Virtual Hub-and-Spoke in BGP/MPLS VPNs

Internet Engineering Task Force (IETF)                           H. Jeng
Request for Comments: 7024                                     J. Uttaro
Category: Standards Track                                           AT&T
ISSN: 2070-1721                                                 L. Jalil
                                                                 Verizon
                                                             B. Decraene
                                                                  Orange
                                                              Y. Rekhter
                                                        Juniper Networks
                                                             R. Aggarwal
                                                                  Arktan
                                                            October 2013


                 Virtual Hub-and-Spoke in BGP/MPLS VPNs

Abstract

   With BGP/MPLS Virtual Private Networks (VPNs), providing any-to-any
   connectivity among sites of a given VPN would require each Provider
   Edge (PE) router connected to one or more of these sites to hold all
   the routes of that VPN.  The approach described in this document
   allows the VPN service provider to reduce the number of PE routers
   that have to maintain all these routes by requiring only a subset of
   these routers to maintain all these routes.

   Furthermore, when PE routers use ingress replication to carry the
   multicast traffic of VPN customers, the approach described in this
   document may, under certain circumstances, reduce bandwidth
   inefficiency associated with ingress replication and redistribute the
   replication load among PE routers.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7024.

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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Overview ........................................................3
   2. Specification of Requirements ...................................4
   3. Routing Information Exchange ....................................5
   4. Forwarding Considerations .......................................7
   5. Internet Connectivity ...........................................9
   6. Deployment Considerations ......................................12
   7. Multicast Considerations .......................................13
      7.1. Terminology ...............................................14
      7.2. Eligible Upstream Multicast Hop (UMH) Routes ..............14
      7.3. Originating VPN-IP Default Route by a V-Hub ...............14
      7.4. Handling C-Multicast Routes ...............................15
      7.5. Originating I-PMSI/S-PMSI/SA A-D Routes by V-Spoke ........15
      7.6. Originating I-PMSI/S-PMSI/SA A-D Routes by V-Hub ..........16
      7.7. Receiving I-PMSI/S-PMSI/SA A-D Routes by V-Spoke ..........17
      7.8. Receiving I-PMSI/S-PMSI/SA A-D Routes by V-Hub ............17
           7.8.1. Case 1 .............................................17
           7.8.2. Case 2 .............................................18
      7.9. Use of Ingress Replication with I-PMSI A-D Routes .........20
   8. An Example of RT Provisioning ..................................21
      8.1. Unicast Routing ...........................................21
      8.2. Multicast Routing .........................................22
   9. Further Refinements ............................................23
   10. Security Considerations .......................................23
   11. Acknowledgements ..............................................23
   12. References ....................................................24
      12.1. Normative References .....................................24
      12.2. Informative References ...................................24

1.  Overview

   With BGP/MPLS VPNs [RFC4364], providing any-to-any connectivity among
   sites of a given VPN is usually accomplished by requiring each
   Provider Edge (PE) router connected to one or more of these sites to
   hold all that VPN's routes.  The approach described in this document
   allows the VPN service provider (SP) to reduce the number of PEs that
   have to maintain all these routes by requiring only a subset of these
   routers to maintain all these routes.

   Consider a set of PEs that maintain VPN Routing and Forwarding tables
   (VRFs) of a given VPN.  In the context of this VPN, we designate a
   subset of these PEs as "Virtual Spoke" PEs (or just Virtual Spokes),
   while some other (non-overlapping) subset of these PEs will be
   "Virtual Hub" PEs (or just Virtual Hubs).  The rest of the PEs in the
   set will be "vanilla" PEs (PEs that implement the procedures
   described in [RFC4364] but that do not implement the procedures
   specified in this document).

   For the sake of brevity, we will use the term "V-hub" to denote a
   Virtual Hub and "V-spoke" to denote a Virtual Spoke.

   For a given VPN, its set of V-hubs may include not only the PEs that
   have sites of that VPN connected to them but also PEs that have no
   sites of that VPN connected to them.  On such PEs, the VRF associated
   with that VPN may import routes from other VRFs of that VPN, even if
   the VRF has no sites of that VPN connected to it.

   Note that while in the context of one VPN a given PE may act as a
   V-hub, in the context of another VPN, the same PE may act as a
   V-spoke, and vice versa.  Thus, a given PE may act as a V-hub only
   for some, but not all, VPNs present on that PE.  Likewise, a given PE
   may act as a V-spoke only for some, but not all, VPNs present on
   that PE.

   For a given VPN, each V-spoke of that VPN is "associated" with one or
   more V-hubs of that VPN (one may use two V-hubs for redundancy to
   avoid a single point of failure).  Note that a given V-hub may have
   no V-spokes associated with it.  For more on how a V-spoke and a
   V-hub become "associated" with each other, see Section 3.

   Consider a set of V-spokes that are associated with a given V-hub,
   V-hub-1.  If one of these V-spokes is also associated with some other
   V-hub, V-hub-2, then other V-spokes in the set need not be associated
   with the same V-hub, V-hub-2, but may be associated with some other
   V-hubs (e.g., V-hub-3, V-hub-4, etc.).

   This document defines a VPN-IP default route as a VPN-IP route whose
   VPN-IP prefix contains only a Route Distinguisher (RD) (for the
   definition of "VPN-IP route", see [RFC4364]).

   A PE that acts as a V-hub of a given VPN maintains all routes of that
   VPN (such a PE imports routes from all other V-hubs and V-spokes, as
   well as from "vanilla" PEs of that VPN).  A PE that acts as a V-spoke
   of a given VPN needs to maintain only the routes of that VPN that are
   originated by the sites of that VPN connected to that PE, plus one or
   more VPN-IP default routes originated by the V-hub(s) associated with
   that V-spoke (such a PE needs to import only VPN-IP default routes
   from certain V-hubs).  This way, only a subset of PEs that maintain
   VRFs of a given VPN -- namely, only the PEs acting as V-hubs of that
   VPN -- has to maintain all routes of that VPN.  PEs acting as
   V-spokes of that VPN need to maintain only a (small) subset of the
   routes of that VPN.

   This document assumes that a given V-hub and its associated
   V-spoke(s) are in the same Autonomous System (AS).  However, if PEs
   that maintain a given VPN's VRFs span multiple ASes, this document
   does not restrict all V-hubs of that VPN to be in the same AS -- the
   V-hubs may be spread among these ASes.

   One could model the approach defined in this document as a two-level
   hierarchy, where the top level consists of V-hubs and the bottom
   level consists of V-spokes.  Generalization of this approach to more
   than two levels of hierarchy is outside the scope of this document.

