +------------+---------+--------+        +------------+---------+--------+
                 Figure 2: Inter-subnet Unicast Example (1)


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   As shown in Figure 2, only one data center (i.e., DC East) is
   deployed with a default gateway (i.e., GW). PE-2 which is connected
   to GW would either be configured with or learn from GW a default
   route with next-hop being pointed to GW. Meanwhile, this route is
   distributed to other PE routers (i.e., PE-1) as per normal [RFC4364]
   operation.  Assume host A sends an ARP request for its default
   gateway (i.e., 1.1.1.4) prior to communicating with a destination
   host outside of its subnet. Upon receiving this ARP request, PE-1
   acting as an ARP proxy returns its own MAC address as a response.
   Host A then sends a packet for Host B to PE-1. PE-1 tunnels such
   packet towards PE-2 according to the default route learnt from PE-2,
   which in turn forwards that packet to GW.
                                  +--------------------+
            +-----------------+   |                    |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |  +------+     \++---+-+                +-+---++/     +------+  |
            |  |Host A+----+-+ PE-1 |                | PE-2 +-+----+Host B|  |
            |  +------+\   | ++-+-+-+                +-+-+-++ |   /+------+  |
            |   1.1.1.2/24 |  | | |                    | | |  | 1.1.1.3/24   |
            |   GW=1.1.1.4 |  | | |                    | | |  | GW=1.1.1.4   |
            |  +------+    |  | | |                    | | |  |    +------+  |
            |--+ GW-1 +----+  | | |                    | | |  +----+ GW-2 +--|
            |  +------+\      | | |                    | | |      /+------+  |
            |   1.1.1.4/24    | | |                    | | |    1.1.1.4/24   |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |      DC East    |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |
        VRF_A :                 V                VRF_A : V
         +------------+---------+--------+        +------------+---------+--------+
         |   Prefix   | Nexthop |Protocol|        |   Prefix   | Nexthop |Protocol|
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.1/32 |127.0.0.1| Direct |        | 1.1.1.1/32 |127.0.0.1| Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.2/32 | 1.1.1.2 | Direct |        | 1.1.1.2/32 |  PE-1   |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.3/32 |   PE-2  |  IBGP  |        | 1.1.1.3/32 | 1.1.1.3 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.4/32 | 1.1.1.4 | Direct |        | 1.1.1.4/32 | 1.1.1.4 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/24 | 1.1.1.1 | Direct |        | 1.1.1.0/24 | 1.1.1.1 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 0.0.0.0/0  | 1.1.1.4 | Static |        | 0.0.0.0/0  | 1.1.1.4 | Static |
         +------------+---------+--------+        +------------+---------+--------+
                 Figure 3: Inter-subnet Unicast Example (2)

   As shown in Figure 3, in the case where each data center is deployed
   with a default gateway, CE hosts will get ARP responses directly from
   their local default gateways, rather than from their local PE routers
   when sending ARP requests for their default gateways.


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                                         +------+
                                  +------+ PE-3 +------+
            +-----------------+   |      +------+      |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |  +------+     \++---+-+                +-+---++/     +------+  |
            |  |Host A+------+ PE-1 |                | PE-2 +------+Host B|  |
            |  +------+\     ++-+-+-+                +-+-+-++     /+------+  |
            |   1.1.1.2/24    | | |                    | | |    1.1.1.3/24   |
            |   GW=1.1.1.1    | | |                    | | |    GW=1.1.1.1   |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |      DC East    |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |
        VRF_A :                 V                VRF_A : V
         +------------+---------+--------+        +------------+---------+--------+
         |   Prefix   | Nexthop |Protocol|        |   Prefix   | Nexthop |Protocol|
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.1/32 |127.0.0.1| Direct |        | 1.1.1.1/32 |127.0.0.1| Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.2/32 | 1.1.1.2 | Direct |        | 1.1.1.2/32 |  PE-1   |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.3/32 |   PE-2  |  IBGP  |        | 1.1.1.3/32 | 1.1.1.3 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/24 | 1.1.1.1 | Direct |        | 1.1.1.0/24 | 1.1.1.1 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 0.0.0.0/0  |   PE-3  |  IBGP  |        | 0.0.0.0/0  |   PE-3  |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
                 Figure 4: Inter-subnet Unicast Example (3)

   Alternatively, as shown in Figure 4, PE routers themselves could be
   directly configured as default gateways of their locally connected CE
   hosts as long as these PE routers have routes for outside networks.

