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MVPN/EVPN Tunnel Aggregation with Common Labels
RFC 9573

Document Type RFC - Proposed Standard (May 2024)
Authors Zhaohui (Jeffrey) Zhang , Eric C. Rosen , Wen Lin , Zhenbin Li , IJsbrand Wijnands
Last updated 2024-05-24
RFC stream Internet Engineering Task Force (IETF)
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RFC 9573


Internet Engineering Task Force (IETF)                          Z. Zhang
Request for Comments: 9573                              Juniper Networks
Updates: 6514, 7432, 7582                                       E. Rosen
Category: Standards Track                                     Individual
ISSN: 2070-1721                                                   W. Lin
                                                        Juniper Networks
                                                                   Z. Li
                                                     Huawei Technologies
                                                            IJ. Wijnands
                                                              Individual
                                                                May 2024

            MVPN/EVPN Tunnel Aggregation with Common Labels

Abstract

   The Multicast VPN (MVPN) specifications allow a single Point-to-
   Multipoint (P2MP) tunnel to carry traffic of multiple IP VPNs
   (referred to as VPNs in this document).  The EVPN specifications
   allow a single P2MP tunnel to carry traffic of multiple Broadcast
   Domains (BDs).  These features require the ingress router of the P2MP
   tunnel to allocate an upstream-assigned MPLS label for each VPN or
   for each BD.  A packet sent on a P2MP tunnel then carries the label
   that is mapped to its VPN or BD (in some cases, a distinct upstream-
   assigned label is needed for each flow.)  Since each ingress router
   allocates labels independently, with no coordination among the
   ingress routers, the egress routers may need to keep track of a large
   number of labels.  The number of labels may need to be as large as,
   or larger than, the product of the number of ingress routers times
   the number of VPNs or BDs.  However, the number of labels can be
   greatly reduced if the association between a label and a VPN or BD is
   made by provisioning, so that all ingress routers assign the same
   label to a particular VPN or BD.  New procedures are needed in order
   to take advantage of such provisioned labels.  These new procedures
   also apply to Multipoint-to-Multipoint (MP2MP) tunnels.  This
   document updates RFCs 6514, 7432, and 7582 by specifying the
   necessary procedures.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9573.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Terminology
   2.  Problem Description
   3.  Proposed Solutions
     3.1.  MP2MP Tunnels
     3.2.  Segmented Tunnels
     3.3.  Summary of Label Allocation Methods
   4.  Specifications
     4.1.  Context-Specific Label Space ID Extended Community
     4.2.  Procedures
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   A Multicast VPN (MVPN) can use Point-to-Multipoint (P2MP) tunnels
   (set up by RSVP-TE, Multipoint LDP (mLDP), or PIM) to transport
   customer multicast traffic across a service provider's backbone
   network.  Often, a given P2MP tunnel carries the traffic of only a
   single VPN.  However, there are procedures defined that allow a
   single P2MP tunnel to carry traffic of multiple VPNs.  In this case,
   the P2MP tunnel is called an "aggregate tunnel".  The Provider Edge
   (PE) router that is the ingress node of an aggregate P2MP tunnel
   allocates an "upstream-assigned MPLS label" [RFC5331] for each VPN,
   and each packet sent on the P2MP tunnel carries the upstream-assigned
   MPLS label that the ingress PE has bound to the packet's VPN.

   Similarly, an EVPN can use P2MP tunnels (set up by RSVP-TE, mLDP, or
   PIM) to transport Broadcast, Unknown Unicast, or Multicast (BUM)
   traffic across the provider network.  Often, a P2MP tunnel carries
   the traffic of only a single Broadcast Domain (BD).  However, there
   are procedures defined that allow a single P2MP tunnel to be an
   aggregate tunnel that carries traffic of multiple BDs.  The
   procedures are analogous to the MVPN procedures -- the PE router that
   is the ingress node of an aggregate P2MP tunnel allocates an
   upstream-assigned MPLS label for each BD, and each packet sent on the
   P2MP tunnel carries the upstream-assigned MPLS label that the ingress
   PE has bound to the packet's BD.