   When PEs use ingress replication to carry the multicast traffic of
   VPN customers, the approach described in this document may, under
   certain circumstances, reduce bandwidth inefficiency associated with
   ingress replication and redistribute the replication load among the
   PEs.  This is because a PE that acts as a V-spoke of a given VPN
   would need to replicate multicast traffic only to other V-hubs (while
   other V-hubs would replicate this traffic to the V-spokes associated
   with these V-hubs), rather than to all PEs of that VPN.  Likewise, a
   PE that acts as a V-hub of a given VPN would need to replicate
   multicast traffic to other V-hubs and the V-spokes, but only the
   V-spokes associated with that V-hub, rather than replicating the
   traffic to all PEs of that VPN.  Limiting replication could be
   especially beneficial if the V-spoke PEs have limited replication
   capabilities and/or have links with limited bandwidth.

2.  Specification of Requirements

   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 [RFC2119].

3.  Routing Information Exchange

   Routing information exchange among all PEs of a given VPN is subject
   to the following rules.

   A PE that has sites of a given VPN connected to it has to retain
   routing information received from these sites, irrespective of
   whether this PE acts as a V-hub or a V-spoke of that VPN and follows
   the rules specified in [RFC4364].

   A PE that has sites of a given VPN connected to it follows the rules
   specified in [RFC4364] when exporting (as VPN-IP routes) the routes
   received from these sites, irrespective of whether this PE acts as a
   V-hub or a V-spoke of that VPN.

   In addition, a V-hub of a given VPN MUST export a VPN-IP default
   route for that VPN.  This route MUST be exported to only the V-spokes
   of that VPN that are associated with that V-hub.

   To enable a given VPN's V-spoke to share its outbound traffic load
   among the V-hubs associated with that V-spoke, each of the VPN's
   V-hubs MUST use a distinct RD (per V-hub, per VPN) when originating a
   VPN-IP default route.  The use of Type 1 RDs may be an attractive
   option for such RDs.

   If a V-spoke imports several VPN-IP default routes, each originated
   by its own V-hub, and these routes have the same preference, then
   traffic from the V-spoke to other sites of that VPN would be load
   shared among the V-hubs.

   Following the rules specified in [RFC4364], a V-hub of a given VPN
   imports all the non-default VPN-IP routes originated by all other PEs
   that have sites of that VPN connected to them (irrespective of
   whether these other PEs act as V-hubs or V-spokes or just "vanilla"
   PEs for that VPN, and irrespective of whether or not these V-spokes
   are associated with the V-hub).

   A V-hub of a given VPN MUST NOT import a VPN-IP default route unless
   the imported route is the Internet VPN-IP default route (for the
   definition of "Internet VPN-IP default route" and information on how
   to distinguish between a VPN-IP default route and the Internet VPN-IP
   default route, see Section 5).

   Within a given VPN, a V-spoke MUST import all VPN-IP default routes
   that have been originated by the V-hubs associated with that V-spoke.

   In addition, a V-spoke of a given VPN MAY import VPN-IP routes for
   that VPN that have been originated by some other V-spokes of that
   VPN, but only by the V-spokes that are associated with the same
   V-hub(s) as the V-spoke itself.

   The above rules are realized by using Route Target (RT) extended
   communities [RFC4360] and VRF export/import policies based on these
   RTs.  This document defines the following procedures for implementing
   the above rules.

   Consider a "vanilla" any-to-any VPN.  This document assumes that all
   the PEs of that VPN (or to be more precise, all VRFs of that VPN) are
   provisioned with the same export and import RT -- we will refer to
   this RT as "RT-VPN" (of course, for a given VPN service provider,
   each VPN would use its own RT-VPN, distinct from RT-VPNs used by
   other VPNs).

   To evolve this VPN into V-hubs and V-spokes, all PEs (or to be more
   precise, all VRFs) that are designated as either V-hubs or V-spokes
   of that VPN keep the same export RT-VPN.  This RT-VPN is attached to
   all VPN-IP routes originated by these PEs.  Also, all the V-hubs keep
   the same import RT-VPN.

   In addition, each of a given VPN's V-hubs is provisioned with its own
   export RT, called RT-VH.  This RT-VH MUST be different from the
   export RT (RT-VPN) provisioned on that V-hub.  Furthermore, for a
   given VPN service provider, no two VPNs can use the same RT-VH.

   A given V-spoke becomes associated with a given V-hub by virtue of
   provisioning the V-spoke to import only the VPN-IP route(s) that
   carry RT-VH provisioned on the V-hub (thus, associating a new V-spoke
   with a given V-hub requires provisioning only on that V-spoke -- no
   provisioning changes are required on the V-hub).

   To avoid the situation where within a given VPN all the V-spokes
   would be associated with every V-hub (in other words, to partition
   V-spokes among V-hubs), different V-hubs within that VPN MAY use
   different RT-VHs.  At one extreme, every V-hub may use a distinct
   RT-VH.  The use of IP-address-specific RTs may be an attractive
   option for this scenario.  However, it is also possible for several
   V-hubs to use the same RT-VH, in which case all of these V-hubs would
   be associated with the same set of V-spokes.

   When a V-hub originates a (non-Internet) VPN-IP default route, the
   V-hub MUST attach RT-VH to that route (the case where a V-hub
   originates the Internet VPN-IP default route is covered in
   Section 5).  Thus, this route is imported by all V-spokes associated
   with the V-hub.

   A V-spoke MAY be provisioned to export VPN-IP routes not just to the
   V-hubs but also to the V-spokes that import the same VPN-IP default
   route(s) as the V-spoke itself.  The V-spoke accomplishes this by
   adding its import RT-VH(s) to the VPN-IP routes exported by the
   V-spoke.

4.  Forwarding Considerations

   This section describes changes/modifications to the forwarding
   procedures specified in [RFC4364].

   For a given VPN, the MPLS label that a V-hub of that VPN advertises
   with a VPN-IP default route MUST be the label that is mapped to a
   Next Hop Label Forwarding Entry (NHLFE) that identifies the VRF of
   the V-hub.  As a result, when the V-hub receives a packet that
   carries such a label, the V-hub pops the label and determines further
   disposition of the packet based on the lookup in the VRF.

   Note that this document does not require the advertisement of labels
   mapped to an NHLFE that identifies a VRF for routes other than the
   VPN-IP default route.

   When a V-hub of a given VPN originates a VPN-IP default route for
   that VPN, the V-hub MUST NOT install in its VRF of that VPN a default
   route, unless that route has been originated as a result of

   a) the V-hub receiving an IP default route from one of the VPN
      Customer Edge (CE) routers connected to it, or

   b) the V-hub receiving (and importing) the Internet VPN-IP default
      route (Section 5) from some other PE, or

   c) the VRF being provisioned with a default route pointing to the
      routing table that maintains the Internet routes.

