Network Working Group R. Aggarwal (Editor)
Internet Draft Juniper Networks
Category: Standards Track
Expiration Date: March 2011 Y. Kamite
NTT Communications
L. Fang
Cisco Systems, Inc
September 01, 2010
Multicast in VPLS
draft-ietf-l2vpn-vpls-mcast-07.txt
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Abstract
This document describes a solution for overcoming a subset of the
limitations of existing VPLS multicast solutions. It describes
procedures for VPLS multicast that utilize multicast trees in the
sevice provider (SP) network. One such multicast tree can be shared
between multiple VPLS instances. Procedures by which a single
multicast tree in the SP network can be used to carry traffic
belonging only to a specified set of one or more IP multicast streams
from one or more VPLSes are also described.
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Table of Contents
1 Specification of requirements ......................... 4
2 Contributors .......................................... 4
3 Terminology ........................................... 5
4 Introduction .......................................... 5
5 Existing Limitations of VPLS Multicast ................ 6
6 Overview .............................................. 6
6.1 Inclusive and Selective Multicast Trees ............... 6
6.2 BGP-Based VPLS Membership Auto-Discovery .............. 8
6.3 IP Multicast Group Membership Discovery ............... 8
6.4 Advertising P-Multicast Tree to VPLS/C-Multicast Binding ..9
6.5 Aggregation ........................................... 9
6.6 Inter-AS VPLS Multicast ............................... 10
7 Intra-AS Inclusive P-Multicast Tree A-D/Binding ....... 11
7.1 Originating intra-AS VPLS auto-discovery routes ....... 12
7.2 Receiving intra-AS VPLS auto-discovery routes ......... 12
8 Demultiplexing P-Multicast Tree Traffic ............... 14
8.1 One P-Multicast Tree - One VPLS Mapping ............... 14
8.2 One P-Multicast Tree - Many VPLS Mapping .............. 14
9 Establishing P-Multicast Trees ........................ 15
9.1 Common Procedures ..................................... 15
9.2 RSVP-TE P2MP LSPs ..................................... 16
9.2.1 P2MP TE LSP - VPLS Mapping ............................ 16
9.3 Receiver Initiated MPLS Trees ......................... 17
9.3.1 P2MP LSP - VPLS Mapping ............................... 17
9.4 Encapsulation of Aggregate P-Multicast Trees .......... 17
10 Inter-AS Inclusive P-Multicast Tree A-D/Binding ....... 17
10.1 VSIs on the ASBRs ..................................... 18
10.1.1 Option (a) ............................................ 18
10.1.2 Option (e) ............................................ 18
10.2 Option (b) - Segmented Inter-AS Trees ................. 19
10.2.1 Segmented Inter-AS Trees VPLS Inter-AS A-D/Binding .... 19
10.2.2 Propagating BGP VPLS A-D routes to other ASes: Overview ...20
10.2.2.1 Propagating Intra-AS VPLS A-D routes in E-BGP ......... 21
10.2.2.2 Inter-AS A-D route received via E-BGP ................. 22
10.2.2.3 Leaf A-D Route received via E-BGP ..................... 24
10.2.2.4 Inter-AS A-D Route received via I-BGP ................. 24
10.3 Option (c) ............................................ 25
11 Optimizing Multicast Distribution via Selective Trees . 26
11.1 Protocol for Switching to Selective Trees ............. 28
11.2 Advertising C-(S, G) Binding to a Selective Tree ...... 29
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11.3 Receiving S-PMSI A-D routes by PEs .................... 31
11.4 Inter-AS Selective Tree ............................... 32
11.4.1 VSIs on the ASBRs ..................................... 33
11.4.1.1 VPLS Inter-AS Selective Tree A-D Binding .............. 33
11.4.2 Inter-AS Segmented Selective Trees .................... 33
11.4.2.1 Handling S-PMSI A-D routes by ASBRs ................... 34
11.4.2.1.1 Merging Selective Tree into an Inclusive Tree ......... 35
11.4.3 Inter-AS Non-Segmented Selective trees ................ 36
12 BGP Extensions ........................................ 36
12.1 Inclusive Tree/Selective Tree Identifier .............. 36
12.2 MCAST-VPLS NLRI ....................................... 37
12.2.1 S-PMSI auto-discovery route ........................... 37
12.2.2 Leaf auto-discovery route ............................. 38
13 Aggregation Methodology ............................... 39
14 Data Forwarding ....................................... 39
14.1 MPLS Tree Encapsulation ............................... 39
14.1.1 Mapping multiple VPLS instances to a P2MP LSP ......... 39
14.1.2 Mapping one VPLS instance to a P2MP LSP ............... 40
15 VPLS Data Packet Treatment ............................ 41
16 Security Considerations ............................... 42
17 IANA Considerations ................................... 43
18 Acknowledgments ....................................... 43
19 Normative References .................................. 43
20 Informative References ................................ 44
21 Author's Address ...................................... 45
1. 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].
2. Contributors
Rahul Aggarwal
Yakov Rekhter
Juniper Networks
Yuji Kamite
NTT Communications
Luyuan Fang
AT&T
Chaitanya Kodeboniya
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3. Terminology
This document uses terminology described in [RFC4761] and [RFC4762].
4. Introduction
[RFC4761] and [RFC4762] describe a solution for VPLS multicast that
relies on ingress replication. This solution has certain limitations
for certain VPLS multicast traffic profiles. For example it may
result in highly non-optimal bandwidth utilization in the MPLS
network when large amount of multicast traffic is to be transported
This document describes procedures for overcoming the limitations of
existing VPLS multicast solutions. It describes procedures for VPLS
multicast that utilize multicast trees in the Sevice Provider (SP)
network. The procedures described in this document are applicable to
both [RFC4761] and [RFC4762].
It provides mechanisms that allow a single multicast distribution
tree in the Service Provider (SP) network to carry all the multicast
traffic from one or VPLS sites connected to a given PE, irrespective
of whether these sites belong to the same or different VPLSes. Such a
tree is referred to as an "Inclusive tree" and more specifically as
an "Aggregate Inclusive tree" when the tree is used to carry
multicast traffic from more than one VPLS.
This document also provides procedures by which a single multicast
distribution tree in the SP network can be used to carry traffic
belonging only to a specified set of IP multicast streams, originated
in one or more VPLS sites connected to a given PE, irrespective of
whether these sites belong to the same or different VPLSes. Such a
tree is referred to as a "Selective tree" and more specifically as an
"Aggregate Selective tree" when the IP multicast streams belong to
different VPLSes. This allows multicast traffic, by default, to be
carried on an Inclusive tree, while traffic from some specific
multicast streams, e.g., high bandwidth streams, could be carried on
one of the "Selective trees".
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5. Existing Limitations of VPLS Multicast
One of the limitations of existing VPLS multicast solutions described
in [RFC4761] and [RFC4762] is that they rely on ingress replication.
Thus the ingress PE replicates the multicast packet for each egress
PE and sends it to the egress PE using a unicast tunnel.
This may be an acceptable model when the bandwidth of the multicast
traffic is low or/and the number of replications performed on an
average on each outgoing interface for a particular customer VPLS
multicast packet is small. If this is not the case it is desirable to
utilize multicast trees in the SP network to transmit VPLS multicast
packets [MCAST-VPLS-REQ]. Note that unicast packets that are flooded
to each of the egress PEs, before the ingress PE learns the
destination MAC address of those unicast packets, MAY still use
ingress replication.
6. Overview
This document describes procedures for using multicast trees in the
SP network to transport VPLS multicast data packets. RSVP-TE P2MP
LSPs described in [RFC4875] are an example of such multicast trees.
The use of multicast trees in the SP network can be beneficial when
the bandwidth of the multicast traffic is high or when it is
desirable to optimize the number of copies of a multicast packet
transmitted on a given link. This comes at a cost of state in the SP
network to build multicast trees and overhead to maintain this state.
This document describes procedures for using multicast trees for VPLS
multicast when the provider tunneling technology is either P2MP RSVP-
PE or mLDP [MLDP]. The protocol architecture described herein is
considered to be flexible to support other P-tunneling technologies
as well.
This document uses the prefix 'C' to refer to the customer control or
data packets and 'P' to refer to the provider control or data
packets. An IP (multicast source, multicast group) tuple is
abbreviated to (S, G).
6.1. Inclusive and Selective Multicast Trees
Multicast trees used for VPLS can be of two types:
1. Inclusive trees. This option supports the use of a single
multicast distribution tree, referred to as an Inclusive P-Multicast
tree, in the SP network to carry all the multicast traffic from a
specified set of VPLS sites connected to a given PE. There is no
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assumption made with respect to whether this traffic is IP
encapsulated or not. A particular P-Multicast tree can be set up to
carry the traffic originated by sites belonging to a single VPLS, or
to carry the traffic originated by sites belonging to different
VPLSes. The ability to carry the traffic of more than one VPLS on the
same tree is termed of the VPLSes that are using the tree. This
implies that a PE may receive multicast traffic for a multicast
stream even if it doesn't have any receivers that are interested in
receiving traffic for that stream.
An Inclusive P-Multicast tree as defined in this document is a P2MP
tree. A P2MP tree is used to carry traffic only for VPLS sites that
are connected to the PE that is the root of the tree.
2. Selective trees. A Selective P-Multicast tree is used by a PE
to send IP multicast traffic for one or IP more specific multicast
streams, originated within sites connected to the PE, that belong to
the same or different VPLSes, to a subset of the PEs that belong to
those VPLSes. Each of the PEs in the subset should be on the path to
a receiver of one or more multicast streams that are mapped onto the
tree. The ability to use the same tree for multicast streams that
belong to different VPLSes is termed a PE the ability to create
separate SP multicast trees for specific multicast streams, e.g. high
bandwidth multicast streams. This allows traffic for these multicast
streams to reach only those PE routers that have receivers in these
streams. This avoids flooding other PE routers in the VPLS.
A SP can use both Inclusive P-Multicast trees and Selective P-
Multicast trees or either of them for a given VPLS on a PE, based on
local configuration. Inclusive P-Multicast trees can be used for
both IP and non-IP data multicast traffic, while Selective P-
Multicast trees can be used only for IP multicast data traffic.
A variety of transport technologies may be used in the SP network.
For inclusive P-Multicast trees, these transport technologies include
point-to-multipoint LSPs created by RSVP-TE or mLDP. For selective P-
Multicast trees, only unicast PE-PE tunnels (using MPLS or IP/GRE
encapsulation) and P2MP LSPs are supported, and the supported P2MP
LSP signaling protocols are RSVP-TE, and mLDP.