   An MVPN or EVPN can also use Bit Index Explicit Replication (BIER)
   [RFC8279] to transmit VPN multicast traffic [RFC8556] or EVPN BUM
   traffic [BIER-EVPN].  Although BIER does not explicitly set up P2MP
   tunnels, from the perspective of an MVPN/EVPN, the use of BIER
   transport is very similar to the use of aggregate P2MP tunnels.  When
   BIER is used, the PE transmitting a packet (the "Bit-Forwarding
   Ingress Router" (BFIR) [RFC8279]) must allocate an upstream-assigned
   MPLS label for each VPN or BD, and the packets transmitted using BIER
   transport always carry the label that identifies their VPN or BD.
   (See [RFC8556] and [BIER-EVPN] for details.)  In the remainder of
   this document, we will use the term "aggregate tunnels" to include
   both P2MP tunnels and BIER transport.

   When an egress PE receives a packet from an aggregate tunnel, it must
   look at the upstream-assigned label carried by the packet and must
   interpret that label in the context of the ingress PE.  Essentially,
   for each ingress PE, the egress PE has a context-specific label space
   [RFC5331] that matches the default label space from which the ingress
   PE assigns the upstream-assigned labels.  When an egress PE looks up
   the upstream-assigned label carried by a given packet, it looks it up
   in the context-specific label space for the ingress PE of the packet.
   How an egress PE identifies the ingress PE of a given packet depends
   on the tunnel type.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   Familiarity with MVPN/EVPN protocols and procedures is assumed.  Some
   terms are listed below for convenience.

   VPN:  Virtual Private Network.  In this document, "VPN" specifically
      refers to an IP VPN [RFC4364].

   BUM [RFC7432]:  Broadcast, Unknown Unicast, or Multicast (traffic).

   BD [RFC7432]:  Broadcast Domain.

   EC [RFC4360]:  Extended Community.

   PMSI [RFC6513]:  Provider Multicast Service Interface.  A pseudo-
      overlay interface for PEs to send certain overlay/customer
      multicast traffic via underlay/provider tunnels.  It includes
      Inclusive/Selective PMSIs (I/S-PMSIs) (often referred to as
      x-PMSIs).  A PMSI is instantiated by the underlay/provider tunnel.

   Inclusive PMSI (I-PMSI):  A PMSI that enables traffic to be sent to
      all PEs of a VPN/BD.  The underlay/provider tunnel that
      instantiates the I-PMSI is referred to as an inclusive tunnel.

   Selective PMSI (S-PMSI):  A PMSI that enables traffic to be sent to a
      subset of PEs of a VPN/BD.  The underlay/provider tunnel that
      instantiates the S-PMSI is referred to as a selective tunnel.

   Aggregate Tunnel:  A tunnel that instantiates x-PMSIs of multiple
      MVPNs or EVPN BDs.

   IMET [RFC7432]:  Inclusive Multicast Ethernet Tag.  An EVPN-specific
      name for an I-PMSI Auto-Discovery (A-D) route.

   PTA [RFC6514]:  PMSI Tunnel Attribute.  A BGP attribute that may be
      attached to a BGP-MVPN/EVPN x-PMSI A-D route.

   ASBR:  Autonomous System Border Router.

   RBR:  Regional Border Router.  A border router between segmentation
      regions that participates in segmentation procedures.

   (C-S,C-G):  A Customer/overlay <S,G> multicast flow.

   (C-*,C-G):  Customer/overlay <*,G> multicast flows.

   (C-*,C-*):  All Customer/overlay multicast flows.

   ES:  Ethernet Segment.

   ESI [RFC7432]:  ES Identifier.

   ESI Label [RFC7432]:  A label that identifies an ES.

   SRGB [RFC8402]:  Segment Routing (SR) Global Block.  The set of
      global segments in the SR domain.  In SR-MPLS [RFC8660], an SRGB
      is a local property of a node and identifies the set of local
      labels reserved for global segments.

   DCB:  Domain-wide Common Block.  A common block of labels reserved on
      all nodes in a domain.

   Context-Specific Label Space [RFC5331]:  A router may maintain
      additional label spaces besides its default label space.  When the
      label at the top of the stack is not from the default label space,
      there must be some context in the packet that identifies which one
      of those additional label spaces is to be used to look up the
      label; hence, those label spaces are referred to as context-
      specific label spaces.