   When a multihomed site is connected to a V-hub and a V-spoke, then
   the V-hub uses the following OPTIONAL procedures to support Internal
   BGP (IBGP) / External BGP (EBGP) load balancing for the site's
   inbound traffic that has been originated by some other V-spoke
   associated with the V-hub.  When the V-hub receives from some other
   PE a packet that carries an MPLS label that the V-hub advertised in
   the VPN-IP default route, then the V-hub uses the label to identify
   the VRF that should be used for further disposition of the packet.
   If (using the information present in the VRF) the V-hub determines
   that the packet has to be forwarded using a non-default route present
   in the VRF, and this route indicates that the packet's destination is
   reachable either over one of the VRF attachment circuits (for the
   definition of "VRF attachment circuits", see [RFC4364]) or via some
   other (V-spoke) PE, the V-hub forwards the packet either over this
   attachment circuit or via that other PE.  The choice between the two
   is a local matter to the V-hub.

   To illustrate this, consider the following example:

                  <- RD:0/0           RD:0/0->

                                   <- RD:192.0.2        <-192.0.2/24
    CE1----PE-S-------------PE-H----------------PE-S1-------------CE2
                           /
                           |    |
                           |    |  192.0.2/24
                           |    |
                          CE4   CE3

   A multihomed site (not shown in the above figure) is connected via
   CE2 and CE3.  Thus, both CE2 and CE3 advertise a route to 192.0.2/24.
   CE2 advertises this route (route to 192.0.2/24) to PE-S1, which in
   turn originates a VPN-IP route RD:192.0.2.  CE3 advertises this route
   to PE-H.

   PE-H is a V-hub, while PE-S and PE-S1 are V-spokes associated with
   that V-hub.  Thus, PE-H originates a VPN-IP default route (RD:0/0),
   and both PE-S and PE-S1 import that route.

   PE-H receives from PE-S1 a VPN-IP route to RD:192.0.2 and from CE3 a
   plain IP route to 192.0.2.  Thus, the VRF entry on PE-H has two
   possible next hops for 192.0.2: CE3 and PE-S1 (the latter is a next
   hop that is not directly connected to PE-H).

   Now consider what happens when CE1 originates a packet destined to
   192.0.2.1.  When PE-S receives this packet, PE-S (following the
   VPN-IP default route) forwards the packet to PE-H.  The MPLS label in
   the packet is the label that PE-H advertised to PE-S in the VPN-IP
   default route.  Thus, following the rule specified above, PE-H may
   forward the packet either via CE3 or via PE-S1 (with PE-S1
   subsequently forwarding the packet to CE2), resulting in IBGP/EBGP
   load balancing.

   Likewise, if CE4 originates a packet destined to 192.0.2.1, PE-H may
   forward the packet either via CE3 or via PE-S1 (with PE-S1
   subsequently forwarding the traffic to CE2), resulting in IBGP/EBGP
   load balancing.

   Note, however, that if there is some other CE, CE5, connected to
   PE-S1, and CE5 sends a packet to 192.0.2.1, then (due to the IP
   longest match rule) PE-S1 will always forward this packet to CE2.

   Thus, for a multihomed site connected to a V-hub and a V-spoke,
   IBGP/EBGP load balancing will be available for some but not all the
   traffic destined to that site.  Specifically, IBGP/EBGP load
   balancing will not be available for the traffic destined to that site
   if this traffic has been originated within some other site that is
   connected to the same V-spoke.

   Moreover, if CE3 advertises 192.0.2.0/25 and 192.0.2/24, while CE2
   advertises 192.0.2.128/25 and 192.0.2/24 (which is yet another form
   of load balancing for a multihomed site), when CE5 sends a packet to
   192.0.2.1, then (due to the IP longest match rule) PE-S1 will always
   forward this packet to CE2, even though the VPN customer would expect
   this traffic to flow via CE3.

   This document proposes two options to address the issues raised in
   the previous two paragraphs.  The first option is to disallow a given
   VPN to provision PEs that have multihomed sites of that VPN connected
   to them as V-spokes (such PEs could be provisioned as either V-hubs
   or plain "vanilla" PEs).  The second option is for the V-spoke, when
   it receives an IP route from a CE, to not install this route in its
   forwarding table but just re-advertise this route as a VPN-IP route,
   together with an MPLS label.  The NHLFE [RFC3031] associated with
   that label MUST specify the CE that advertises the IP route as the
   next hop.  As a result, when the PE receives data that carries that
   label, the PE just forwards the data to the CE without performing an
   IP lookup on the data.  Note that doing this would result in forcing
   the traffic between a pair of sites connected to the same V-spoke to
   go through the V-hub of that V-spoke.

   An implementation that supports IBGP/EBGP load balancing, as
   specified above, SHOULD support the second option.  If the
   implementation does not support the second option, then deploying
   this implementation to support IBGP/EBGP load balancing, as specified
   above, would either (a) restrict the set of PEs that could be
   provisioned as V-spokes (any PE that has a multihomed site connected
   to it cannot be provisioned as a V-spoke) or (b) result in IBGP/EBGP
   load balancing not being available for certain scenarios (the
   scenarios that the second option is intended to cover).

5.  Internet Connectivity

   This document specifies two possible alternatives for providing
   Internet connectivity for a given VPN.

   The first alternative is when a PE that maintains Internet routes
   also maintains a VRF of a given VPN.  In this case, the Internet
   connectivity for that VPN MAY be provided by provisioning a default
   route in the VPN's VRF on that PE pointing to the routing table on
   that PE that maintains the Internet routes.  This PE MUST NOT be
   provisioned as a V-spoke for that VPN (this PE may be provisioned as
   either a V-hub or a "vanilla" PE).  If this PE is provisioned as a
   V-hub, then this PE MUST originate a VPN-IP default route.  The route
   MUST carry both RT-VPN and RT-VH of the V-hub (see Section 3 for the
   definitions of "RT-VPN" and "RT-VH").  Thus, this route will be
   imported by all the V-spokes associated with the V-hub, as well as by
   other V-hubs and "vanilla" PEs.  An implementation MUST support the
   first alternative.

   The second alternative is when a site of a given VPN has connection
   to the Internet, and a CE of that site advertises an IP default route
   to the PE connected to that CE.  This alternative has two subcases:
   (a) the PE is provisioned as a V-hub, and (b) the PE is provisioned
   as a V-spoke.  An implementation MUST support subcase (a).  An
   implementation MAY support subcase (b).

   If a PE is provisioned as a V-hub, then the PE re-advertises this IP
   default route as a VPN-IP default route and installs in its VRF an IP
   default route with the next hop specifying the CE(s) that advertise
   the IP default route to the PE.  Note that when re-advertising the
   VPN-IP default route, the route MUST carry both RT-VPN and RT-VH of
   the V-hub (see Section 3 for the definitions of "RT-VPN" and
   "RT-VH").  Thus, this route will be imported by all the V-spokes
   associated with the V-hub, as well as by other V-hubs and
   "vanilla" PEs.