This document also describes the data plane encapsulations for
supporting the various SP multicast transport options.
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6.2. BGP-Based VPLS Membership Auto-Discovery
In order to establish Inclusive P-Multicast trees for one or more
VPLSes, when Aggregation is performed or when the tunneling
technology is P2MP RSVP-TE, the root of the tree must be able to
discover the other PEs that have membership in one or more of these
VPLSes. This document uses the BGP-based procedures described in
[RFC4761] and [L2VPN-SIG] for discovering the VPLS membership of all
PEs.
The leaves of the Inclusive P-Multicast trees must also be able to
auto-discover the identifier of the tree. This is described in
section 6.4.
6.3. IP Multicast Group Membership Discovery
The setup of a Selective P-Multicast tree for one or more IP
multicast (S, G)s, requires the ingress PE to learn the PEs that have
receivers in one or more of these (C-S, C-G)s, in the following
cases:
+ When aggregation is used OR
+ When the tunneling technology is P2MP RSVP-TE
+ If ingress replication is used and the ingress PE wants to send
traffic for (C-S, C-G)s to only those PEs that are on the path to
receivers to the (C-S,C-G)s.
For discovering the IP multicast group membership, this document
describes procedures that allow an ingress PE to enable explicit
tracking. Thus an ingress PE can request the IP multicast membership
from egress PEs for one or more C-multicast streams. These procedures
are described in section "Optimizing Multicast Distribution via
Selective Trees".
These procedures are applicable when IGMP is used as the multicast
routing protocol between the VPLS CEs. They are also applicable when
PIM is used as the multicast routing protocol between the VPLS CEs
and PIM join suppression is disabled on all the CEs. However these
procedures do not apply when PIM is used as the multicast routing
protocol between the VPLS CEs and it not possible to disable PIM join
suppression on all the CEs. Procedures for this case are for further
study.
The leaves of the Selective P-Multicast trees must also be able to
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discover the identifier of the tree. This is described in section
6.4.
6.4. Advertising P-Multicast Tree to VPLS/C-Multicast Binding
This document describes procedures based on BGP VPLS Auto-Discovery
(A-D) that are used by the root of an Aggregate P-Multicast tree to
advertise the Inclusive or Selective P-Multicast tree binding and the
de-multiplexing information to the leaves of the tree. This document
uses the PMSI Tunnel Attribute [BGP-MVPN] for this purpose.
Once a PE decides to bind a set of VPLSes or customer multicast
groups to an Inclusive P-Multicast tree or a Selective P-Multicast
tree, it needs to announce this binding to other PEs in the network.
This procedure is referred to as Inclusive P-Multicast tree or
Selective P-Multicast tree binding distribution and is performed
using BGP.
When an Aggregated Inclusive P-Multicast tree is used by an ingress
PE, this discovery implies that an ingress PE MUST announce the
binding of all VPLSes bound to the Inclusive P-Multicast tree to the
other PEs. The inner label assigned by the ingress PE for each VPLS
MUST be included, if more than one VPLS is bound to the same P-
Multicast tree. The Inclusive P-Multicast tree Identifier MUST be
included.
For a Selective P-Multicast tree this discovery implies announcing
all the specific <C-S, C-G> entries bound to this P-Multicast tree
along with the Selective P-Multicast tree Identifier. The inner label
assigned for each <C-S, C-G> MUST be included if <C-S, C-G>s from
different VPLSes are bound to the same P-Multicast tree. The labels
MUST be distinct on a per VPLS basis and MAY be distinct on a per <C-
S, C-G> basis. The Selective P-Multicast tree Identifier MUST be
included.
6.5. Aggregation
As described above the ability to carry the traffic of more than one
VPLS on the same P-Multicast tree is termed 'Aggregation'. Both
Inclusive and Selective P-Multicast trees support aggregation.
Aggregation enables the SP to place a bound on the amount of
multicast tree forwarding and control plane state which the P routers
must have. Let us call the number of VPLSes aggregated onto a single
P-Multicast tree as the "Aggregation Factor". When Inclusive source
P-Multicast trees are used the number of trees that a PE is the root
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of is proportional to:
+ (Number of VPLSes on the PE / Aggregation Factor).
In this case the state maintained by a P router, is proportional to:
+ ((Average number of VPLSes on a PE / Aggregation Factor) * number
of PEs) / (Average number of P-Multicast trees that transit a
given P router)
Thus the state does not grow linearly with the number of VPLSes.
Aggregation requires a mechanism for the egresses of the P-Multicast
tree to demultiplex the multicast traffic received over the P-
Multicast tree. This document describes how upstream-assigned labels
can be assigned and distributed by the root of aggregate P-Multicast
tree and then used by the egresses to perform this demultiplexing.
6.6. Inter-AS VPLS Multicast
This document supports three models of inter-AS VPLS service, option
(a), (b) and (c) which are very similar conceptually to option (a),
(b) and (c) specified in [RFC4364] for IP VPNs. The three options
described here are also similar to the three options described in in
[RFC4761], which in turn extends the concepts of [RFC4364] to inter-
AS VPLS.
For option (a) and option (b) support this document specifies a model
where Inter-AS VPLS service can be offered without requiring a single
P-Multicast tree to span multiple ASes. There are two variants of
this model and they are described in section 10.
For option (c) support this document specifies a model where Inter-AS
VPLS service is offered by requiring a single P-Multicast tree to
span multiple ASs. This is because in the case of option (c) the
ASBRs do not exchange BGP-VPLS NLRIs or A-D routes.
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7. Intra-AS Inclusive P-Multicast Tree A-D/Binding
This section specifies procedures for the intra-AS auto-discovery (A-
D) of VPLS membership and the distribution of information used to
instantiate P-Multicast Tunnels.
VPLS auto-discovery/binding consists of two components: intra-AS and
inter-AS. The former provides VPLS auto-discovery/binding within a
single AS. The latter provides VPLS auto-discovery/binding across
multiple ASes. Inter-AS auto-discovery/binding is described in
section 10.
VPLS auto-discovery using BGP as described in [RFC4761, L2VPN-SIG]
enables a PE to learn the VPLS membership of other PEs. A PE that
belongs to a particular VPLS announces a BGP Network Layer
Reachability Information (NLRI) that identifies the Virtual Switch
Instance (VSI). This NLRI is constructed from the <Route-
Distinguisher (RD), VPLS Edge Device Identifier (VE-ID)> tuple. The
NLRI defined in [RFC4761] comprises the <RD, VE-ID> tuple and label
blocks for PW signaling. The VE-ID in this case is a two octet
number. The NLRI defined in [L2VPN-SIG] comprises only the <RD, VE-
ID> where the VE-ID is a four octet number.
The procedures for constructing Inclusive intra-AS and inter-AS trees
as specified in this document require the BGP A-D NLRI to carry only
the <RD, VE-ID>. Hence these procedures can be used for both BGP-VPLS
and LDP-VPLS with BGP A-D.
It is to be noted that BGP A-D is an inherent feature of BGP-VPLS.
However it is not an inherent feature of LDP-VPLS. Infact there are
deployments and/or implementations of LDP-VPLS that require
configuration to enable a PE in a particular VPLS to determine other
PEs in the VPLS and exchange PW labels using FEC 128 [RFC4447]. The
use of BGP A-D for LDP-VPLS [L2VPN-SIG], to enable automatic setup of
PWs, requires FEC 129 [RFC4447]. However FEC 129 is not required in
order to use BGP A-D for the setup of P-Multicast trees for LDP-VPLS
as described in this document. An LDP-VPLS implementation that
supports P-Multicast trees described in this document, MUST support
the BGP A-D procedures to setup P-Multicast trees and it MAY support
FEC 129 to automate the signaling of PWs.
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7.1. Originating intra-AS VPLS auto-discovery routes
To participate in the VPLS auto-discovery/binding a PE router that
has a given VSI of a given VPLS originates an BGP VPLS intra-AS auto-
discovery route and advertises this route in Multi-Protocol (MP) I-
BGP. The route is constructed as described in [RFC4761] and [L2VPN-
SIG].
The route carries a single L2VPN NLRI with the RD set to the RD of
the VSI, and the VE-ID set to the VE-ID of the VSI.
If an Inclusive P-Multicast tree is used to instantiate the provider
tunnel for VPLS multicast on the PE, the advertising PE MUST
advertise the type and the identity of the P-Multicast tree in the
the PMSI Tunnel attribute [BGP-MVPN]. This attribute is described in
section 12.1.
A PE that uses an Inclusive P-Multicast tree to instantiate the
provider tunnel MAY aggregate two or more VPLSes present on the PE
onto the same tree. If the PE decides to perform aggregation after it
has already advertised the intra-AS VPLS auto-discovery routes for
these VPLSes, then aggregation requires the PE to re-advertise these
routes. The re-advertised routes MUST be the same as the original
ones, except for the PMSI Tunnel attribute. If the PE has not
previously advertised intra-AS auto-discovery routes for these
VPLSes, then the aggregation requires the PE to advertise (new)
intra-AS auto-discovery routes for these VPLSes. The P-Tunnel
attribute in the newly advertised/re-advertised routes MUST carry the
identity of the P-Multicast tree that aggregates the VPLSes, as well
as an MPLS upstream-assigned label [RFC5331]. Each re-advertised
route MUST have a distinct label.
Discovery of PE capabilities in terms of what tunnels types they
support is outside the scope of this document. Within a given AS PEs
participating in a VPLS are expected to advertise tunnel bindings
whose tunnel types are supported by all other PEs that are
participating in this VPLS and are part of the same AS.
7.2. Receiving intra-AS VPLS auto-discovery routes
When a PE receives a BGP Update message that carries an intra-AS A-D
route such that (a) the route was originated by some other PE within
the same AS as the local PE, (b) at least one of the Route Targets of
the route matches one of the import Route Targets configured for a
particular VSI on the local PE, (c) the BGP route selection
determines that this is the best route with respect to the NLRI
carried by the route, and (d) the route carries the PMSI Tunnel
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attribute, the PE performs the following.
If the route carries the PMSI Tunnel attribute then:
+ If the Tunnel Type in the PMSI Tunnel attribute is set to LDP
P2MP LSP, the PE SHOULD join the P-Multicast tree whose identity
is carried in the PMSI Tunnel Attribute.
+ If the Tunnel Type in the PMSI Tunnel attribute is set to RSVP-TE
P2MP LSP, the receiving PE has to establish the appropriate state
to properly handle the traffic received over that LSP. The PE
that originated the route MUST establish an RSVP-TE P2MP LSP with
the local PE as a leaf. This LSP MAY have been established before
the local PE receives the route.