   Upstream Assigned [RFC5331]:  When the label at the top of the label
      stack is not assigned by the router receiving the traffic from its
      default label space, the label is referred to as upstream
      assigned.  Otherwise, it is downstream assigned.  An upstream-
      assigned label must be looked up in a context-specific label space
      specific for the assigner.

2.  Problem Description

   Note that the upstream-assigned label procedures may require a very
   large number of labels.  Suppose that an MVPN or EVPN deployment has
   1001 PEs, each hosting 1000 VPNs/BDs.  Each ingress PE has to assign
   1000 labels, and each egress PE has to be prepared to interpret 1000
   labels from each of the ingress PEs.  Since each ingress PE allocates
   labels from its own label space and does not coordinate label
   assignments with others, each egress PE must be prepared to interpret
   1,000,000 upstream-assigned labels (across 1000 context-specific
   label spaces -- one for each ingress PE).  This is an evident scaling
   problem.

   So far, few if any MVPN/EVPN deployments use aggregate tunnels, so
   this problem has not surfaced.  However, the use of aggregate tunnels
   is likely to increase due to the following two factors:

   *  In an EVPN, a single customer ("tenant") may have a large number
      of BDs, and the use of aggregate RSVP-TE or mLDP P2MP tunnels may
      become important, since each tunnel creates state at the
      intermediate nodes.

   *  The use of BIER as the transport for an MVPN/EVPN is becoming more
      and more attractive and feasible.

   A similar problem also exists with EVPN ESI labels used for
   multihoming.  A PE attached to a multihomed ES advertises an ESI
   label in its Ethernet A-D per ES route.  The PE imposes the label
   when it sends frames received from the ES to other PEs via a P2MP/
   BIER tunnel.  A receiving PE that is attached to the source ES will
   know from the ESI label that the packet originated on the source ES
   and thus will not transmit the packet on its local Attachment Circuit
   to that ES.  From the receiving PE's point of view, the ESI label is
   (upstream) assigned from the source PE's label space, so the
   receiving PE needs to maintain context-specific label tables, one for
   each source PE, just like the VPN/BD label case above.  If there are
   1001 PEs, each attached to 1000 ESs, this can require each PE to
   understand 1,000,000 ESI labels.  Notice that the issue exists even
   when no P2MP tunnel aggregation (i.e., one tunnel used for multiple
   BDs) is used.

3.  Proposed Solutions

   The number of labels could be greatly reduced if a central entity in
   the provider network assigned a label to each VPN, BD, or ES and if
   all PEs used that same label to represent a given VPN, BD, or ES.
   Then, the number of labels needed would just be the sum of the number
   of VPNs, BDs, and/or ESs.

   One method of achieving this is to reserve a portion of the default
   label space for assignment by a central entity.  We refer to this
   reserved portion as the DCB of labels.  This is analogous to the
   concept of using identical SRGBs on all nodes, as described in
   [RFC8402].  A PE that is attached (via L3VPN Virtual Routing and
   Forwarding (VRF) interfaces or EVPN Attachment Circuits) would know
   by provisioning which label from the DCB corresponds to which of its
   locally attached VPNs, BDs, or ESs.

   For example, all PEs could reserve a DCB [1000~2000], and they would
   all be provisioned so that label 1000 maps to VPN 0, label 1001 maps
   to VPN 1, and so forth.  Now, only 1000 labels instead of 1,000,000
   labels are needed for 1000 VPNs.

   In this document, "domain" is defined loosely; it simply includes all
   the routers that share the same DCB, and it only needs to include all
   PEs of an MVPN/EVPN.

   The "domain" could also include all routers in the provider network,
   making it not much different from a common SRGB across all the
   routers.  However, that is not necessary, as the labels used by PEs
   for the purposes defined in this document will only rise to the top
   of the label stack when traffic arrives at the PEs.  Therefore, it is
   better to not include internal P routers in the "domain".  That way,
   they do not have to set aside the same DCB used for the purposes
   defined in this document.