3.2. Multicast

   To support IP multicast between CE hosts of the same virtual subnet,
   MVPN technology [MVPN] could be directly reused. For example, PE
   routers attached to a given VPN join a default provider multicast
   distribution tree which is dedicated for that VPN. Ingress PE routers,
   upon receiving multicast packets from their local CE hosts, forward
   them towards remote PE routers through the corresponding default
   provider multicast distribution tree.

   More details about how to support multicast and broadcast in VS will
   be explored in a later version of this document.

   3.3. CE Host Discovery

   PE routers SHOULD be able to discover their local CE hosts and keep
   the list of these hosts up to date in a timely manner so as to ensure


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   the availability and accuracy of the corresponding host routes
   originated from them. PE routers could accomplish local CE host
   discovery by some traditional host discovery mechanisms using ARP or
   ND protocols. Furthermore, Link Layer Discovery Protocol (LLDP)
   described in [802.1AB] or VSI Discovery and Configuration Protocol
   (VDP) described in [802.1Qbg], or even interaction with the data
   center orchestration system could also be considered as a means to
   dynamically discover local CE hosts.

   3.4. ARP/ND Proxy

   Acting as ARP or ND proxies, PE routers SHOULD only respond to an ARP
   request or Neighbor Solicitation (NS) message for the target host
   when there is a corresponding host route in the associated VRF and
   the outgoing interface of that route is different from the one over
   which the ARP request or the NS message arrived.

   In the scenario where a given VPN site (i.e., a data center) is
   multi-homed to more than one PE router via an Ethernet switch or an
   Ethernet network, Virtual Router Redundancy Protocol (VRRP) [RFC5798]
   is usually enabled on these PE routers. In this case, only the PE
   router being elected as the VRRP Master is allowed to perform the
   ARP/ND proxy function.

   3.5. CE Host Mobility

   During the VM migration process, the PE router to which the moving VM
   is now attached would create a host route for that CE host upon
   receiving a notification message of VM attachment while the PE router
   to which the moving VM was previously attached would withdraw the
   corresponding host route when receiving a notification message of VM
   detachment. Meanwhile, the latter PE router could optionally
   broadcast a gratuitous ARP/ND message on behalf of that CE host with
   source MAC address being one of its own. In the way, the ARP/ND entry
   of that moved CE host which has been cached on any local CE host
   would be updated accordingly.

   3.6. Forwarding Table Scalability

   3.6.1. MAC Table Reduction on Data Center Switches

   In a VS environment, the MAC learning domain associated with a given
   virtual subnet which has been extended across multiple data centers
   is partitioned into segments and each segment is confined within a
   single data center. Therefore data center switches only need to learn
   local MAC addresses, rather than learning both local and remote MAC
   addresses.