+ If the PMSI Tunnel attribute does not carry a label, then all
packets that are received on the P-Multicast tree, as identified
by the PMSI Tunnel attribute, are forwarded using the VSI that
has at least one of its import Route Targets that matches one of
the Route Targets of the received auto-discovery route.
+ If the PMSI Tunnel attribute has the Tunnel Type set to LDP P2MP
LSP or RSVP-TE P2MP LSP, and the attribute also carries an MPLS
label, then the egress PE MUST treat this as an upstream-assigned
label, and all packets that are received on the P-Multicast tree,
as identified by the PMSI Tunnel attribute, with that upstream
label are forwarded using the VSI that has at least one of its
import Route Target that matches one of the Route Targets of the
received intra-AS auto-discovery route.
If the local PE uses RSVP-TE P2MP LSP for sending (multicast)
traffic, originated by VPLS sites connected to the PE, to the sites
attached to other PEs then the local PE MUST use the Originating
Router's IP address information carried in the intra-AS A-D route to
add the PE, that originated the route, as a leaf node to the LSP.
This MUST be done irrespective of whether the received Intra-AS A-D
route carries the PMSI Tunnel attribute or not.
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8. Demultiplexing P-Multicast Tree Traffic
Demultiplexing received VPLS traffic requires the receiving PE to
determine the VPLS instance the packet belongs to. The egress PE can
then perform a VPLS lookup to further forward the packet. It also
requires the egress PE to determine the identity of the ingress PE
for MAC learning, as described in section 15.
8.1. One P-Multicast Tree - One VPLS Mapping
When a P-Multicast tree is mapped to only one VPLS, determining the
tree on which the packet is received is sufficient to determine the
VPLS instance on which the packet is received. The tree is determined
based on the tree encapsulation. If MPLS encapsulation is used, eg:
RSVP-TE P2MP LSPs, the outer MPLS label is used to determine the
tree. Penultimate-hop-popping MUST be disabled on the MPLS LSP (RSVP-
TE P2MP LSP or LDP P2MP LSP).
8.2. One P-Multicast Tree - Many VPLS Mapping
As traffic belonging to multiple VPLSes can be carried over the same
tree, there is a need to identify the VPLS the packet belongs to.
This is done by using an inner label that determines to the VPLS for
which the packet is intended. The ingress PE uses this label as the
inner label while encapsulating a customer multicast data packet.
Each of the egress PEs must be able to associate this inner label
with the same VPLS and use it to demultimplex the traffic received
over the Aggregate Inclusive tree or the Aggregate Selective tree.
This document requires the use of upstream label assignment by the
ingress PE [RFC5331]. Hence the inner label is assigned by the
ingress PE. When the egress PE receives a packet over an Aggregate
tree, the outer encapsulation [in the case of MPLS P2MP LSPs, the
outer MPLS label] specifies the label space to perform the inner
label lookup. The same label space MUST be used by the egress PE for
all P-Multicast trees that have the same root [RFC5331].
If the tree uses MPLS encapsulation, as in RSVP-TE P2MP LSPs, the
outer MPLS label and the incoming interface provides the label space
of the label beneath it. This assumes that penultimate-hop-popping is
disabled. The egress PE MUST NOT advertise IMPLICIT NULL or EXPLICIT
NULL for that tree. Once the label representing the tree is popped
off the MPLS label stack, the next label is the demultiplexing
information that allows the proper MVPN to be determined.
The ingress PE informs the egress PEs about the inner label as part
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of the tree binding procedures described in section 12.
9. Establishing P-Multicast Trees
This document does not place any fundamental restrictions on the
multicast technology used to setup P-Multicast trees. However
specific procedures are specified only for RSVP-TE P2MP LSPs and LDP
P2MP LSPs. An implementation that supports this document MUST support
RSVP-TE P2MP LSPs and MAY support LDP P2MP LSPs.
The P-Multicast trees supported in this document are P2MP trees. A
P2MP tree is used to carry traffic originated in sites connected to
the PE which is the root of the tree, irrespective of whether these
sites belong to the same or different VPLSes.
9.1. Common Procedures
The following procedures apply to both RSVP-TE P2MP and LDP P2MP
LSPs.
Demultiplexing the C-multicast data packets at the egress PE requires
that the PE must be able to determine the P2MP LSP that the packets
are received on. This enables the egress PE to determine the VPLS
that the packet belongs to. To achieve this the LSP MUST be signaled
with penultimate-hop-popping (PHP) off as described in section 8. In
other words an egress PE MUST NOT advertise IMPLICIT NULL or EXPLICIT
NULL for a P2MP LSP that is carrying traffic for one or more VPLSes.
This is because the egress PE needs to rely on the MPLS label, that
it advertises to its upstream neighbor, to determine the P2MP LSP
that a C-multicast data packet is received on.
The egress PE also needs to identify the ingress PE to perform MAC
learning. When P2MP LSPs are used as P2MP trees, determining the
P2MP LSP that the packets are received on, is sufficient to determine
the ingress PE. This is because the ingress PE is the root of the
P2MP LSP.
The egress PE relies on receiving the PMSI Tunnel Attribute in BGP to
determine the VPLS instance to P2MP LSP mapping.
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9.2. RSVP-TE P2MP LSPs
This section describes procedures that are specific to the usage of
RSVP-TE P2MP LSPs for instantiating a P-Multicast tree. Procedures in
[RFC4875] are used to signal the P2MP LSP. The LSP is signaled after
the root of the P2MP LSP discovers the leaves. The egress PEs are
discovered using the procedures described in section 7. Aggregation
as described in this document is supported.
9.2.1. P2MP TE LSP - VPLS Mapping
P2MP TE LSP to VPLS mapping is learned at the egress PEs using BGP
based advertisements of the P2MP TE LSP - VPLS mapping. They require
that the root of the tree include the P2MP TE LSP identifier as the
tunnel identifier in the BGP advertisements. This identifier contains
the following information elements:
- The type of the tunnel is set to RSVP-TE P2MP LSP
- RSVP-TE P2MP LSP's SESSION Object
This Tunnel Identifier is described in section 12.1.
Once the egress PE receives the P2MP TE LSP to VPLS mapping:
+ If the egress PE already has RSVP-TE state for the P2MP TE LSP,
it MUST begin to assign a MPLS label from the non-reserved label
range, for the P2MP TE LSP and signal this to the previous hop of
the P2MP TE LSP. Further it MUST create forwarding state to
forward packets received on the P2MP LSP.
+ If the egress PE does not have RSVP-TE state for the P2MP TE LSP,
it MUST retain this mapping. Subsequently when the egress PE
receives the RSVP-TE P2MP signaling message, it creates the RSVP-
TE P2MP LSP state. It MUST then assign a MPLS label from the
non-reserved label range, for the P2MP TE LSP, and signal this to
the previous hop of the P2MP TE LSP.
Note that if the signaling to set up an RSVP-TE P2MP LSPis
completed before a given egress PE learns, via a PMSI Tunnel
attribute, of the VPLS or set of VPLSes to which the LSP is
bound, the PE MUST discard any traffic received on that LSP until
the binding is received. In order for the egress PE to be able to
discard such traffic it needs to know that the LSP is associated
with one or more VPLSes and that the VPLS A-D route that binds
the LSP to a VPLS has not yet been received. This is provided by
extending [RFC4875] with [RSVP-OBB].
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9.3. Receiver Initiated MPLS Trees
Receiver initiated P2MP MPLS trees signaled using LDP [mLDP] can also
be used. Procedures in [MLDP] MUST be used to signal the P2MP LSP.
The LSP is signaled once the leaves receive the LDP FEC for the tree
from the root as described in section 7. An ingress PE is required to
discover the egress PEs when aggregation is used and this is achieved
using the procedures in section 7.
9.3.1. P2MP LSP - VPLS Mapping
P2MP LSP to VPLS mapping is learned at the egress PEs using BGP based
advertisements of the P2MP LSP - VPLS mapping. They require that the
root of the tree include the P2MP LSP identifier as the tunnel
identifier in the BGP advertisements. This identifier contains the
following information elements:
- The type of the tunnel is set to LDP P2MP LSP
- LDP P2MP FEC which includes an identifier generated by the
root.
Each egress PE SHOULD "join" the P2MP MPLS tree by sending LDP label
mapping messages for the LDP P2MP FEC, that was learned in the BGP
advertisement, using procedures described in [MLDP].
9.4. Encapsulation of Aggregate P-Multicast Trees
An Aggregate Inclusive P-Multicast tree or an Aggregate Selective P-
Multicast tree MUST use a MPLS encapsulation. The protocol type in
the data link header is as described in [RFC5332].
10. Inter-AS Inclusive P-Multicast Tree A-D/Binding
This document supports three models of inter-AS VPLS service, option
(a), (b) and (c) which are very similar conceptually to option (a),
(b) and (c) specified in [RFC4364] for IP VPNs. The three options
described here are also similar to the three options described in
[RFC4761], which in turn extends the concepts of [RFC4364] to inter-
AS VPLS. An implementation MUST support all three of these models.
When there are multiple options for implementing one of these models,
this section specifies which option is mandatory.
For option (a) and option (b) support this section specifies a model
where inter-AS VPLS service can be offered without requiring a single
P-Multicast tree to span multiple ASes. This allows individual ASes
to potentially use different P-tunneling technologies. There are two
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variants of this model. One that requires MAC lookup on the ASBRs and
another that does not require MAC lookup on the ASBRs and instead
builds segmented inter-AS trees. This applies to both Inclusive and
Selective trees.
For option (c) support this document specifies a model where Inter-AS
VPLS service is offered by requiring a single Inclusive P-Multicast
tree to span multiple ASs. This is referred to as a non-segmented P-
Multicast tree. This is because in the case of option (c) the ASBRs
do not exchange BGP-VPLS NLRIs or VPLS A-D routes. Selective inter-AS
trees for option (c) support may be segmented or non-segmented.
10.1. VSIs on the ASBRs
In this variant, the ASBRs MUST perform a MAC lookup, in addition to
any MPLS lookups, to determine the forwarding decision on a VPLS
packet. The P-Multicast trees are confined to an AS. An ASBR on
receiving a VPLS packet from another ASBR is required to perform a
MAC lookup to determine how to forward the packet. Thus an ASBR is
required to keep a VSI for the VPLS and MUST be configured with its
own VE ID for the VPLS. In this variant the BGP VPLS A-D routes
generated by PEs in an AS MUST NOT be propagated outside the AS.