   If a PE is provisioned as a V-spoke, then receiving a default route
   from a CE MUST NOT cause the V-spoke to install an IP default route
   in its VRF.  The V-spoke MUST originate a VPN-IP default route with a
   (non-null) MPLS label.  The route MUST carry only RT-VPN (as a
   result, this route is not imported by any of the V-spokes but is
   imported by V-hubs).  The packet's next hop of the NHLFE [RFC3031]
   associated with that label MUST specify the CE that advertises the IP
   default route.  As a result, when the V-spoke receives data that
   carries that label, it just forwards the data to the CE without
   performing an IP lookup on the data.  Note that in this case, the VRF
   on the V-spoke will have an IP default route, but this route would be
   created as a result of receiving a VPN-IP default route from one of
   the V-hubs associated with that V-spoke (and not as a result of
   receiving the IP default route from the CE).  Note also that if this
   V-spoke has other sites of that VPN connected to it, then traffic
   from these sites to the Internet would go to that V-spoke, then to
   the V-hub selected by the V-spoke, then from that V-hub back to the
   V-spoke, and then to the CE that advertises an IP default route to
   the V-spoke.

   If a PE is provisioned as a V-spoke of a given VPN, and if a CE of
   that VPN advertises an IP default route to the PE (as the CE belongs
   to the site that provides the Internet connectivity for the VPN),
   then the PE MUST NOT advertise an IP default route back to that CE.
   Yet, the CE has to specify that PE as the next hop for all the
   traffic to other sites of that VPN.  A way to accomplish this is to
   require the V-spoke to implement procedures specified in Section 9.

   In all the scenarios described above in this section, we refer to the
   originated VPN-IP default route as the "Internet VPN-IP default
   route".  Specifically, the Internet VPN-IP default route is a VPN-IP
   default route originated by a PE (this PE could be either a V-hub or
   a V-spoke) as a result of (a) receiving an IP default route from a CE
   or (b) the PE maintaining Internet routes and also provisioning in
   the VRF of its VPN a default route pointing to its (the PE's) routing
   table that contains Internet routes.

   The difference between the Internet VPN-IP default route and a
   non-Internet VPN-IP default route originated by a V-hub is in the RTs
   carried by the route -- for a given VPN and a given V-hub of that
   VPN, the Internet VPN-IP default route carries both RT-VPN and RT-VH
   of that V-hub, while the non-Internet VPN-IP default route carries
   just RT-VH of that V-hub.

   When a V-hub originates the Internet VPN-IP default route, the V-hub
   MUST withdraw the non-Internet VPN-IP default route that has been
   originated by the V-hub.  When a V-hub withdraws the Internet VPN-IP
   default route that has been originated by the V-hub, the V-hub MUST
   originate a non-Internet VPN-IP default route.  That is, at any given
   point in time, a given V-hub originates either the Internet VPN-IP
   default route or a non-Internet VPN-IP default route.

   As a result of the rules specified above, if a V-hub originates the
   Internet VPN-IP default route, then all the V-spokes associated with
   that V-hub MUST import that route.  In addition (and in contrast with
   a non-Internet VPN-IP default route), other V-hubs MAY import that
   route.  A V-hub MAY also import the Internet VPN-IP default routes
   originated by V-spoke(s).  A V-spoke MUST NOT import the Internet
   VPN-IP default route originated by any other V-spoke.  Such a route
   MAY be imported only by V-hubs.

   If the Internet VPN-IP default route originated by a V-hub has the
   same preference as the (non-Internet) VPN-IP default route originated
   by some other V-hub, then a V-spoke that imports VPN-IP default
   routes originated by both of these V-hubs would load share the
   outgoing Internet traffic between these two V-hubs (and thus some of
   the outgoing Internet traffic from that V-spoke will first be routed
   to the V-hub that does not originate the Internet VPN-IP default
   route, then from that V-hub to the V-hub that does originate the
   Internet VPN-IP default route).

   If taking an extra-hub hop for the Internet traffic is viewed as
   undesirable, then it is RECOMMENDED that the Internet VPN-IP default
   route be of higher preference than a (non-Internet) VPN-IP default
   route originated by some other V-hub.  However, in this case the
   traffic from the V-spokes to other sites of that VPN will not be load
   shared between these two V-hubs.

6.  Deployment Considerations

   For a given VPN, a V-hub and a set of V-spokes associated with that
   V-hub should be chosen in a way that minimizes the additional network
   distance/latency penalty, given that VPN geographic footprint.

   For a given VPN, some or all of its V-spokes could be grouped into
   geographically based clusters (e.g., V-spokes within a given cluster
   could be in close geographical proximity to each other) with
   any-to-any connectivity within each cluster.  Note that the V-spokes
   within a given cluster need not be associated with the same V-hub(s).
   Likewise, not all V-spokes associated with a given V-hub need to be
   in the same cluster.  A use case for this would be a VPN for a large
   retail chain in which data traffic is hub/spoke between each store
   and centralized datacenters, but there is a need for direct Voice
   over IP (VoIP) traffic between stores within the same geographical
   area.

   The use of constrained route distribution for BGP/MPLS IP VPNs
   ("RT constrains") [RFC4684] may further facilitate/optimize routing
   exchange in support of V-hubs and V-spokes.

   Introducing a V-spoke PE in a VPN may introduce the following changes
   for the customer of that VPN:

   +  Traceroute from a CE connected to a V-spoke may report an
      additional hop: the V-hub PE.

   +  Latency for traffic sent from a CE connected to a V-spoke may
      increase, depending on the location of the V-hub in the layer 3
      and layer 1 network topology of the SP.

7.  Multicast Considerations

   This section describes procedures for supporting Multicast VPN (MVPN)
   in the presence of Virtual Hub-and-Spoke.  The procedures rely on
   MVPN specifications as defined in [RFC6513], [RFC6514], and
   [RFC6625].

   The procedures assume that for the purpose of ensuring
   non-duplication, both V-hubs and V-spokes can discard packets from a
   "wrong" PE, as specified in Section 9.1.1 of [RFC6513].  The existing
   procedures for Selective Provider Multicast Service Interface
   (S-PMSI) auto-discovery (A-D) routes [RFC6513] [RFC6514] [RFC6625]
   are sufficient to discard packets coming from a "wrong" PE for all
   types of provider tunnels (P-tunnels) specified in [RFC6514]
   (including Ingress Replication).  The existing procedures for
   Inclusive Provider Multicast Service Interface (I-PMSI) A-D routes
   [RFC6513] [RFC6514] are sufficient to discard packets coming from a
   "wrong" PE for all types of P-tunnels specified in [RFC6514], except
   for Ingress Replication.  Section 7.9 of this document specifies
   changes to the procedures in [RFC6514], to enable the discarding of
   packets from a "wrong" PE when Ingress Replication is used for I-PMSI
   P-tunnels.