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   3.6.2. PE Router FIB Reduction
                                         +------+
                                  +------+RR/APR+------+
            +-----------------+   |      +------+      |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |  +------+     \++---+-+                +-+---++/     +------+  |
            |  |Host A+------+ PE-1 |                | PE-2 +------+Host B|  |
            |  +------+\     ++-+-+-+                +-+-+-++     /+------+  |
            |   1.1.1.2/24    | | |                    | | |    1.1.1.3/24   |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |      DC East    |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |
        VRF_A :                 V                VRF_A : V
      +------------+---------+--------+------+ +------------+---------+--------+------+
      |   Prefix   | Nexthop |Protocol|In_FIB| |   Prefix   | Nexthop |Protocol|In_FIB|
      +------------+---------+--------+------+ +------------+---------+--------+------+
      | 1.1.1.1/32 |127.0.0.1| Direct |  Yes | | 1.1.1.1/32 |127.0.0.1| Direct |  Yes |
      +------------+---------+--------+------+ +------------+---------+--------+------+
      | 1.1.1.2/32 | 1.1.1.2 | Direct |  Yes | | 1.1.1.2/32 |  PE-1   |  IBGP  |  No  |
      +------------+---------+--------+------+ +------------+---------+--------+------+
      | 1.1.1.3/32 |   PE-2  |  IBGP  |  No  | | 1.1.1.3/32 | 1.1.1.3 | Direct |  Yes |
      +------------+---------+--------+------+ +------------+---------+--------+------+
      | 1.1.1.0/25 |    RR   |  IBGP  |  Yes | | 1.1.1.0/25 |    RR   |  IBGP  |  Yes |
      +------------+---------+--------+------+ +------------+---------+--------+------+
      |1.1.1.128/25|    RR   |  IBGP  |  Yes | |1.1.1.128/25|    RR   |  IBGP  |  Yes |
      +------------+---------+--------+------+ +------------+---------+--------+------+
      | 1.1.1.0/24 | 1.1.1.1 | Direct |  Yes | | 1.1.1.0/24 | 1.1.1.1 | Direct |  Yes |
      +------------+---------+--------+------+ +------------+---------+--------+------+
                      Figure 5: FIB Reduction Example

   To reduce the FIB size of PE routers, Virtual Aggregation (VA) [VA-
   AUTO] technology can be used. Take the VPN instance A shown in Figure
   5 as an example, the procedures of FIB reduction are as follows:

   1) Multiple more specific prefixes (e.g., 1.1.1.0/25 and 1.1.1.128/25)
      corresponding to the prefix of virtual subnet (i.e., 1.1.1.0/24)
      are configured as Virtual Prefixes (VPs) and a Route-Reflector (RR)
      is configured as an Aggregation Point Router (APR) for these VPs.
      PE routers as RR clients advertise host routes for their own local
      CE hosts to the RR which in turn, as an APR, installs those host
      routes into its FIB and then attach the "can-suppress" tag to those
      host routes before reflecting them to its clients.

   2) Those host routes which have been attached with the "can suppress"
      tag would not be installed into FIBs by clients who are VA-aware
      since they are not APRs for those host routes. In addition, the RR
      as an APR would advertise the corresponding VP routes to all of its



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      clients, and those of which who are VA-aware in turn would install
      these VP routes into their FIBs.

   3) Upon receiving a packet from a local CE host, if no matching host
      route found, the ingress PE router will forward the packet to the
      RR according to one of the VP routes learnt from the RR, which in
      turn forwards the packet to the relevant egress PE router according
      to the host route learnt from that egress PE router. In a word, the
      FIB table size of PE routers can be greatly reduced at the cost of
      path stretch. Note that in the case where the RR is not available
      for transferring L3VPN traffic between PE routers for some reason
      (e.g., the RR is implemented on a server, rather than a router),
      the APR function could actually be performed by a given PE router
      other than the RR as long as that PE router has installed all host
      routes belonging to the virtual subnet into its FIB. Thus, the RR
      only needs to attach a "can-suppress" tag to the host routes learnt
      from its clients before reflecting them to the other clients.
      Furthermore, PE routers themselves could directly attach the "can-
      suppress" tag to those host routes for their local CE hosts before
      distributing them to remote peers as well.

   4) Provided a given local CE host sends an ARP request for a remote
      CE host, the PE router that receives such request will install the
      host route for that remote CE host into its FIB, in case there is a
      host route for that CE host in its RIB and has not yet been
      installed into the FIB. Therefore, the subsequent packets destined
      for that remote CE host will be forwarded directly to the egress PE
      router. To save the FIB space, FIB entries corresponding to remote
      host routes which have been attached with "can-suppress" tags would
      expire if they have not been used for forwarding packets for a
      certain period of time.