   In some deployments, it may be impractical to allocate a DCB that is
   large enough to contain labels for all the VPNs/BDs/ESs.  In this
   case, it may be necessary to allocate those labels from one or a few
   context-specific label spaces that are independent of each PE.  For
   example, if it is too difficult to have a DCB of 10,000 labels across
   all PEs for all the VPNs/BDs/ESs that need to be supported, a
   separate context-specific label space can be dedicated to those
   10,000 labels.  Each separate context-specific label space is
   identified in the forwarding plane by a label from the DCB (which
   does not need to be large).  Each PE is provisioned with the label-
   space-identifying DCB label and the common VPN/BD/ES labels allocated
   from that context-specific label space.  When sending traffic, an
   ingress PE imposes all necessary service labels (for the VPN/BD/ES)
   first, then imposes the label-space-identifying DCB label.  From the
   label-space-identifying DCB label, an egress PE can determine the
   label space where the inner VPN/BD/ES label is looked up.

   The MVPN/EVPN signaling defined in [RFC6514] and [RFC7432] assumes
   that certain MPLS labels are allocated from a context-specific label
   space for a particular ingress PE.  In this document, we augment the
   signaling procedures so that it is possible to signal that a
   particular label is from the DCB, rather than from a context-specific
   label space for an ingress PE.  We also augment the signaling so that
   it is possible to indicate that a particular label is from an
   identified context-specific label space that is not for an ingress
   PE.

   Notice that the VPN/BD/ES-identifying labels from the DCB or from
   those few context-specific label spaces are very similar to Virtual
   eXtensible Local Area Network (VXLAN) Network Identifiers (VNIs) in
   VXLANs.  Allocating a label from the DCB or from a context-specific
   label space and communicating the label to all PEs is not different
   from allocating VNIs and is especially feasible with controllers.

3.1.  MP2MP Tunnels

   Multipoint-to-Multipoint (MP2MP) tunnels present the same problem
   (Section 2) that can be solved the same way (Section 3), with the
   following additional requirement.

   Per [RFC7582] ("Multicast Virtual Private Network (MVPN): Using
   Bidirectional P-Tunnels"), when MP2MP tunnels are used for an MVPN,
   the root of the MP2MP tunnel is required to allocate and advertise
   "PE Distinguisher Labels" (Section 4 of [RFC6513]).  These labels are
   assigned from the label space used by the root node for its upstream-
   assigned labels.

   It is REQUIRED by this document that the PE Distinguisher Labels
   allocated by a particular node come from the same label space that
   the node uses to allocate its VPN-identifying labels.

3.2.  Segmented Tunnels

   There are some additional issues to be considered when an MVPN or
   EVPN is using "tunnel segmentation" (see [RFC6514], [RFC7524], and
   Sections 5 and 6 of [RFC9572]).

3.2.1.  Selective Tunnels

   For selective tunnels that instantiate S-PMSIs (see Sections 2.1.1
   and 3.2.1 of [RFC6513] and Section 4 of [RFC9572]), the procedures
   outlined above work only if tunnel segmentation is not used.

   A selective tunnel carries one or more particular sets of flows to a
   particular subset of the PEs that attach to a given VPN or BD.  Each
   set of flows is identified by an S-PMSI A-D route [RFC6514].  The PTA
   of the S-PMSI route identifies the tunnel used to carry the
   corresponding set of flows.  Multiple S-PMSI routes can identify the
   same tunnel.

   When tunnel segmentation is applied to an S-PMSI, certain nodes are
   "segmentation points".  A segmentation point is a node at the
   boundary between two segmentation regions.  Let's call these "region
   A" and "region B".  A segmentation point is an egress node for one or
   more selective tunnels in region A and an ingress node for one or
   more selective tunnels in region B.  A given segmentation point must
   be able to receive traffic on a selective tunnel from region A and
   label-switch the traffic to the proper selective tunnel in region B.

   Suppose that one selective tunnel (call it "T1") in region A is
   carrying two flows, Flow-1 and Flow-2, identified by S-PMSI routes
   Route-1 and Route-2, respectively.  However, it is possible that in
   region B, Flow-1 is not carried by the same selective tunnel that
   carries Flow-2.  Let's suppose that in region B, Flow-1 is carried by
   tunnel T2 and Flow-2 by tunnel T3.  Then, when the segmentation point
   receives traffic from T1, it must be able to label-switch Flow-1 from
   T1 to T2, while also label-switching Flow-2 from T1 to T3.  This
   implies that Route-1 and Route-2 must signal different labels in the
   PTA.  For comparison, when segmentation is not used, they can all use
   the common per-VPN/BD DCB label.