   The V-hub/V-spoke architecture, as specified in this document,
   affects certain multicast scenarios.  In particular, it affects
   multicast scenarios where the source of a multicast flow is at a site
   attached to a V-hub and a receiver of that flow is at a site attached
   to a V-spoke that is not associated with that same V-hub.  It also
   affects multicast scenarios where the source of a multicast flow is
   at a site attached to a V-spoke, a receiver of that flow is at a site
   attached to a different V-spoke, and the set intersection between the
   V-hub(s) associated with the first V-spoke and the V-hub(s)
   associated with the second V-spoke is empty.  It may also affect
   multicast scenarios where the source of a multicast flow is at a site
   connected to a V-spoke, a receiver of that flow is at a site attached
   to a different V-spoke, and the set intersection between the V-hub(s)
   associated with the first V-spoke and the V-hub(s) associated with
   the second V-spoke is non-empty (the multicast scenarios are affected
   if the I-PMSI/S-PMSI A-D routes originated by the first V-spoke are
   not imported by the second V-spoke).

   The use of Virtual Hub-and-Spoke in conjunction with seamless MPLS
   multicast [MPLS-MCAST] is outside the scope of this document.

7.1.  Terminology

   We will speak of a P-tunnel being "bound" to a particular
   I-PMSI/S-PMSI A-D route if the P-tunnel is specified in that route's
   PMSI Tunnel attribute.

   When Ingress Replication is used, the P-tunnel bound to a particular
   I-PMSI/S-PMSI A-D route is actually a set of unicast tunnels
   (procedures differ from [RFC6514] for the case of I-PMSI and are
   specified in Section 7.9 of this document).  The PE originating the
   I-PMSI/S-PMSI A-D route uses these unicast tunnels to carry traffic
   to the PEs that import the route.  The PEs that import the route
   advertise labels for the unicast tunnels in Leaf A-D routes
   originated in response to the I-PMSI/S-PMSI A-D route.  When we say
   that traffic has been received by a PE on a P-tunnel "bound" to a
   particular I-PMSI/S-PMSI A-D route imported by that PE, we refer to
   the unicast tunnel for which the label was advertised in a Leaf A-D
   route by the PE that imported the I-PMSI/S-PMSI route; the PE that
   originated that route uses this tunnel to send traffic to the PE that
   imported the I-PMSI/S-PMSI route.

7.2.  Eligible Upstream Multicast Hop (UMH) Routes

   On a V-spoke, the set of Eligible UMH routes consists of all the
   unicast VPN-IP routes received by the V-spoke, including the default
   VPN-IP routes received from its V-hub(s).  Note that such routes MAY
   include routes received from other V-spokes.  The routes received
   from other V-spokes could be either "vanilla" VPN-IP routes (routes
   using the IPv4 or IPv6 Address Family Identifier (AFI) and Subsequent
   Address Family Identifier (SAFI) set to 128 "MPLS-labeled VPN
   address" [IANA-SAFI]) or routes using the IPv4 or IPv6 AFI (as
   appropriate) but with the SAFI set to SAFI 129 "Multicast for
   BGP/MPLS IP Virtual Private Networks (VPNs)" [IANA-SAFI].

   The default VPN-IP routes received from the V-hub(s) may be either
   Internet default VPN-IP routes or non-Internet default VPN-IP routes.

7.3.  Originating VPN-IP Default Route by a V-Hub

   When originating a VPN-IP default route, a V-hub, in addition to
   following the procedures specified in Section 3, also follows the
   procedures specified in Sections 6 and 7 of [RFC6514] (see also
   Section 5.1 of [RFC6513]).  Specifically, the V-hub MUST add the VRF
   Route Import Extended Community that embeds the V-hub's IP address.
   The route also MUST include the Source AS extended community.

7.4.  Handling C-Multicast Routes

   In the following, the term "C-multicast routes" refers to BGP routes
   that carry customer multicast routing information [RFC6514].

   Origination of C-multicast routes follows the procedures specified in
   [RFC6514] (irrespective of whether these routes are originated by a
   V-hub or a V-spoke).

   When a V-spoke receives a C-multicast route, the V-spoke follows the
   procedures described in [RFC6514].

   When a V-hub receives a C-multicast route, the V-hub determines
   whether the customer Rendezvous Point (C-RP) or the customer source
   (C-S) of the route is reachable via one of its VRF interfaces; if
   yes, then the V-hub follows the procedures described in [RFC6514].

   Otherwise, the C-RP/C-S of the route is reachable via some other PE
   (this is the case where the received route was originated by a
   V-spoke that sees the V-hub as the "upstream PE" for a given source,
   but the V-hub sees some other PE -- either V-hub or V-spoke -- as the
   "upstream PE" for that source).  In this case, the V-hub uses the
   type (Source Tree Join vs Shared Tree Join), the Multicast Source,
   and Multicast Group from the received C-multicast route to construct
   a new route of the same type, with the same Multicast Source and
   Multicast Group.  The hub constructs the rest of the new route
   following procedures specified in Section 11.1.3 of [RFC6514].  The
   hub also creates the appropriate (C-*, C-G) or (C-S, C-G) state in
   its MVPN Tree Information Base (MVPN-TIB).

7.5.  Originating I-PMSI/S-PMSI/SA A-D Routes by V-Spoke

   When a V-spoke originates an I-PMSI, an S-PMSI, or Source Active (SA)
   A-D route, the V-spoke follows the procedures specified in [RFC6514]
   (or in the case of a wildcard S-PMSI A-D route, the procedures
   specified in [RFC6625]), including the procedures for constructing
   RT(s) carried by the route.  Note that as a result, such a route will
   be imported by the V-hubs.  In the case of an I-PMSI/S-PMSI A-D
   route, the P-tunnel bound to this route is used to carry to these
   V-hubs traffic originated by the sites connected to the V-spoke.

   If the V-spoke exports its (unicast) VPN-IP routes not just to the
   V-hubs but also to some other V-spokes (as described in Section 3),
   then (as a result of following the procedures specified in [RFC6514]
   or, in the case of a wildcard S-PMSI A-D route, the procedures
   specified in [RFC6625]) the I-PMSI/S-PMSI/SA A-D route originated by
   the V-spoke will be imported not just by the V-hubs but also by the
   other V-spokes.  This is because in this scenario, the route will
   carry more than one RT; one of these RTs, RT-VPN, will result in
   importing the route by the V-hubs, while other RT(s) will result in
   importing the route by the V-spokes (the other RT(s) are the RT(s)
   that the V-spoke uses for importing the VPN-IP default route).  In
   this case, the P-tunnel bound to this I-PMSI/S-PMSI A-D route is also
   used to carry traffic originated by the sites connected to the
   V-spoke that originates the route to these other V-spokes.