   3.6.3. PE Router RIB Reduction


                                         +------+
                                  +------+  RR  +------+
            +-----------------+   |      +------+      |   +-----------------+
            |VPN_A:1.1.1.1/24 |   |                    |   |VPN_A:1.1.1.1/24 |
            |              \  |   |                    |   |  /              |
            |  +------+     \++---+-+                +-+---++/     +------+  |
            |  |Host A+------+ PE-1 |                | PE-2 +------+Host B|  |
            |  +------+\     ++-+-+-+                +-+-+-++     /+------+  |
            |   1.1.1.2/24    | | |                    | | |    1.1.1.3/24   |
            |                 | | |                    | | |                 |
            |     DC West     | | |  IP/MPLS Backbone  | | |      DC East    |
            +-----------------+ | |                    | | +-----------------+
                                | +--------------------+ |
                                |                        |


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       VRF_A :                 V                VRF_A : V
         +------------+---------+--------+        +------------+---------+--------+
         |   Prefix   | Nexthop |Protocol|        |   Prefix   | Nexthop |Protocol|
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.1/32 |127.0.0.1| Direct |        | 1.1.1.1/32 |127.0.0.1| Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.2/32 | 1.1.1.2 | Direct |        | 1.1.1.3/32 | 1.1.1.3 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/25 |    RR   |  IBGP  |        | 1.1.1.0/25 |    RR   |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         |1.1.1.128/25|    RR   |  IBGP  |        |1.1.1.128/25|    RR   |  IBGP  |
         +------------+---------+--------+        +------------+---------+--------+
         | 1.1.1.0/24 | 1.1.1.1 | Direct |        | 1.1.1.0/24 | 1.1.1.1 | Direct |
         +------------+---------+--------+        +------------+---------+--------+
                      Figure 6: RIB Reduction Example

   To reduce the RIB size of PE routers, BGP Outbound Route Filtering
   (ORF) mechanism is used to realize on-demand route announcement. Take
   the VPN instance A shown in Figure 6 as an example, the procedures of
   RIB reduction are as follows:

   1) PE routers as RR clients advertise host routes for their local CE
      hosts to a RR which however doesn't reflect these host routes by
      default unless it receives explicit ORF requests for them from its
      clients. The RR is configured with routes for more specific subnets
      (e.g., 1.1.1.0/25 and 1.1.1.128/25) corresponding to the virtual
      subnet (i.e., 1.1.1.0/24) with next-hop being pointed to Null0 and
      then advertises these routes to its clients via BGP.

   2) Upon receiving a packet from a local CE host, if no matching host
      route found, the ingress PE router will forward the packet to the
      RR according to one of the subnet routes learnt from the RR, which
      in turn forwards the packet to the relevant egress PE router
      according to the host route learnt from that egress PE router. In a
      word, the RIB table size of PE routers can be greatly reduced at
      the cost of path stretch.

   3) Just as the approach mentioned in section 3.6.2, in the case where
      the RR is not available for transferring L3VPN traffic between PE
      routers for some reason, a PE router other than the RR could
      advertise the more specific subnet routes as long as that PE router
      has installed all host routes belonging to that virtual subnet into
      its FIB.

   4) Provided a given local CE host sends an ARP request for a remote
      CE host, the ingress PE router that receives such request will
      request the corresponding host route from its RR by using the ORF
      mechanism (e.g., a group ORF containing Route-Target (RT) and
      prefix information) in case there is no host route for that CE host
      in its RIB yet. Once the host route for the remote CE host is


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      learnt from the RR, the subsequent packets destined for that CE
      host would be forwarded directly to the egress PE router. Note that
      the RIB entries of remote host routes could expire if they have not
      been used for forwarding packets for a certain period of time. Once
      the expiration time for a given RIB entry is approaching, the PE
      router would notify its RR not to pass the updates for
      corresponding host route by using the ORF mechanism.

   3.7. ARP/ND Cache Table Scalability on Default Gateways

   In case where data center default gateway functions are implemented
   on PE routers of the VS as shown in Figure 4, since the ARP/ND cache
   table on each PE router only needs to contain ARP/ND entries of local
   CE hosts, the ARP/ND cache table size will not grow as the number of
   data centers to be connected increases.

   3.8. ARP/ND and Unknown Uncast Flood Avoidance

   In VS, the flooding domain associated with a given virtual subnet
   that has been extended across multiple data centers, has been
   partitioned into segments and each segment is confined within a
   single data center. Therefore, the performance impact on networks and
   servers caused by the flooding of ARP/ND broadcast/multicast and
   unknown unicast traffic is alleviated.

