Virtual Subnet: A L3VPN-based Subnet Extension Solution
draft-xu-virtual-subnet-10

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Document Type Active Internet-Draft (individual)
Authors Xiaohu Xu  , Susan Hares  , Fan Yongbing  , Christian Jacquenet 
Last updated 2013-02-24
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Network working group                                             X. Xu  
Internet Draft                                                 S. Hares         
Category: Informational                             Huawei Technologies 
                                                                 Y. Fan 
                                                          China Telecom  
                                                           C. Jacquenet 
                                                         France Telecom 
                                                         
Expires: August 2013                                  February 25, 2013 
                                                                                
                                      
          Virtual Subnet: A L3VPN-based Subnet Extension Solution 
                                      
                        draft-xu-virtual-subnet-10 

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.  

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   This Internet-Draft will expire on August 25, 2013. 

 
 
 
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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 ................................................ 3 
   2. Terminology ................................................. 5 
   3. Solution Description......................................... 5 
      3.1. Unicast ................................................ 5 
         3.1.1. Intra-subnet Unicast .............................. 5 
         3.1.2. Inter-subnet Unicast .............................. 6 
      3.2. Multicast .............................................. 8 
      3.3. CE Host Discovery ...................................... 9 
      3.4. ARP/ND Proxy ........................................... 9 
      3.5. CE Host Mobility ....................................... 9 
      3.6. Forwarding Table Scalability .......................... 10 
         3.6.1. MAC Table Reduction on Data Center Switches ...... 10 
         3.6.2. PE Router FIB Reduction .......................... 10 
         3.6.3. PE Router RIB Reduction .......................... 11 
      3.7. ARP/ND Cache Table Scalability on Default Gateways .... 13 
      3.8. ARP/ND and Unknown Uncast Flood Avoidance ............. 13 
      3.9. Path Optimization ..................................... 13 
   4. Security Considerations .................................... 14 
   5. IANA Considerations ........................................ 14 
   6. Acknowledgements ........................................... 14 
   7. References ................................................. 14 
      7.1. Normative References .................................. 14 
      7.2. Informative References ................................ 14 
   Authors' Addresses ............................................ 15 

 
 
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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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      [NARTEN-ARMD] notes that the ARP/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 
   requirements of cloud data center interconnect as described above. 

 
 
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   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.   

   Please note that VS is targeted at scenarios where the traffic 
   across data centers is routable IP traffic.  

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. Strictly according 
   to the normal L3VPN forwarding procedure, 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. In fact, such subnet is a virtual subnet which is 
   emulated by using host routes. 

   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 | 
        +------------+---------+--------+        +------------+---------+--------+ 

 
 
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        | 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) 

   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) 

 
 
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   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.   
                                        +------+ 
                                 +------+ 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.  

 
 
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   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 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, 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.  

 
 
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   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.  

   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., 
 
 
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      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 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  
                                    
                                    
 
 
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                                        +------+ 
                                 +------+  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    | 
           +-----------------+ | |                    | | +-----------------+ 
                               | +--------------------+ | 
                               |                        | 
        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.  

 
 
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   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 
      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).   

 
 
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   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. 

4. 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. 

5. IANA Considerations 

   There is no requirement for any IANA action.  

6. Acknowledgements 

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

7. References 

7.1. Normative References 

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

7.2. 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. 

   [NARTEN-ARMD] Narten, T., Karir, M., and I. Foo, "Problem Statement 
             for ARMD", draft-ietf-armd-problem-statement-01.txt, Work 
             in Progress, February 2012. 

Authors' Addresses 

   Xiaohu Xu 
   Huawei Technologies, 
   Beijing, China. 
   Phone: +86 10 60610041 
   Email: xuxiaohu@huawei.com 
    
   Susan Hares 
   Huawei Technologies (FutureWei group) 

 
 
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   2330 Central Expressway 
   Santa Clara, CA 95050 
   Phone: +1-734-604-0332 
   Email: Susan.Hares@huawei.com 
          shares@ndzh.com 
    
   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

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