10.1.1. Option (a)
When this variant is used with option (a) an ASBR in one AS treats an
adjoining ASBR in another AS as a CE and determines the VSI for
packets received from another ASBR based on the incoming ethernet
interface. In the case of option (a) the ASBRs do not exchange VPLS
A-D routes.
An implementation MUST support this variant for option (a).
10.1.2. Option (e)
The VSIs on the ASBRs variant can be used such that the interconnect
between the ASBRs is a PW and MPLS encapsulation is used between the
ASBRs. An ASBR in one AS treats an adjoining ASBR in another AS as a
CE and determines the VSI for packets received from another ASBR
based on the incoming MPLS encapsulation. The only VPLS A-D routes
that are propagated outside the AS are the ones originated by ASBRs.
This MPLS PW connects the VSIs on the ASBRs and MUST be signaled
using the procedures defined in [RFC4761] or [RFC4762].
The P-Multicast trees for a VPLS are confined to each AS and the VPLS
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auto-discovery/binding MUST follow the intra-AS procedures described
in section 8. An implementation MAY support option (e).
10.2. Option (b) - Segmented Inter-AS Trees
In this variant, an inter-AS P-Multicast tree, rooted at a particular
PE for a particular VPLS instance, consists of a number of
"segments", one per AS, which are stitched together at ASBRs. These
are known as "segmented inter-AS trees". Each segment of a segmented
inter-AS tree may use a different multicast transport technology. In
this variant, an ASBR is not required to keep a VSI for the VPLS and
is not required to perform a MAC lookup in order to forward the VPLS
packet. This implies that an ASBR is not required to be configured
with a VE ID for the VPLS. This variant is applicable to option (b).
An implementation MUST support this variant.
The construction of segmented Inter-AS trees requires the BGP-VPLS A-
D NLRI described in [RFC4761, RFC4762]. A BGP VPLS A-D route for a
<RD, VE ID> tuple advertised outside the AS, to which the originating
PE belongs, will be referred to as an inter-AS VPLS auto-discovery
route (Though this route is originated by a PE as an intra-AS route
and is referred to as an inter-AS route outside the AS).
In addition to this, segmented inter-AS trees require support for the
PMSI Tunnel Attribute described in section 12.1. They also require
additional procedures in BGP to signal leaf A-D routes between ASBRs
as explained in subsequent sections.
10.2.1. Segmented Inter-AS Trees VPLS Inter-AS A-D/Binding
This section specifies the procedures for inter-AS VPLS A-D/binding
for segmented inter-AS trees.
An ASBR must be configured to support a particular VPLS as follows:
+ An ASBR MUST be be configured with a set of (import) Route
Targets (RTs) that specifies the set of VPLSes supported by the
ASBR. These Route Targets control acceptance of BGP VPLS auto-
discovery routes by the ASBR. Note that instead of being
configured, the ASBR MAY obtain this set of (import) Route
Targets (RTs) by using Route Target Constrain [RFC4684].
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+ The ASBR MUST be configured with the tunnel types for the intra-
AS segments of the VPLSes supported by the ASBR, as well as
(depending on the tunnel type) the information needed to create
the PMSI Tunnel attribute for these tunnel types. Note that
instead of being configured, the ASBR MAY derive the tunnel types
from the intra-AS auto-discovery routes received by the ASBR from
the PEs in its own AS.
If an ASBR is configured to support a particular VPLS, the ASBR MUST
participate in the intra-AS VPLS auto-discovery/binding procedures
for that VPLS within the ASBR's own AS, as defined in this document.
Moreover, in addition to the above the ASBR performs procedures
specified in the next section.
10.2.2. Propagating BGP VPLS A-D routes to other ASes: Overview
An auto-discovery route for a given VPLS, originated by an ASBR
within a given AS, is propagated via BGP to other ASes. The precise
rules for distributing and processing the inter-AS auto-discovery
routes are given in subsequent sections.
Suppose that an ASBR A receives and installs an auto-discovery route
for VPLS "X" and VE ID "V" that originated at a particular PE, PE1.
The BGP next hop of that received route becomes A's "upstream
neighbor" on a multicast distribution tree for (X, V) that is rooted
at PE1. When the auto-discovery routes have been distributed to all
the necessary ASes, they define a "reverse path" from any AS that
supports VPLS X and VE ID V back to PE1. For instance, if AS2
supports VPLS X, then there will be a reverse path for VPLS X and VE
ID V from AS2 to AS1. This path is a sequence of ASBRs, the first of
which is in AS2, and the last of which is in AS1. Each ASBR in the
sequence is the BGP next hop of the previous ASBR in the sequence on
the given auto-discovery route.
This reverse path information can be used to construct a
unidirectional multicast distribution tree for VPLS X and VE ID V,
containing all the ASes that support X, and having PE1 at the root.
We call such a tree an "inter-AS tree". Multicast data originating in
VPLS sites for VPLS X connected to PE1 will travel downstream along
the tree which is rooted at PE1.
The path along an inter-AS tree is a sequence of ASBRs. It is still
necessary to specify how the multicast data gets from a given ASBR to
the set of ASBRs which are immediately downstream of the given ASBR
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along the tree. This is done by creating "segments": ASBRs in
adjacent ASes will be connected by inter-AS segments, ASBRs in the
same AS will be connected by "intra-AS segments".
For a given inter-AS tree and a given AS there MUST be only one ASBR
within that AS that accepts traffic flowing on that tree. Further for
a given inter-AS tree and a given AS there MUST be only one ASBR in
that AS that sends the traffic flowing on that tree to a particular
adjacent AS. The precise rules for accomplishing this are given in
subsequent sections.
An ASBR initiates creation of an intra-AS segment when the ASBR
receives an inter-AS auto-discovery route from an E-BGP neighbor.
Creation of the segment is completed as a result of distributing, via
I-BGP, this route within the ASBR's own AS.
For a given inter-AS tunnel each of its intra-AS segments could be
constructed by its own independent mechanism. Moreover, by using
upstream-assigned labels within a given AS multiple intra-AS segments
of different inter-AS tunnels of either the same or different VPLSes
may share the same P-Multicast tree.
If the P-Multicast tree instantiating a particular segment of an
inter-AS tunnel is created by a multicast control protocol that uses
receiver-initiated joins (e.g, mLDP), and this P-Multicast tree does
not aggregate multiple segments, then all the information needed to
create that segment will be present in the inter-AS auto-discovery
routes received by the ASBR from the neighboring ASBR. But if the P-
Multicast tree instantiating the segment is created by a protocol
that does not use receiver-initiated joins (e.g., RSVP-TE, ingress
unicast replication), or if this P-Multicast tree aggregates multiple
segments (irrespective of the multicast control protocol used to
create the tree), then the ASBR needs to learn the leaves of the
segment. These leaves are learned from A-D routes received from other
PEs in the AS, for the same VPLS (i.e. same VE-ID) as the one that
the segment belongs to.
The following sections specify procedures for propagation of inter-AS
auto-discovery routes across ASes in order to construct inter-AS
segmented trees.
10.2.2.1. Propagating Intra-AS VPLS A-D routes in E-BGP
For a given VPLS configured on an ASBR when the ASBR receives intra-
AS A-D routes originated PEs in its own AS, the ASBR MUST propagate
each of these route in E-BGP. This procedure MUST be performed for
each of the VPLSes configurd on the ASBR. Each of these routes is
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constructed as follows:
+ The route carries a single BGP VPLS A-D NLRI with the RD and VE
ID being the same as the NLRI in the received intra-AS A-D route.
+ The Next Hop field of the MP_REACH_NLRI attribute is set to a
routable IP address of the ASBR.
+ The route carries the PMSI Tunnel attribute with the Tunnel Type
set to Ingress Replication; the attribute carries no MPLS labels.
+ The route MUST carry the export Route Target used by the VPLS.
10.2.2.2. Inter-AS A-D route received via E-BGP
When an ASBR receives from one of its E-BGP neighbors a BGP Update
message that carries an inter-AS auto-discovery route, if (a) at
least one of the Route Targets carried in the message matches one of
the import Route Targets configured on the ASBR, and (b) the ASBR
determines that the received route is the best route to the
destination carried in the NLRI of the route, the ASBR re-advertises
this inter-AS auto-discovery route to other PEs and ASBRs within its
own AS. The best route selection procedures MUST ensure that for the
same destination, all ASBRs in an AS pick the same route as the best
route. The best route selection procedures are specified in
[RFC4761] and clarified in [MULTI-HOMING]. The best route procedures
ensure that if multiple ASBRs, in an AS, receive the same inter-AS A-
D route from their E-BGP neighbors, only one of these ASBRs
propagates this route in I-BGP. This ASBR becomes the root of the
intra-AS segment of the inter-AS tree and ensures that this is the
only ASBR that accepts traffic into this AS from the inter-AS tree.
When re-advertising an inter-AS auto-discovery route the ASBR MUST
set the Next Hop field of the MP_REACH_NLRI attribute to a routable
IP address of the ASBR.
Depending on the type of a P-Multicast tree used to instantiate the
intra-AS segment of the inter-AS tunnel, the PMSI Tunnel attribute of
the re-advertised inter-AS auto-discovery route is constructed as
follows:
+ If the ASBR uses ingress replication to instantiate the intra-AS
segment of the inter-AS tunnel, the re-advertised route MUST NOT
carry the PMSI Tunnel attribute.
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+ If the ASBR uses a P-Multicast tree to instantiate the intra-AS
segment of the inter-AS tunnel, the PMSI Tunnel attribute MUST
contain the identity of the tree that is used to instantiate the
segment (note that the ASBR could create the identity of the tree
prior to the actual instantiation of the segment). If in order to
instantiate the segment the ASBR needs to know the leaves of the
tree, then the ASBR obtains this information from the auto-
discovery routes received from other PEs/ASBRs in ASBR's own AS.
+ An ASBR that uses a P-Multicast tree to instantiate the intra-AS
segment of the inter-AS tunnel MAY aggregate two or more VPLSes
present on the ASBR onto the same tree. If the ASBR already
advertises inter-AS auto-discovery routes for these VPLSes, then
aggregation requires the ASBR to re-advertise these routes. The
re-advertised routes MUST be the same as the original ones,
except for the PMSI Tunnel attribute. If the ASBR has not
previously advertised inter-AS auto-discovery routes for these
VPLSes, then the aggregation requires the ASBR to advertise (new)
inter-AS auto-discovery routes for these VPLSes. The PMSI Tunnel
attribute in the newly advertised/re-advertised routes MUST carry
the identity of the P-Multicast tree that aggregates the VPLSes,
as well as an MPLS upstream-assigned label [RFC5331]. Each re-
advertised route MUST have a distinct label.