   3.9. Path Optimization

   Take the scenario shown in Figure 4 as an example, to optimize the
   forwarding path for traffic between cloud users and cloud data
   centers, PE routers located at cloud data centers (i.e., PE-1 and PE-
   2), which are also data center default gateways, propagate host
   routes for their local CE hosts respectively to remote PE routers
   which are attached to cloud user sites (i.e., PE-3).

   As such, traffic from cloud user sites to a given server on the
   virtual subnet which has been extended across data centers would be
   forwarded directly to the data center location where that server
   resides, since traffic is now forwarded according to the host route
   for that server, rather than the subnet route.

   Furthermore, for traffic coming from cloud data centers and forwarded
   to cloud user sites, each PE router acting as a default gateway would
   forward the traffic received from its local CE hosts according to the
   best-match route in the corresponding VRF. As a result, traffic from
   data centers to cloud user sites is forwarded along the optimal path
   as well.




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4. Considerations for Non-IP traffic

   Although most traffic within and across data centers is IP traffic,
   there may still be a few legacy clustering applications which rely on
   non-IP communications (e.g., heartbeat messages between cluster
   nodes). To support those few non-IP traffic (if present) in the
   Virtual Subnet solution, the approach following the idea of "route
   all IP traffic, bridge non-IP traffic" could be considered as an
   enhancement to the original Virtual Subnet solution.

   Note that more and more cluster vendors are offering clustering
   applications based on Layer 3 interconnection.

5. Security Considerations

   This document doesn't introduce additional security risk to BGP/MPLS
   L3VPN, nor does it provide any additional security feature for
   BGP/MPLS L3VPN.

6. IANA Considerations

   There is no requirement for any IANA action.

7. Acknowledgements

   Thanks to Dino Farinacci, Himanshu Shah, Nabil Bitar, Giles Heron,
   Ronald Bonica, Monique Morrow, Rajiv Asati and Eric Osborne for their
   valuable comments and suggestions on this document.

8. References

8.1. Normative References

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

8.2. Informative References

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

   [MVPN] Rosen. E and Aggarwal. R, "Multicast in MPLS/BGP IP VPNs",
             draft-ietf-l3vpn-2547bis-mcast-10.txt, Work in Progress,
             Janurary 2010.






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   [VA-AUTO] Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
             L. Zhang, "Auto-Configuration in Virtual Aggregation",
             draft-ietf-grow-va-auto-05.txt, Work in Progress, December
             2011.

   [RFC925] Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
             Information Sciences Institute, October 1984.

   [RFC1027] Smoot Carl-Mitchell, John S. Quarterman, "Using ARP to
             Implement Transparent Subnet Gateways", RFC 1027, October
             1987.

   [RFC4389] D. Thaler, M. Talwar, and C. Patel, "Neighbor Discovery
             Proxies (ND Proxy) ", RFC 4389, April 2006.

   [RFC5798] S. Nadas., "Virtual Router Redundancy Protocol", RFC 5798,
             March 2010.

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

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

   [802.1AB] IEEE Standard 802.1AB-2009, "Station and Media Access
             Control Connectivity Discovery", September 17, 2009.

   [802.1Qbg] IEEE Draft Standard P802.1Qbg/D2.0, "Virtual Bridged Local
             Area Networks -Amendment XX: Edge Virtual Bridging", Work
             in Progress, December 1, 2011.

   [RFC6820] Narten, T., Karir, M., and I. Foo, "Problem Statement for
             ARMD", RFC 6820, January 2013.

Authors' Addresses

   Xiaohu Xu
   Huawei Technologies,
   Beijing, China.
   Phone: +86 10 60610041
   Email: xuxiaohu@huawei.com

   Susan Hares
   Email: shares@ndzh.com




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   Yongbing Fan
   Guangzhou Institute, China Telecom
   Guangzhou, China.
   Phone: +86 20 38639121
   Email: fanyb@gsta.com

   Christian Jacquenet
   France Telecom
   Rennes
   France
   Email: christian.jacquenet@orange.com