7.6.  Originating I-PMSI/S-PMSI/SA A-D Routes by V-Hub

   When a V-hub originates an I-PMSI/S-PMSI/SA A-D route, the V-hub
   follows the procedures specified in [RFC6514] (or in the case of a
   wildcard S-PMSI A-D route, the procedures specified in [RFC6625]),
   except that in addition to the RT(s) constructed following these
   procedures, the route MUST also carry the RT of the VPN-IP default
   route advertised by the V-hub (RT-VH).  Note that as a result, such a
   route will be imported by other V-hubs and also by the V-spokes, but
   only by the V-spokes that are associated with the V-hub (the V-spokes
   that import the VPN-IP default route originated by the V-hub).  In
   the case of an I-PMSI/S-PMSI A-D route, the P-tunnel bound to this
   route is used to carry to these other V-hubs and V-spokes the traffic
   originated by the sites connected to the V-hub that originates the
   route.

   In addition, if a V-hub originates an I-PMSI A-D route following
   the procedures specified in [RFC6514], the V-hub MUST originate
   another I-PMSI A-D route -- we'll refer to this route as an
   "Associated-V-spoke-only I-PMSI A-D route".  The RT carried by this
   route MUST be the RT that is carried in the VPN-IP default route
   advertised by the V-hub (RT-VH).  Therefore, this route will be
   imported only by the V-spokes associated with the V-hub (the V-spokes
   that import the VPN-IP default route advertised by this V-hub).  The
   P-tunnel bound to this route is used to carry to these V-spokes
   traffic originated by the sites connected to either (a) other V-hubs,
   (b) other V-spokes, including the V-spokes that import the VPN-IP
   default route from the V-hub, or (c) "vanilla" PEs.

   More details on the use of this P-tunnel are described in
   Section 7.8.

   As a result, a V-hub originates not one, but two I-PMSI A-D routes --
   one is a "vanilla" I-PMSI A-D route and another is an
   Associated-V-spoke-only I-PMSI A-D route.  Each of these routes MUST
   have a distinct RD.

   When a V-hub receives traffic from one of the sites connected to the
   V-hub, and the V-hub determines (using some local policies) that this
   traffic should be transmitted using an I-PMSI, the V-hub forwards
   this traffic on the P-tunnel bound to the "vanilla" I-PMSI A-D route
   but MUST NOT forward it on the P-tunnel bound to the
   Associated-V-spoke-only I-PMSI A-D route.

7.7.  Receiving I-PMSI/S-PMSI/SA A-D Routes by V-Spoke

   When a V-spoke receives an I-PMSI/S-PMSI/SA A-D route, the V-spoke
   follows the procedures specified in [RFC6514] (or in the case of a
   wildcard S-PMSI A-D route, the procedures specified in [RFC6625]).
   As a result, a V-spoke that is associated with a given V-hub (the
   V-spoke that imports the VPN-IP default route originated by that
   V-hub) will also import I-PMSI/S-PMSI/SA A-D routes originated by
   that V-hub.  Specifically, the V-spoke will import both the "vanilla"
   I-PMSI A-D route and the Associated-V-spoke-only I-PMSI A-D route
   originated by the V-hub.

   In addition, if a V-spoke imports the (unicast) VPN-IP routes
   originated by some other V-spokes (as described in Section 3), then
   the V-spoke will also import I-PMSI/S-PMSI/SA A-D routes originated
   by these other V-spokes.

7.8.  Receiving I-PMSI/S-PMSI/SA A-D Routes by V-Hub

   The following describes procedures that a V-hub MUST follow when it
   receives an I-PMSI/S-PMSI/SA A-D route.

7.8.1.  Case 1

   This is the case where a V-hub receives an I-PMSI/S-PMSI/SA A-D
   route, and one of the RT(s) carried in the route is the RT that the
   V-hub uses for advertising its VPN-IP default route (RT-VH).

   In this case, the receiving route was originated either

   +  by a V-spoke associated with the V-hub (the V-spoke that imports
      the VPN-IP default route originated by the V-hub), or

   +  by some other V-hub that uses the same RT as the receiving V-hub
      for advertising the VPN-IP default route.

   In this case, the received I-PMSI/S-PMSI/SA A-D route carries more
   than one RT.  One of these RTs results in importing this route by the
   V-hubs.  Another of these RTs is the RT that the V-hub uses when
   advertising its VPN-IP default route (RT-VH).  This RT results in
   importing the received I-PMSI/S-PMSI/SA A-D route by all the V-spokes
   associated with the V-hub (the V-spokes that import the VPN-IP
   default route originated by the V-hub).

   In handling such an I-PMSI/S-PMSI/SA A-D route, the V-hub simply
   follows the procedures specified in [RFC6514] (or in the case of a
   wildcard S-PMSI A-D route, the procedures specified in [RFC6625]).

   Specifically, the V-hub MUST NOT reoriginate this route as done in
   Case 2 below.

   The following specifies the rules that the V-hub MUST follow when
   handling traffic that the V-hub receives on a P-tunnel bound to this
   I-PMSI/S-PMSI A-D route.  The V-hub may forward this traffic to only
   the sites connected to that V-hub (forwarding this traffic to these
   sites follows the procedures specified in [RFC6514] or, in the case
   of a wildcard S-PMSI A-D route, the procedures specified in
   [RFC6625]).  The V-hub MUST NOT forward the traffic received on this
   P-tunnel to any other V-hubs or V-spokes, including the V-spokes that
   import the VPN-IP default route originated by the V-hub (V-spokes
   associated with the V-hub).  Specifically, the V-hub MUST NOT forward
   the traffic received on the P-tunnel advertised in the received
   I-PMSI A-D route over the P-tunnel that the V-hub binds to its
   Associated-V-spoke-only I-PMSI A-D route.

7.8.2.  Case 2

   This is the case where a V-hub receives an I-PMSI/S-PMSI/SA A-D
   route, and the route does not carry the RT that the receiving V-hub
   uses when advertising its VPN-IP default route (RT-VH).

   In this case, the receiving I-PMSI/S-PMSI/SA A-D route was originated
   by either some other V-hub or a V-spoke.  The I-PMSI/S-PMSI/SA A-D
   route is imported by the V-hub (as well as by other V-hubs) but not
   by any of the V-spokes associated with the V-hub (V-spokes that
   import the VPN-IP default route originated by the V-hub).

   In this case, the V-hubs follow the procedures specified in [RFC6514]
   (or in the case of a wildcard S-PMSI A-D route, the procedures
   specified in [RFC6625]), with the following additions.

   Once a V-hub accepts an I-PMSI A-D route, when the V-hub receives
   data on the P-tunnel bound to that I-PMSI A-D route, the V-hub
   follows the procedures specified in [RFC6513] and [RFC6514] to
   determine whether to accept the data.  If the data is accepted, then
   the V-hub further forwards the data over the P-tunnel bound to the
   Associated-V-spoke-only I-PMSI A-D route originated by the V-hub.
   Note that in deciding whether to forward the data over the P-tunnel
   bound to the Associated-V-spoke-only I-PMSI A-D route originated by
   the V-hub, the V-hub SHOULD take into account the (multicast) state
   present in its MVPN-TIB that has been created as a result of
   receiving C-multicast routes from the V-spokes associated with the
   V-hub.  If (using the information present in the MVPN-TIB) the V-hub
   determines that none of these V-spokes have receivers for the data,
   the V-hub SHOULD NOT forward the data over the P-tunnel bound to the
   Associated-V-spoke-only I-PMSI A-D route originated by the V-hub.

