Internet Engineering Task Force (IETF) H. Jeng

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.

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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

Jeng, et al. Standards Track [Page 2]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.).

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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].

Jeng, et al. Standards Track [Page 4]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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

Jeng, et al. Standards Track [Page 9]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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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

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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.

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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.

Jeng, et al. Standards Track [Page 13]

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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.

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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

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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.

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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

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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

Jeng, et al. Standards Track [Page 18]

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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

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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.

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RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

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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

Jeng, et al. Standards Track [Page 22]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

Jeng, et al. Standards Track [Page 23]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

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.

Jeng, et al. Standards Track [Page 24]

RFC 7024 Virtual Hub-and-Spoke in BGP/MPLS VPNs October 2013

Authors' Addresses

Huajin Jeng
AT&T

EMail: [email protected]

James Uttaro
AT&T

EMail: [email protected]

Luay Jalil
Verizon

EMail: [email protected]

Bruno Decraene
Orange

EMail: [email protected]

Yakov Rekhter
Juniper Networks, Inc.

EMail: [email protected]

Rahul Aggarwal
Arktan

EMail: [email protected]

Jeng, et al. Standards Track [Page 25]