In addition the ASBR MUST send to the E-BGP neighbor, from whom it
receives the inter-AS auto-discovery route, a BGP Update message that
carries a "leaf auto-discovery route". The exact encoding of this
route is described in section 12. This route contains the following
information elements:
+ The route carries a single NLRI with the Route Key field set to
the <RD, VE ID> tuple of the BGP VPLS A-D NLRI of the inter-AS
auto-discovery route received from that neighbor. The NLRI also
carries the IP address of the ASBR (this MUST be a routable IP
address).
+ The leaf auto-discovery route MUST include the PMSI Tunnel
attribute with the Tunnel Type set to Ingress Replication, and
the Tunnel Identifier set to a routable address of the
advertising router. The PMSI Tunnel attribute MUST carry a
downstream assigned MPLS label that is used to demultiplex the
VPLS traffic received over a unicast tunnel by the advertising
router.
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+ The Next Hop field of the MP_REACH_NLRI attribute of the route
SHOULD be set to the same IP address as the one carried in the
Originating Router's IP Address field of the route.
+ To constrain the distribution scope of this route the route MUST
carry the NO_ADVERTISE BGP community ([RFC1997]).
+ The ASBR constructs an IP-based Route Target extended community
by placing the IP address carried in the next hop of the received
Inter-AS VPLS A-D route in the Global Administrator field of the
community, with the Local Administrator field of this community
set to 0, and sets the Extended Communities attribute of the Leaf
A-D route to that community. Note that this Route Target is the
same as the ASBR Import RT of the EBGP neighbor from which the
ASBR received the inter-AS VPLS A-D route.
10.2.2.3. Leaf A-D Route received via E-BGP
When an ASBR receives via E-BGP a leaf auto-discovery route, the ASBR
accepts the route only if if (a) at least one of the Route Targets
carried in the message matches one of the import Route Targets
configured on the ASBR, and (b) the ASBR determines that the received
route is the best route to the destination carried in the NLRI of the
route.
If the ASBR accepts the leaf auto-discovery route, the ASBR finds an
auto-discovery route whose BGP-VPLS A-D NLRI has the same value as
the <RD, VE-ID> field of the the leaf auto-discovery route.
The MPLS label carried in the PMSI Tunnel attribute of the leaf auto-
discovery route is used to stitch a one hop ASBR-ASBR LSP to the tail
of the intra-AS tunnel segment associated with the found auto-
discovery route.
10.2.2.4. Inter-AS A-D Route received via I-BGP
In the context of this section we use the term "PE/ASBR router" to
denote either a PE or an ASBR router.
Note that a given inter-AS auto-discovery route is advertised within
a given AS by only one ASBR as described above.
When a PE/ASBR router receives from one of its I-BGP neighbors a BGP
Update message that carries an inter-AS auto-discovery route, if (a)
at least one of the Route Targets carried in the message matches one
of the import Route Targets configured on the PE/ASBR, and (b) the
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PE/ASBR determines that the received route is the best route to the
destination carried in the NLRI of the route, the PE/ASBR performs
the following operations. The best route determination is based as
described in [RFC4761] and clarified in [MULTI-HOMING].
If the router is an ASBR then the ASBR propagates the route to its E-
BGP neighbors. When propagating the route to the E-BGP neighbors the
ASBR MUST set the Next Hop field of the MP_REACH_NLRI attribute to a
routable IP address of the ASBR.
If the received inter-AS auto-discovery route carries the PMSI Tunnel
attribute with the Tunnel Type set to LDP P2MP LSP, the PE/ASBR
SHOULD join the P-Multicast tree whose identity is carried in the
PMSI Tunnel Attribute.
If the received inter-AS auto-discovery route carries the PMSI Tunnel
attribute with the Tunnel Identifier set to RSVP-TE P2MP LSP, then
the ASBR that originated the route MUST establish an RSVP-TE P2MP LSP
with the local PE/ASBRas a leaf. This LSP MAY have been established
before the local PE/ASBR receives the route, or MAY be established
after the local PE receives the route.
If the received inter-AS auto-discovery route carries the PMSI Tunnel
attribute with the Tunnel Type set to LDP P2MP LSP, or RSVP-TE P2MP
LSP, but the attribute does not carry a label, then the P-Multicast
tree, as identified by the PMSI Tunnel Attribute, is an intra-AS LSP
segment that is part of the inter-AS Tunnel for the <VPLS, VE ID>
advertised by the inter-AS auto-discovery route and rooted at the PE
that originated the auto-discovery route. If the PMSI Tunnel
attribute carries a (upstream-assigned) label, then a combination of
this tree and the label identifies the intra-AS segment. If the
received router is an ASBR, this intra-AS segment may further be
stitched to ASBR-ASBR inter-AS segment of the inter-AS tunnel. If the
PE/ASBR has local receivers in the VPLS, packets received over the
intra-AS segment must be forwarded to the local receivers using the
local VSI.
10.3. Option (c)
In this method, there is a multi-hop E-BGP peering between the PEs
(or a Route Reflector) in one AS and the PEs (or Route Reflector) in
another AS. The PEs exchange BGP-VPLS NLRI or BGP-VPLS A-D NLRI,
along with PMSI Tunnel Attribute, as in the intra-AS case described
in section 8. An implementation MUST support this method.
The PEs in different ASs use a non-segmented inter-AS P2MP tunnel for
VPLS multicast. A non-segmented inter-AS tunnel is a single tunnel
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which spans AS boundaries. The tunnel technology cannot change from
one point in the tunnel to the next, so all ASes through which the
tunnel passes must support that technology. In essence, AS boundaries
are of no significance to a non-segmented inter-AS P2MP tunnel.
This method requires no VPLS A-D routes in the control or VPLS MAC
address learning in the data plane on the ASBRs. The ASBRs only need
to participate in the non-segmented P2MP tunnel setup in the control
plane, and do MPLS label forwarding in the data plane.
The setup of non-segmented inter-AS P2MP tunnels MAY require the P-
routers in one AS to have IP reachability to the loopback addresses
of the PE routers in another AS, depending on the tunneling
technology chosen. If this is the case, reachability to the loopback
addresses of PE routers in one AS MUST be present in the IGP in
another AS.
The data forwarding in this model is the same as in the intra-AS case
described in section 8.
11. Optimizing Multicast Distribution via Selective Trees
Whenever a particular multicast stream is being sent on an Inclusive
P-Multicast tree, it is likely that the data of that stream is being
sent to PEs that do not require it as the sites connected to these
PEs may have no receivers for the stream. If a particular stream has
a significant amount of traffic, it may be beneficial to move it to a
Selective P-Multicast tree which has at its leaves only those PEs,
connected to sites that have receivers for the multicast stream (or
at least includes fewer PEs that are attached to sites with no
receivers compared to an Inclusive tree).
A PE connected to the multicast source of a particular multicast
stream may be performing explicit tracking i.e. it may know the PEs
that have receivers in the multicast stream. Section 11.2.1 describes
procedures that enable explicit tracking. If this is the case
Selective P-Multicast trees can also be triggered on other criteria.
For instance there could be a "pseudo wasted bandwidth" criteria:
switching to a Selective tree would be done if the bandwidth
multiplied by the number of uninterested PEs (PE that are receiving
the stream but have no receivers) is above a specified threshold. The
motivation is that (a) the total bandwidth wasted by many sparsely
subscribed low-bandwidth groups may be large, and (b) there's no
point to moving a high-bandwidth group to a Selective tree if all the
PEs have receivers for it.
Switching a (C-S, C-G) stream to a Selective P-Multicast tree may
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require the root of the tree to determine the egress PEs that need to
receive the (C-S, C-G) traffic. This is true in the following cases:
+ If the tunnel is a P2MP tree, such as a RSVP-TE P2MP Tunnel, the
PE needs to know the leaves of the tree before it can instantiate
the Selective tree.
+ If a PE decides to send traffic for multicast streams, belonging
to different VPLSes, using one P-Multicast Selective tree, such a
tree is termed an Aggregate tree with a selective mapping. The
setting up of such an Aggregate tree requires the ingress PE to
know all the other PEs that have receivers for multicast groups
that are mapped onto the tree.
+ If ingress replication is used and the ingress PE wants to send
traffic for (C-S, C-G)s to only those PEs that are on the path to
receivers to the (C-S,C-G)s.
For discovering the IP multicast group membership, for the above
cases, this document describes procedures that allow an ingress PE to
enable explicit tracking. Thus an ingress PE can request the IP
multicast membership from egress PEs for one or more C-multicast
streams. These procedures are described in section 11.2.1.
These procedures are applicable when IGMP is used as the multicast
routing protocol between the VPLS CEs. They are also applicable when
PIM is used as the multicast routing protocol between the VPLS CEs
and PIM join suppression is disabled on all the CEs. However these
procedures do not apply when PIM is used as the multicast routing
protocol between the VPLS CEs and it not possible to disable PIM join
suppression on all the CEs. Procedures that allow the setup of
Selective trees for this case are for further study.
The root of the Selective P-Multicast tree MAY decide to do explicit
tracking of the IP multicast stream only after it has determined to
move the stream to a Selective tree, or it MAY have been doing
explicit tracking all along. This document also describes explicit
tracking for a wild-card source and/or group in section 11.2.1, which
facilitates a Selective P-Multicast tree only mode in which IP
multicast streams are always carried on a Selective P-Multicast tree.
In the description on Selective P-Multicast trees the notation C-S,
is intended to representeither a specific source address or a
wildcard. Similarly C-G is intended to represent either a specific
group address or a wildcard.
The PE at the root of the tree MUST signal the leaves of the tree
that the (C-S, C-G) stream is now bound to the to the Selective Tree.
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Note that the PE could create the identity of the P-Multicast tree
prior to the actual instantiation of the tunnel.
If the Selective tree is instantiated by a RSVP-TE P2MP LSP the PE at
the root of the tree MUST establish the P2MP RSVP-TE LSP to the
leaves. This LSP MAY have been established before the leaves receive
the Selective tree binding, or MAY be established after the leaves
receives the binding. A leaf MUST not switch to the Selective tree
until it receives the binding and the RSVP-TE P2MP LSP is setup to
the leaf.
11.1. Protocol for Switching to Selective Trees
Selective trees provide a PE the ability to create separate P-
Multicast trees for certain <C-S, C-G> streams. The source PE, that
originates the Selective tree, and the egress PEs, MUST use the
Selective tree for the <C-S, C-G> streams that are mapped to it. This
may require the source and egress PEs to switch to the Selective tree
from an Inclusive tree if they were already using an Inclusive tree
for the <C-S, C-G> streams mapped to the Selective tree.