   Whenever a V-hub imports an S-PMSI A-D route (respectively, SA A-D
   route) in a VRF, the V-hub, in contrast to Case 1 above, MUST
   originate an S-PMSI A-D route (respectively, SA A-D route) targeted
   to its V-spokes.  To accomplish this, the V-hub replaces the RT(s)
   carried in the route with the RT that the V-hub uses when originating
   its VPN-IP default route (RT-VH), changes the RD of the route to the
   RD that the V-hub uses when originating its Associated-V-spoke-only
   I-PMSI A-D route, and sets Next Hop to the IP address that the V-hub
   places in the Global Administrator field of the VRF Route Import
   Extended Community of the VPN-IP routes advertised by the V-hub.  For
   S-PMSI A-D routes, the V-hub also changes the Originating Router's IP
   address in the MCAST-VPN NLRI (Network Layer Reachability
   Information) of the route to the same address as the one in the Next
   Hop.  Moreover, before advertising the new S-PMSI A-D route, the
   V-hub modifies its PMSI Tunnel attribute as appropriate (e.g., by
   replacing the P-tunnel rooted at the originator of this route with a
   P-tunnel rooted at the V-hub).

   Note that a V-hub of a given MVPN may receive and accept multiple
   (C-*, C-*) wildcard S-PMSI A-D routes [RFC6625], each originated by
   its own PE.  Yet, even if the V-hub receives and accepts such
   multiple (C-*, C-*) S-PMSI A-D routes, the V-hub re-advertises just
   one (C-*, C-*) S-PMSI A-D route, thus aggregating the received (C-*,
   C-*) S-PMSI A-D routes.  The same applies for (C-*, C-G) S-PMSI A-D
   routes.

   Whenever a V-hub receives data on the P-tunnel bound to a received
   S-PMSI A-D route, the V-hub follows the procedures specified in
   [RFC6513] and [RFC6514] (or in the case of a wildcard S-PMSI A-D
   route, the procedures specified in [RFC6625]) to determine whether to
   accept the data.  If the data is accepted, then the V-hub further
   forwards it over the P-tunnel bound to the S-PMSI A-D route that has
   been re-advertised by the V-hub.

   If multiple S-PMSIs received by a V-hub have been aggregated into the
   same P-tunnel, then the V-hub, prior to forwarding to the V-spokes
   associated with that V-hub the data received on this P-tunnel, MAY
   de-aggregate and then reaggregate (in a different way) this data
   using the state present in its MVPN-TIB that has been created as a
   result of receiving C-multicast routes from the V-spokes.  Even if
   S-PMSIs received by the V-hub each have their own P-tunnel, the
   V-hub, prior to forwarding to the V-spokes the data received on these
   P-tunnels, MAY aggregate these S-PMSIs using the state present in its
   MVPN-TIB that has been created as a result of receiving C-multicast
   routes from the V-spokes.

7.9.  Use of Ingress Replication with I-PMSI A-D Routes

   The following modifications to the procedures specified in [RFC6514]
   for originating/receiving I-PMSI A-D routes enable the discarding of
   packets coming from a "wrong" PE when Ingress Replication is used for
   I-PMSI P-tunnels (for other types of P-tunnels, the procedures
   specified in [RFC6513] and [RFC6514] are sufficient).

   The modifications to the procedures are required to be implemented
   (by all the PEs of a given MVPN) only under the following conditions:

   +  At least one of those PEs is a V-hub or V-spoke PE for the given
      MVPN.

   +  The given MVPN is configured to use the optional procedure of
      using Ingress Replication to instantiate an I-PMSI.

   If Ingress Replication is used with I-PMSI A-D routes, when a PE
   advertises such routes, the Tunnel Type in the PMSI Tunnel attribute
   MUST be set to Ingress Replication; the Leaf Information Required
   flag MUST be set to 1; the attribute MUST carry no MPLS labels.

   A PE that receives such an I-PMSI A-D route MUST respond with a Leaf
   A-D route.  The PMSI Tunnel attribute of that Leaf A-D route is
   constructed as follows:

   o  The Tunnel Type is set to Ingress Replication.

   o  The Tunnel Identifier MUST carry a routable address of the PE that
      originates the Leaf A-D route.

   o  The PMSI Tunnel attribute MUST carry a downstream-assigned MPLS
      label that is used to demultiplex the traffic received over a
      unicast tunnel by the PE.

   o  The receiving PE MUST assign the label in such a way as to enable
      the receiving PE to identify (a) the VRF on that PE that should be
      used to process the traffic received with this label and (b) the
      PE that sends the traffic with this label.

   This document assumes that for a given MVPN, all the PEs that have
   sites of that MVPN connected to them implement the procedures
   specified in this section.

8.  An Example of RT Provisioning

   Consider a VPN A that consists of 9 sites -- site-1 through site-9.
   Each site is connected to its own PE -- PE-1 through PE-9.

   We designate PE-3, PE-6, and PE-9 as V-hubs.

   To simplify the presentation, the following example assumes that each
   V-spoke is associated with just one V-hub.  However, as mentioned
   earlier, in practice each V-spoke should be associated with two or
   more V-hubs.

   PE-1 and PE-2 are V-spokes associated with PE-3.  PE-4 and PE-5 are
   V-spokes associated with PE-6.  PE-7 and PE-8 are V-spokes associated
   with PE-9.

8.1.  Unicast Routing

   All the PEs (both V-hubs and V-spokes) are provisioned to export
   routes using RT-A (just as with "vanilla" any-to-any VPN).

   All the V-hubs (PE-3, PE-6, and PE-9) are provisioned to import
   routes with RT-A (just as with "vanilla" any-to-any VPN).

   In addition, PE-3 is provisioned to originate a VPN-IP default route
   with RT-A-VH-1 (but not with RT-A), while PE-1 and PE-2 are
   provisioned to import routes with RT-A-VH-1.

   Likewise, PE-6 is provisioned to originate a VPN-IP default route
   with RT-A-VH-2 (but not with RT-A), while PE-4 and PE-5 are
   provisioned to import routes with RT-A-VH-2.

   Finally, PE-9 is provisioned to originate a VPN-IP default route with
   RT-A-VH-3 (but not with RT-A), while PE-7 and PE-8 are provisioned to
   import routes with RT-A-VH-3.