   In this case, it is not practical to have a central entity assign
   domain-wide unique labels to individual S-PMSI routes.  To address
   this problem, all PEs can be assigned their own disjoint label blocks
   in those few context-specific label spaces; each PE will
   independently allocate labels for a segmented S-PMSI from its own
   assigned label block.  For example, PE1 allocates from label block
   [101~200], PE2 allocates from label block [201~300], and so on.

   Allocating from disjoint label blocks can be used for VPN/BD/ES
   labels as well, though it does not address the original scaling
   issue, because there would be 1,000,000 labels allocated from those
   few context-specific label spaces in the original example, instead of
   just 1000 common labels.

3.2.2.  Per-PE/Region Tunnels

   Similarly, for segmented per-PE (MVPN (C-*,C-*) S-PMSI or EVPN IMET)
   or per-AS/region (MVPN Inter-AS I-PMSI or EVPN per-region I-PMSI)
   tunnels [RFC6514] [RFC7432] [RFC9572], labels need to be allocated
   per PMSI route.  In the case of a per-PE PMSI route, the labels
   should be allocated from the label block allocated to the advertising
   PE.  In the case of a per-AS/region PMSI route, different ASBRs/RBRs
   attached to the same source AS/region will advertise the same PMSI
   route.  The same label could be used when the same route is
   advertised by different ASBRs/RBRs, though that requires
   coordination; a simpler way is for each ASBR/RBR to allocate a label
   from the label block allocated to itself (see Section 3.2.1).

   In the rest of this document, we call the label allocated for a
   particular PMSI a "(per-)PMSI label", just like we have (per-)VPN/BD/
   ES labels.  Notice that using a per-PMSI label in the case of a per-
   PE PMSI still has the original scaling issue associated with the
   upstream-assigned label, so per-region PMSIs are preferred.  Within
   each AS/region, per-PE PMSIs are still used, though they do not go
   across borders and per-VPN/BD labels can still be used.

   Note that when a segmentation point re-advertises a PMSI route to the
   next segment, it does not need to re-advertise a new label unless the
   upstream or downstream segment uses ingress replication.

3.2.3.  Alternative to Per-PMSI Label Allocation

   Per-PMSI label allocation in the case of segmentation, whether for
   S-PMSIs or per-PE/region I-PMSIs, is applied so that segmentation
   points are able to label-switch traffic without having to do IP or
   Media Access Control (MAC) lookups in VRFs (the segmentation points
   typically do not have those VRFs at all).  Alternatively, if the
   label scaling becomes a concern, the segmentation points could use
   (C-S,C-G) lookups in VRFs for flows identified by the S-PMSIs.  This
   allows the S-PMSIs for the same VPN/BD to share a VPN/BD-identifying
   label that leads to lookups in the VRFs.  That label needs to be
   different from the label used in the per-PE/region I-PMSIs though, so
   that the segmentation points can label-switch other traffic (not
   identified by those S-PMSIs).  However, this moves the scaling
   problem from the number of labels to the number of (C-S/*,C-G) routes
   in VRFs on the segmentation points.

3.3.  Summary of Label Allocation Methods

   In summary, labels can be allocated and advertised in the following
   ways:

   1.  A central entity allocates per-VPN/BD/ES labels from the DCB.
       PEs advertise the labels with an indication that they are from
       the DCB.

   2.  A central entity allocates per-VPN/BD/ES labels from a few common
       context-specific label spaces and allocates labels from the DCB
       to identify those context-specific label spaces.  PEs advertise
       the VPN/BD labels along with the context-identifying labels.

   3.  A central entity assigns disjoint label blocks from a few
       context-specific label spaces to each PE and allocates labels
       from the DCB to identify those context-specific label spaces.  A
       PE independently allocates a label for a segmented S-PMSI from
       its assigned label block and advertises the label along with the
       context-identifying label.

   Option 1 is simplest, but it requires that all the PEs set aside a
   common label block for the DCB that is large enough for all the
   VPNs/BDs/ESs combined.  Option 3 is needed only for segmented
   selective tunnels that are set up dynamically.  Multiple options
   could be used in any combination, depending on the deployment
   situation.