Once a source PE decides to setup an Selective tree, it MUST announce
the mapping of the <C-S, C-G> streams (which may be in different
VPLSes) that are mapped to the tree to the other PEs using BGP. After
the egress PEs receive the announcement they setup their forwarding
path to receive traffic on the Selective tree if they have one or
more receivers interested in the <C-S, C-G> streams mapped to the
tree. Setting up the forwarding path requires setting up the
demultiplexing forwarding entries based on the top MPLS label (if
there is no inner label) or the inner label (if present) as described
in section 9. The egress PEs MAY perform this switch to the Selective
tree once the advertisement from the ingress PE is received or wait
for a preconfigured timer to do so, after receiving the
advertisement, when the P2MP LSP protocol is mLDP. When the P2MP LSP
protocol is P2MP RSVP-TE an egress PE MUST perform this switch to the
Selective tree only after the advertisement from the ingress PE is
received and the RSVP-TE P2MP LSP has been setup to the egress PE.
This switch MAY be done after waiting for a preconfigured timer after
these two steps have been accomplished.
A source PE MUST use the following approach to decide when to start
transmitting data on the Selective tree, if it was already using an
Inclusive tree. A certain pre-configured delay after advertising the
<C-S, C-G> streams mapped to an Selective tree, the source PE begins
to send traffic on the Selective tree. At this point it stops to send
traffic for the <C-S, C-G> streams, that are mapped on the Selective
tree, on the Inclusive tree. This traffic is instead transmitted on
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the Selective tree.
11.2. Advertising C-(S, G) Binding to a Selective Tree
The ingress PE informs all the PEs that are on the path to receivers
of the (C-S, C-G) of the binding of the Selective tree to the (C-S,
C-G), using BGP. The BGP announcement is done by sending update for
the MCAST-VPLS address family using what is referred to as the S-PMSI
A-D route. The format of the NLRI is described in section 12.1. The
NLRI MUST be constructed as follows:
+ The RD MUST be set to the RD configured locally for the VPLS.
This is required to uniquely identify the <C-S, C-G> as the
addresses could overlap between different VPLSes. This MUST be
the same RD value used in the VPLS auto-discovery process.
+ The Multicast Source field MUST contain the source address
associated with the C-multicast stream, and the Multicast Source
Length field is set appropriately to reflect this. If the source
address is a wildcard the source address is set to 0.
+ The Multicast Group field MUST contain the group address
associated with the C-multicast stream, and the Multicast Group
Length field is set appropriately to reflect this. If the group
address is a wildcard the group address is set to 0.
+ The Originating Router's IP Address field MUST be set to the IP
address that the (local) PE places in the BGP next-hop of the
BGP-VPLS A-D routes. Note that the <RD, Originating Router's IP
address> tuple uniquely identifies a given VPLS.
The PE constructs the rest of the Selective A-D route as follows.
Depending on the type of a P-Multicast tree used for the P-tunnel,
the PMSI tunnel attribute of the S-PMSI A-D route is constructed as
follows:
+ The PMSI tunnel attribute MUST contain the identity of the P-
Multicast tree (note that the PE could create the identity of the
tree prior to the actual instantiation of the tree).
+ If in order to establish the P-Multicast tree the PE needs to
know the leaves of the tree within its own AS, then the PE
obtains this information from the Leaf A-D routes received from
other PEs/ASBRs within its own AS (as other PEs/ASBRs originate
Leaf A-D routes in response to receiving the S-PMSI A-D route) by
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setting the Leaf Information Required flag in the PMSI Tunnel
attribute to 1. This enables explicit tracking for the multicast
stream(s) advertised by the S-PMSI A-D route.
+ If a PE originates S-PMSI A-D routes with the Leaf Information
Required flag in the PMSI Tunnel attribute set to 1, then the PE
MUST be (auto)configured with an import Route Target, which
controls acceptance of Leaf A-D routes by the PE. (Procedures for
originating Leaf A-D routes by the PEs that receive the S-PMSI A-
D route are described in section "Receiving S-PMSI A-D routes by
PEs.)
This Route Target is IP address specific. The Global
Administrator field of this Route Target MUST be set to the IP
address carried in the Next Hop of all the S-PMSI A-D routes
advertised by this PE (if the PE uses different Next Hops, then
the PE MUST be (auto)configured with multiple import RTs, one per
each such Next Hop). The Local Administrator field of this Route
Target MUST be set to 0.
If the PE supports Route Target Constrain [RFC4684], the PE
SHOULD advertise this import Route Target within its own AS using
Route Target Constrains. To constrain distribution of the Route
Target Constrain routes to the AS of the advertising PE these
routes SHOULD carry the NO_EXPORT Community ([RFC1997]).
+ A PE MAY aggregate two or more S-PMSIs originated by the PE onto
the same P-Multicast tree. If the PE already advertises S-PMSI A-
D routes for these S-PMSIs, then aggregation requires the PE to
re-advertise these routes. The re-advertised routes MUST be the
same as the original ones, except for the PMSI tunnel attribute.
If the PE has not previously advertised S-PMSI A-D routes for
these S-PMSIs, then the aggregation requires the PE to advertise
(new) S-PMSI A-D routes for these S-PMSIs. The PMSI Tunnel
attribute in the newly advertised/re-advertised routes MUST carry
the identity of the P-Multicast tree that aggregates the S-PMSIs.
If at least some of the S-PMSIs aggregated onto the same P-
Multicast tree belong to different MVPNs, then all these routes
MUST carry an MPLS upstream assigned label [RFC5331]. If all
these aggregated S-PMSIs belong to the same MVPN, then the routes
MAY carry an MPLS upstream assigned label [RFC5331]. The labels
MUST be distinct on a per MVPN basis, and MAY be distinct on a
per route basis.
The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD
be set to the same IP address as the one carried in the Originating
Router's IP Address field.
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By default the set of Route Targets carried by the route MUST be the
same as the Route Targets carried in the BGP-VPLS A-D route
originated from the VSI. The default could be modified via
configuration.
11.3. Receiving S-PMSI A-D routes by PEs
Consider a PE that receives an S-PMSI A-D route. If one or more of
the VSIs on the PE have their import Route Targets that contain one
or more of the Route Targets carried by the received S-PMSI A-D
route, then for each such VSI the PE performs the following.
Procedures for receiving an S-PMSI A-D route by a PE (both within and
outside of the AS of the PE that originates the route) are the same
as specified in Section "Inter-AS A-D route received via IBGP" except
that (a) instead of Inter-AS I-PMSI A-D routes the procedures apply
to S-PMSI A-D routes, and (b) the rules for determining whether the
received S-PMSI A-D route is the best route to the destination
carried in the NLRI of the route, are the same as BGP path selection
rules and may be modified by policy, and (c) a PE performs procedures
specified in that section only if in addition to the criteria
specified in that section the following is true:
+ If as a result of IGMP or PIM snooping on the PE-CE interfaces,
the PE has snooped state for at least one multicast join that
matches the multicast source and group advertised in the S-PMSI
A-D route. Further if the oif (outgoing interfaces) for this
state contains one or more interfaces to the locally attached
CEs.
The snooped state is said to "match" the S-PMSI A-D route if any of
the following is true:
+ The S-PMSI A-D route carries (C-S, C-G) and the snooped state is
for (C-S, C-G). OR
+ The S-PMSI A-D route carries (C-*, C-G) and (a) the snooped
state is for (C-*, C-G) OR (b) the snooped state is for at least
one multicast join with the multicast group address equal to C-G
and there doesn't exist another S-PMSI A-D route that carries (C-
S, C-G) where C-S is the source address of the snooped state.
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+ The S-PMSI A-D route carries (C-S, C-*) and (a) the snooped
state is for at least one multicast join with the multicast
source address equal to C-S, and (b) there doesn't exist another
S-PMSI A-D route that carries (C-S, C-G) where C-G is the group
address of the snooped state.
+ The S-PMSI A-D route carries (C-*, C-*)
Note if the above conditions are true, and if the received S-PMSI A-D
route has a PMSI Tunnel attribute with the Leaf Information Required
flag set to 1, then the PE originates a Leaf A-D route. The Route Key
of the Leaf A-D route is set to the MCAST-VPLS NLRI of the S-PMSI A-D
route. The rest of the Leaf A-D route is constructed using the same
procedures as specified in section "Originating Leaf A-D route into
IBGP", except that instead of originating Leaf A-D routes in response
to receiving Inter-AS A-D routes the procedures apply to originating
Leaf A-D routes in response to receiving S-PMSI A-D routes.
In addition to the procedures specified in Section "Inter-AS A-D
route received via IBGP" the PE MUST set up its forwarding path to
receive traffic, for each multicast stream in the matching snooped
state, from the tunnel advertised by the S-PMSI A-D route (the PE
MUST switch to the Selective tree).
11.4. Inter-AS Selective Tree
Inter-AS Selective trees support all three models of inter-AS VPLS
service, option (a), (b) and (c), that are supported by Inter-AS
Inclusive trees. They are constructed in a manner that is very
similar to Inter-AS Inclusive trees.
For option (a) and option (b) support inter-AS Selective trees are
constructed without requiring a single P-Multicast tree to span
multiple ASes. This allows individual ASes to potentially use
different P-tunneling technologies.There are two variants of this
model. One that requires MAC and IP multicast lookup on the ASBRs and
another that does not require MAC/IP multicast lookup on the ASBRs
and instead builds segmented inter-AS Selective trees.
Segmented Inter-AS Selective trees can also be used with option (c)
unlike Segmented Inter-AS Inclusive trees. This is because the S-PMSI
A-D routes can be exchanged via ASBRs (even though BGP VPLS A-D
routes are not exchanged via ASBRs).
In the case of Option (c) an Inter-AS Selective tree may also be a
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non-segmented P-Multicast tree that spans multiple ASs.
11.4.1. VSIs on the ASBRs
The requirements on ASBRs in this model include the requirements
presented in section 10. The source ASBR (that receives traffic from
another AS) may independently decide whether it wishes to use
Selective trees or not. If it uses Selective trees the source ASBR
MUST perform a MAC lookup to determine the Selective tree to forward
the VPLS packet on.
11.4.1.1. VPLS Inter-AS Selective Tree A-D Binding
The mechanisms for propagating S-PMSI A-D routes are the same as the
intra-AS case described in section 12.2. The BGP Selective tree A-D
routes generated by PEs in an AS MUST NOT be propagated outside the
AS.