   Now let's modify the example above a bit by assuming that site-3 has
   Internet connectivity.  Thus, site-3 advertises an IP default route
   to PE-3.  PE-3 in turn originates a VPN-IP default route.  In this
   case, the VPN-IP default route carries RT-A and RT-A-VH-1 (rather
   than just RT-A-VH-1, as before), which results in importing this
   route to PE-6 and PE-9, as well as to PE-1 and PE-2.

   If PE-7 and PE-8, in addition to importing a VPN-IP default route
   from PE-9, also want to import each other's VPN-IP routes, then PE-7
   and PE-8 export their VPN-IP routes with two RTs: RT-A and RT-A-VH-3.

8.2.  Multicast Routing

   All the PEs designated as V-spokes (PE-1, PE-2, PE-4, PE-5, PE-7, and
   PE-8) are provisioned to export their I-PMSI/S-PMSI/SA A-D routes
   using RT-A (just as with "vanilla" any-to-any MVPN).  Thus, these
   routes could be imported by all the V-hubs (PE-3, PE-6, and PE-9).

   The V-hub on PE-3 is provisioned to export its I-PMSI/S-PMSI/SA A-D
   routes with two RTs: RT-A and RT-A-VH-1.  Thus, these routes could be
   imported by all the other V-hubs (PE-6 and PE-9) and also by the
   V-spokes, but only by the V-spokes associated with the V-hub on PE-3
   (PE-1 and PE-2).  In addition, the V-hub on PE-3 originates the
   Associated-V-spoke-only I-PMSI A-D route with RT-A-VH-1.  This route
   could be imported only by the V-spokes associated with the V-hub on
   PE-3 (PE-1 and PE-2).

   The V-hub on PE-6 is provisioned to export its I-PMSI/S-PMSI/SA A-D
   routes with two RTs: RT-A and RT-A-VH-2.  Thus, these routes could be
   imported by all the other V-hubs (PE-3 and PE-9) and also by the
   V-spokes, but only by the V-spokes associated with the V-hub on PE-6
   (PE-4 and PE-5).  In addition, the V-hub on PE-6 originates the
   Associated-V-spoke-only I-PMSI A-D route with RT-A-VH-2.  This route
   could be imported only by the V-spokes associated with the V-hub on
   PE-6 (PE-4 and PE-5).

   The V-hub on PE-9 is provisioned to export its I-PMSI/S-PMSI/SA A-D
   routes with two RTs: RT-A and RT-A-VH-3.  Thus, these routes could be
   imported by all the other V-hubs (PE-3 and PE-6) and also by the
   V-spokes, but only by the V-spokes associated with the V-hub on PE-9
   (PE-7 and PE-8).  In addition, the V-hub on PE-9 originates the
   Associated-V-spoke-only I-PMSI A-D route with RT-A-VH-3.  This route
   could be imported only by the V-spokes associated with the V-hub on
   PE-9 (PE-7 and PE-8).

   If PE-7 and PE-8, in addition to importing a VPN-IP default route
   from PE-9, also want to import each other's VPN-IP routes, then PE-7
   and PE-8 export their I-PMSI/S-PMSI/SA A-D routes with two RTs: RT-A
   and RT-A-VH-3.

   If the V-hub on PE-9 imports an S-PMSI A-D route or SA A-D route
   originated by either some other V-hub (PE-3 or PE-6) or a V-spoke
   that is not associated with this V-hub (PE-1, or PE-2, or PE-4, or
   PE-5), the V-hub originates an S-PMSI A-D route (respectively, SA A-D
   route).  The V-hub constructs this route from the imported route
   following the procedures specified in Section 7.8.2.  Specifically,
   the V-hub replaces the RT(s) carried in the imported route with just
   one RT -- RT-A-VH-3.  Thus, the originated route could be imported
   only by the V-spokes associated with the V-hub on PE-9 (PE-7
   and PE-8).

9.  Further Refinements

   In some cases, a VPN customer may not want to rely solely on an (IP)
   default route being advertised from a V-spoke to a CE, but may want
   CEs to receive all the VPN routes (e.g., for the purpose of faster
   detection of VPN connectivity failures and activating some backup
   connectivity).

   In this case, an OPTIONAL approach would be to install in the
   V-spoke's data plane only the VPN-IP default route advertised by the
   V-hub associated with the V-spoke, even if the V-spoke receives an IP
   default route from the CE, and to keep all the VPN-IP routes in the
   V-spoke's control plane (thus being able to advertise these routes as
   IP routes from the V-spoke to the CEs).  Granted, this would not
   change control-plane resource consumption but would reduce forwarding
   state on the data plane.

10.  Security Considerations

   This document introduces no new security considerations above and
   beyond those already specified in [RFC4364].

11.  Acknowledgements

   We would like to acknowledge Han Nguyen (AT&T) for his contributions
   to this document.  We would like to thank Eric Rosen (Cisco) for his
   review and comments.  We would also like to thank Samir Saad (AT&T),
   Jeffrey (Zhaohui) Zhang (Juniper), and Thomas Morin (Orange) for
   their review and comments.

12.  References

12.1.  Normative References

   [IANA-SAFI]  IANA Subsequent Address Family Identifiers (SAFI)
                Parameters,
                <http://www.iana.org/assignments/safi-namespace/>.

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

   [RFC3031]    Rosen, E., Viswanathan, A., and R. Callon,
                "Multiprotocol Label Switching Architecture", RFC 3031,
                January 2001.

   [RFC4360]    Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
                Communities Attribute", RFC 4360, February 2006.

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

   [RFC4684]    Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
                R., Patel, K., and J. Guichard, "Constrained Route
                Distribution for Border Gateway Protocol/MultiProtocol
                Label Switching (BGP/MPLS) Internet Protocol (IP)
                Virtual Private Networks (VPNs)", RFC 4684, November
                2006.

   [RFC6513]    Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
                MPLS/BGP IP VPNs", RFC 6513, February 2012.

   [RFC6514]    Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
                Encodings and Procedures for Multicast in MPLS/BGP IP
                VPNs", RFC 6514, February 2012.

   [RFC6625]    Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R.
                Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes",
                RFC 6625, May 2012.

12.2.  Informative References

   [MPLS-MCAST] Rekhter, Y., Aggarwal, R., Morin, T., Grosclaude, I.,
                Leymann, N., and S. Saad, "Inter-Area P2MP Segmented
                LSPs", Work in Progress, May 2013.

Authors' Addresses

   Huajin Jeng
   AT&T

   EMail: hj2387@att.com


   James Uttaro
   AT&T

   EMail: ju1738@att.com


   Luay Jalil
   Verizon

   EMail: luay.jalil@verizon.com


   Bruno Decraene
   Orange

   EMail: bruno.decraene@orange.com


   Yakov Rekhter
   Juniper Networks, Inc.

   EMail: yakov@juniper.net


   Rahul Aggarwal
   Arktan

   EMail: raggarwa_1@yahoo.com