11.4.2. Inter-AS Segmented Selective Trees
Inter-AS Segmented Selective trees MUST be used when option (b) is
used to provide the inter-AS VPLS service. They MAY be used when
option (c) is used to provide the inter-AS VPLS service.
A Segmented inter-AS Selective Tunnel is constructed similar to an
inter-AS Segmented Inclusive Tunnel. Namely, such a tunnel is
constructed as a concatenation of tunnel segments. There are two
types of tunnel segments: an intra-AS tunnel segment (a segment that
spans ASBRs within the same AS), and inter-AS tunnel segment (a
segment that spans adjacent ASBRs in adjacent ASes). ASes that are
spanned by a tunnel are not required to use the same tunneling
mechanism to construct the tunnel - each AS may pick up a tunneling
mechanism to construct the intra-AS tunnel segment of the tunnel, in
its AS.
The PE that decides to set up a Selective tree, advertises the
Selective tree to multicast stream binding using a S-PMSI A-D route
as per procedures in section 11.2, to the routers in its own AS.
A S-PMSI A-D route advertised outside the AS, to which the
originating PE belongs, will be referred to as an inter-AS Selective
Tree A-D route (Although this route is originated by a PE as an
intra-AS route it is referred to as an inter-AS route outside the
AS).
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11.4.2.1. Handling S-PMSI A-D routes by ASBRs
Procedures for handling an S-PMSI A-D route by ASBRs (both within and
outside of the AS of the PE that originates the route) are the same
as specified in Section "Propagating VPLS BGP A-D routes to other
ASes", except that instead of Inter-AS BGP-VPLS A-D routes and the
BGP-VPLS A-D NLRI these procedures apply to S-PMSI A-D routes and the
S-PMSI A-D NLRI.
In addition to these procedures an ASBR advertises a Leaf A-D route
in response to a S-PMSI A-D route only if:
+ The S-PMSI A-D route was received via EBGP from another ASBR and
the ASBR merges the S-PMSI A-D route into an Inter-AS BGP VPLS A-
D route as described in the next section. OR
+ The ASBR receives a Leaf A-D route from a downstream PE or ASBR
in response to the S-PMSI A-D route, received from an upstream PE
or ASBR, that the ASBR propagated inter-AS to downstream ASBRs
and PEs.
+ The ASBR has snooped state from local CEs that matches the NLRI
carried in the S-PMSI A-D route as per the following rules:
i) The NLRI encodes (C-S, C-G) which is the same as the snooped
(C-S, C-G) ii) The NLRI encodes (*, C-G) and there is snooped
state for at least one (C-S, C-G) and there is no other matching
SPMSI A-D route for (C-S, C-G) OR there is snooped state for (*,
C-G) iii) The NLRI encodes (*, *) and there is snooped state for
at least one (C-S, C-G) or (*, C-G) and there is no other
matching SPMSI A-D route for that (C-S, C-G) or (*, C-G)
respecively.
The C-multicast data traffic is sent on the Selective tree by the
originating PE. When it reaches an ASBR that is on the Inter-AS
segmented tree, it is delivered to local receivers, if any. It is
then forwarded on any inter-AS or intra-AS segments that exist on the
Inter-AS Selective Segmented tree. If the Inter-AS Segmented
Selective Tree is merged onto an Inclusive tree, as described in the
next section, the data traffic is forwarded onto the Inclusive tree.
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11.4.2.1.1. Merging Selective Tree into an Inclusive Tree
Consider the situation where:
+ An ASBR is receiving (or expecting to receive) inter-AS (C-S, C-
G) data from upstream via a Selective tree.
+ The ASBR is sending (or expecting to send) the inter-AS (C-S,
C-G) data downstream via an Inclusive tree.
This situation may arise if the upstream providers have a policy of
using Selective trees but the downstream providers have a policy of
using Inclusive trees. To support this situation, an ASBR MAY,
under certain conditions, merge one or more upstream Selective trees
into a downstream Inclusive tree. Note that this can be the case only
for option (b) and not for option (c) as for option (c) the ASBRs do
not have Inclusive tree state.
A Selective tree (corresponding to a particular S-PMSI A-D route) MAY
be merged by a particular ASBR into an Inclusive tree
(corresponding to a particular Inter-AS BGP VPLS A-D route) if and
only if the following conditions all hold:
+ The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route
originate in the same AS. The Inter-AS BGP VPLS A-D route carries
the originating AS in the AS_PATH attribute of the route. The S-
PMSI A-D route carries the originating AS in the AS_PATH
attribute of the route.
+ The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route have
exactly the same set of RTs.
An ASBR performs merging by stitching the tail end of the P-tunnel,
as specified in the the PMSI Tunnel Attribute of the S-PMSI A-D route
received by the ASBR, to the to the head of the P-tunnel, as
specified in the PMSI Tunnel Attribute of the Inter-AS BGP VPLS A-D
route re-advertised by the ASBR.
An ASBR that merges an S-PMSI A-D route into an Inter-AS BGP VPLS A-D
route MUST NOT re-advertise the S-PMSI A-D route.
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11.4.3. Inter-AS Non-Segmented Selective trees
Inter-AS Non-segmented Selective trees MAY be used in the case of
option (c).
In this method, there is a multi-hop E-BGP peering between the PEs
(or a Route Reflector) in one AS and the PEs (or Route Reflector) in
another AS. The PEs exchange BGP Selective tree A-D routes, along
with PMSI Tunnel Attribute, as in the intra-AS case described in
section 10.3.
The PEs in different ASs use a non-segmented Selective inter-AS P2MP
tunnel for VPLS multicast.
This method requires no VPLS information (in either the control or
the data plane) on the ASBRs. The ASBRs only need to participate in
the non-segmented P2MP tunnel setup in the control plane, and do MPLS
label forwarding in the data plane.
The data forwarding in this model is the same as in the intra-AS case
described in section 9.
12. BGP Extensions
This section describes the encoding of the BGP extensions required by
this document.
12.1. Inclusive Tree/Selective Tree Identifier
Inclusive P-Multicast tree and Selective P-Multicast tree
advertisements carry the P-Multicast tree identifier.
This document reuses the BGP attribute, called PMSI Tunnel Attribute
that is defined in [MVPN].
Only the following Tunnel Types MUST be used when PMSI Tunnel
attribute is carried in VPLS A-D or VPLS S-PMSI A-D routes:
+ 0 - No tunnel information present
+ 1 - RSVP-TE P2MP LSP
+ 2 - LDP P2MP LSP
+ 6 - Ingress Replication
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12.2. MCAST-VPLS NLRI
This document defines a new BGP NLRI, called the MCAST-VPLS NLRI.
Following is the format of the MCAST-VPLS NLRI:
+-----------------------------------+
| Route Type (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| Route Type specific (variable) |
+-----------------------------------+
The Route Type field defines encoding of the rest of MCAST-VPLS NLRI
(Route Type specific MCAST-VPLS NLRI).
The Length field indicates the length in octets of the Route Type
specific field of MCAST-VPLS NLRI.
This document defines the following Route Types for auto-discovery
routes:
+ 3 - Selective Tree auto-discovery route;
+ 4 - Leaf auto-discovery route.
The MCAST-VPLS NLRI is carried in BGP using BGP Multiprotocol
Extensions [RFC4760] with an AFI of 25 (L2VPN AFI), and an SAFI of
MCAST-VPLS [To be assigned by IANA]. The NLRI field in the
MP_REACH_NLRI/MP_UNREACH_NLRI attribute contains the MCAST-VPLS NLRI
(encoded as specified above).
In order for two BGP speakers to exchange labeled MCAST-VPLS NLRI,
they must use BGP Capabilities Advertisement to ensure that they both
are capable of properly processing such NLRI. This is done as
specified in [RFC4760], by using capability code 1 (multiprotocol
BGP) with an AFI of 25 and an SAFI of MCAST-VPLS.
The following describes the format of the Route Type specific MCAST-
VPLS NLRI for various Route Types defined in this document.
12.2.1. S-PMSI auto-discovery route
An S-PMSI A-D route type specific MCAST-VPLS NLRI consists of the
following:
+-----------------------------------+
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| RD (8 octets) |
+-----------------------------------+
| Multicast Source Length (1 octet) |
+-----------------------------------+
| Multicast Source (Variable) |
+-----------------------------------+
| Multicast Group Length (1 octet) |
+-----------------------------------+
| Multicast Group (Variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+
The RD is encoded as described in [RFC4364].
The Multicast Source field contains the C-S address i.e the address
of the multicast source. This may be 0 to indicate a wildcard. If the
Multicast Source field contains an IPv4 address, then the value of
the Multicast Source Length field is 32. If the Multicast Source
field contains an IPv6 address, then the value of the Multicast
Source Length field is 128.
The Multicast Group field contains the C-G address i.e. the address
of the multicast group. This may be 0 to indicate a wildcard. If the
Multicast Group field contains an IPv4 address, then the value of the
Multicast Group Length field is 32. If the Multicast Group field
contains an IPv6 address, then the value of the Multicast Group
Length field is 128.
Usage of Selective Tree auto-discovery routes is described in Section
12.
12.2.2. Leaf auto-discovery route
A leaf auto-discovery route type specific MCAST-VPLS NLRI consists of
the following:
+-----------------------------------+
| Route Key (variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+
Usage of Leaf auto-discovery routes is described in sections "Inter-
AS Inclusive P-Multicast tree A-D/Binding" and "Optimizing Multicast
Distribution via Selective trees".
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13. Aggregation Methodology
In general the herustics used to decide which VPLS instances or <C-S,
C-G> entries to aggregate is implementation dependent. It is also
conceivable that offline tools can be used for this purpose. This
section discusses some tradeoffs with respect to aggregation.
The "congruency" of aggregation is defined by the amount of overlap
in the leaves of the client trees that are aggregated on a SP tree.
For Aggregate Inclusive trees the congruency depends on the overlap
in the membership of the VPLSes that are aggregated on the Aggregate
Inclusive tree. If there is complete overlap aggregation is perfectly
congruent. As the overlap between the VPLSes that are aggregated
reduces, the congruency reduces.
If aggregation is done such that it is not perfectly congruent a PE
may receive traffic for VPLSes to which it doesn't belong. As the
amount of multicast traffic in these unwanted VPLSes increases
aggregation becomes less optimal with respect to delivered traffic.
Hence there is a tradeoff between reducing state and delivering
unwanted traffic.
An implementation should provide knobs to control the congruency of
aggregation. This will allow a SP to deploy aggregation depending on
the VPLS membership and traffic profiles in its network. If
different PEs or shared roots' are setting up Aggregate Inclusive
trees this will also allow a SP to engineer the maximum amount of
unwanted VPLSes that a particular PE may receive traffic for.
The state/bandwidth optimality trade-off can be further improved by
having a versatile many-to-many association between client trees and
provider trees. Thus a VPLS can be mapped to multiple Aggregate
trees. The mechanisms for achieving this are for further study. Also
it may be possible to use both ingress replication and an Aggregate
tree for a particular VPLS. Mechanisms for achieving this are also
for further study.
14. Data Forwarding
14.1. MPLS Tree Encapsulation
14.1.1. Mapping multiple VPLS instances to a P2MP LSP
The following diagram shows the progression of the VPLS IP multicast
packet as it enters and leaves the SP network when MPLS trees are
being used for multiple VPLS instances. RSVP-TE P2MP LSPs are
examples of such trees.
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Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
|MPLS Tree Label|
+---------------+
| VPLS Label |
++=============++ ++=============++ ++=============++
||C-Ether Hdr || || C-Ether Hdr || || C-Ether Hdr ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
The receiver PE does a lookup on the outer MPLS tree label and
determines the MPLS forwarding table in which to lookup the inner
MPLS label. This table is specific to the tree label space. The inner
label is unique within the context of the root of the tree (as it is
assigned by the root of the tree, without any coordination with any
other nodes). Thus it is not unique across multiple roots. So, to
unambiguously identify a particular VPLS one has to know the label,
and the context within which that label is unique. The context is
provided by the outer MPLS label [RFC5331].
The outer MPLS label is stripped. The lookup of the resulting MPLS
label determines the VSI in which the receiver PE needs to do the C-
multicast data packet lookup. It then strips the inner MPLS label and
sends the packet to the VSI for multicast data forwarding.
14.1.2. Mapping one VPLS instance to a P2MP LSP
The following diagram shows the progression of the VPLS IP multicast
packet as it enters and leaves the SP network when a given MPLS tree
is being used for a single VPLS instance. RSVP-TE P2MP LSPs are
examples of such trees.
Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
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+---------------+
|MPLS Tree Label|
++=============++ ++=============++ ++=============++
||C-Ether Hdr || || C-Ether Hdr || || C-Ether Hdr ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
The receiver PE does a lookup on the outer MPLS tree label and
determines the VSI in which the receiver PE needs to do the C-
multicast data packet lookup. It then strips the inner MPLS label and
sends the packet to the VSI for multicast data forwarding.
15. VPLS Data Packet Treatment
If the destination MAC address of a VPLS packet received by a PE from
a VPLS site is a multicast adddress, a P-Multicast tree SHOULD be
used to transport the packet, if possible. If the packet is an IP
multicast packet and a Selective tree exists for that multicast
stream, the Selective tree SHOULD be used. Else if an Inclusive tree
exists for the VPLS, it SHOULD be used.
If the destination MAC address of a VPLS packet is a broadcast
address, it is flooded. If Inclusive tree is already established, PE
SHOULD flood over it. If Inclusive tree cannot be used for some
reason, PE MUST flood over multiple PWs, based on [RFC4761] or
[RFC4762].
If the destination MAC address of a packet is a unicast address and
it has not been learned, the packet MUST be sent to all PEs in the
VPLS. Inclusive P-Multicast trees or a Selective P-Multicast tree
bound to (C-*, C-*) SHOULD be used for sending unknown unicast MAC
packets to all PEs. When this is the case the receiving PEs MUST
support the ability to perform MAC address learning for packets
received on a multicast tree. In order to perform such learning, the
receiver PE MUST be able to determine the sender PE when a VPLS
packet is received on a P-Multicast tree. This further implies that
the MPLS P-Multicast tree technology MUST allow the egress PE to
determine the sender PE from the received MPLS packet.
When a receiver PE receives a VPLS packet with a source MAC address,
that has not yet been learned, on a P-Multicast tree, the receiver PE
determines the PW to the sender PE. The receiver PE then creates
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forwarding state in the VPLS instance with a destination MAC address
being the same as the source MAC address being learned, and the PW
being the PW to the sender PE.
It should be noted that when a sender PE that is sending packets
destined to an unknown unicast MAC address over a P-Multicast tree
learns the PW to use for forwarding packets destined to this unicast
MAC address, it might immediately switch to transport such packets
over this particular PW. Since the packets were initially being
forwarded using a P-Multicast tree, this could lead to packet
reordering. This constraint should be taken into consideration if
unknown unicast frames are forwarded using a P-Multicast tree,
instead of multiple PWs based on [RFC4761] or [RFC4762].
An implementation MUST support the ability to transport unknown
unicast traffic over Inclusive P-Multicast trees. Further an
implementation MUST support the ability to perform MAC address
learning for packets received on a P-Multicast tree.
16. Security Considerations
Security considerations discussed in [RFC4761] and [RFC4762] apply to
this document. This section describes additional considerations.
As mentioned in [RFC4761], there are two aspects to achieving data
privacy in a VPLS: securing the control plane and protecting the
forwarding path. Compromise of the control plane could result in a PE
sending multicast data belonging to some VPLS to another VPLS, or
blackholing VPLS multicast data, or even sending it to an
eavesdropper; none of which are acceptable from a data privacy point
of view. The mechanisms in this document use BGP for the control
plane. Hence techniques such as in [RFC2385] help authenticate BGP
messages, making it harder to spoof updates (which can be used to
divert VPLS traffic to the wrong VPLS) or withdraws (denial-of-
service attacks). In the multi-AS methods (b) and (c) described in
Section 11, this also means protecting the inter-AS BGP sessions,
between the ASBRs, the PEs, or the Route Reflectors.
Note that [RFC2385] will not help in keeping MPLS labels, associated
with P2MP LSPs or the upstream MPLS labels used for aggregation,
private -- knowing the labels, one can eavesdrop on VPLS traffic.
However, this requires access to the data path within a Service
Provider network.
One of the requirements for protecting the data plane is that the
MPLS labels are accepted only from valid interfaces. This applies
both to MPLS labels associated with P2MP LSPs and also applies to the
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upstream assigned MPLS labels. For a PE, valid interfaces comprise
links from P routers. For an ASBR, a valid interface is a link from
another ASBR in an AS that is part of a given VPLS. It is especially
important in the case of multi-AS VPLSes that one accept VPLS packets
only from valid interfaces.
17. IANA Considerations
This document defines a new NLRI, called MCAST-VPLS, to be carried in
BGP using multiprotocol extensions. It requires assignment of a new
SAFI. This is to be assigned by IANA.
This document defines a BGP optional transitive attribute, called
PMSI Attribute. This is the same attribute as the one defined in
[BGP-MVPN] and the code point for this attribute has already been
assigned by IANA as 22 [BGP-IANA]. Hence no further action is
required from IANA regarding this attribute.
18. Acknowledgments
Many thanks to Thomas Morin for his support of this work. We would
also like to thank authors of [BGP-MVPN] and [MVPN] as the details of
the inter-AS segmented tree procedures in this document have
benefited from those in [BGP-MVPN] and [MVPN]. We would also like to
thank Wim Henderickx for his comments.
19. Normative References
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, March 1997
[RFC4761] K. Kompella, Y. Rekther, "Virtual Private LAN Service",
draft-ietf-l2vpn-vpls-bgp-02.txt
[RFC4762] M. Lasserre, V. Kompella, "Virtual Private LAN Services
over MPLS", draft-ietf-l2vpn-vpls-ldp-03.txt
[RFC4760] T. Bates, et. al., "Multiprotocol Extensions for BGP-4",
January 2007
[RFC5331] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS Upstream Label
Assignment and Context Specific Label Space", RFC 5331, August 2008
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20. Informative References
[L2VPN-SIG] E. Rosen et. al., "Provisioning, Autodiscovery, and
Signaling in L2VPNs", draft-ietf-l2vpn-signaling-08.txt
[RFC5332] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter, "MPLS
Multicast Encapsulations", RFC 5332, August 2008
[MVPN] E. Rosen, R. Aggarwal, "Multicast in 2547 VPNs", draft-ietf-
l3vpn-2547bis-mcast-08.txt"
[BGP-MVPN] R. Aggarwal, E. Rosen, Y. Rekhter, T. Morin, C.
Kodeboniya. "BGP Encodings for Multicast in 2547 VPNs", draft-ietf-
l3vpn-2547bis-mcast-bgp-06.txt
[RFC4875] R. Aggarwal et. al, "Extensions to RSVP-TE for Point to
Multipoint TE LSPs", draft-ietf-mpls-rsvp-te-p2mp-07.txt
[RSVP-OBB] Z. Ali, G. Swallow, R. Aggarwal, "Non PHP behavior and
out-of-band mapping for RSVP-TE LSPs", draft-ietf-mpls-rsvp-te-no-
php-obb-mapping, work in progress.
[MLDP] I. Minei et. al, "Label Distribution Protocol Extensions for
Point-to-Multipoint and Multipoint-to-Multipoint Label Switched
Paths", draft-ietf-mpls-ldp-p2mp, work in progress.
[RFC4364] "BGP MPLS VPNs", E. Rosen, Y.Rekhter, February 2006
[MCAST-VPLS-REQ] Y. kamite, et. al., "Requirements for Multicast
Support in Virtual Private LAN Services", draft-ietf-l2vpn-vpls-
mcast-reqts-05.txt
[RFC1997] R. Chandra, et. al., "BGP Communities Attribute", August
1996
[BGP-IANA] http://www.iana.org/assignments/bgp-parameters
[RFC4684] P. Marques et. al., "Constrained Route Distribution for
Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS)
Internet Protocol (IP) Virtual Private Networks (VPNs)", RFC 4684,
November 2006
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC4447] L. Martini et. al., "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447 April 2006
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[MULTI-HOMING] K. Kompella et. al., "Multi-homing in BGP-based
Virtual Private LAN Service", draft-kompella-l2vpn-vpls-
multihoming-02.txt
21. Author's Address
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
USA
Phone: +1-408-936-2720
Email: rahul@juniper.net
Yuji Kamite
NTT Communications Corporation
Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku,
Tokyo 163-1421,
Japan
Email: y.kamite@ntt.com
Luyuan Fang
Cisco Systems
300 Beaver Brook Road
BOXBOROUGH, MA 01719
USA
Email: lufang@cisco.com
Yakov Rekhter
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: yakov@juniper.net
Chaitanya Kodeboniya
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Raggarwa, Kamite & Fang [Page 46]