4.  Specifications

4.1.  Context-Specific Label Space ID Extended Community

   The Context-Specific Label Space ID Extended Community (EC) is a new
   Transitive Opaque EC with the following structure:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | 0x03 or 0x43  |      8        |      ID-Type                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         ID-Value                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   ID-Type:  A 2-octet field that specifies the type of Label Space ID.
      In this document, the ID-Type is 0, indicating that the ID-Value
      field is a label.

   ID-Value:  A 4-octet field that specifies the value of the Label
      Space ID.  When it is a label (with ID-Type 0), the most
      significant 20-bit portion is set to the label value.

   This document introduces a DCB-flag (Bit 47 as assigned by IANA) in
   the "Additional PMSI Tunnel Attribute Flags" BGP Extended Community
   [RFC7902].

   In the remainder of this document, when we say that a BGP-MVPN/EVPN
   A-D route carries a DCB-flag or has a DCB-flag attached to it, we
   mean the following:

   *  The route carries a PTA and its Flags field has the Extension bit
      set, AND

   *  The route carries an "Additional PMSI Tunnel Attribute Flags" EC
      and its DCB-flag is set.

4.2.  Procedures

   The protocol and procedures specified in this section MAY be used
   when BIER or P2MP/MP2MP tunnel aggregation is used for an MVPN/EVPN
   or when BIER/P2MP/MP2MP tunnels are used with EVPN multihoming.  When
   these procedures are used, all PE routers and segmentation points
   MUST support the procedures.  How to ensure this behavior is outside
   the scope of this document.

   Via methods outside the scope of this document, each VPN/BD/ES is
   assigned a label from the DCB or one of those few context-specific
   label spaces, and every PE that is part of the VPN/BD/ES is aware of
   the assignment.  The ES label and the BD label MUST be assigned from
   the same label space.  If PE Distinguisher Labels are used [RFC7582],
   they MUST be allocated from the same label space as well.

   In the case of tunnel segmentation, each PE is also assigned a
   disjoint label block from one of those few context-specific label
   spaces, and it allocates labels for its segmented PMSI routes from
   its assigned label block.

   When a PE originates/re-advertises an x-PMSI/IMET route, the route
   MUST carry a DCB-flag if and only if the label in its PTA is assigned
   from the DCB.

   If the VPN/BD/ES/PMSI label is assigned from one of those few
   context-specific label spaces, a Context-Specific Label Space ID EC
   MUST be attached to the route.  The ID-Type in the EC is set to 0,
   and the ID-Value is set to a label allocated from the DCB and
   identifies the context-specific label space.  When an ingress PE
   sends traffic, it imposes the DCB label that identifies the context-
   specific label space after it imposes the label (that is advertised
   in the Label field of the PTA in the x-PMSI/IMET route) for the VPN/
   BD and/or the label (that is advertised in the ESI Label EC) for the
   ESI, and then imposes the encapsulation for the transport tunnel.

   When a PE receives an x-PMSI/IMET route with the Context-Specific
   Label Space ID EC, it MUST place an entry in its default MPLS
   forwarding table to map the label in the EC to a corresponding
   context-specific label table.  That table is used for the next label
   lookup for incoming data traffic with the label signaled in the EC.

   Then, the receiving PE MUST place an entry for the label that is in
   the PTA or ESI Label EC in either the default MPLS forwarding table
   (if the route carries the DCB-flag) or the context-specific label
   table (if the Context-Specific Label Space ID EC is present)
   according to the x-PMSI/IMET route.

   An x-PMSI/IMET route MUST NOT carry both the DCB-flag and the
   Context-Specific Label Space ID EC.  A received route with both the
   DCB-flag set and the Context-Specific Label Space ID EC attached MUST
   be treated as withdrawn.  If neither the DCB-flag nor the Context-
   Specific Label Space ID EC is attached, the label in the PTA or ESI
   Label EC MUST be treated as the upstream-assigned label from the
   label space of the source PE, and procedures provided in [RFC6514]
   and [RFC7432] MUST be followed.

   If a PE originates two x-PMSI/IMET routes with the same tunnel, it
   MUST ensure that one of the following scenarios applies, so that the
   PE receiving the routes can correctly interpret the label that
   follows the tunnel encapsulation of data packets arriving via the
   tunnel:

   *  They MUST all have the DCB-flag,

   *  They MUST all carry the Context-Specific Label Space ID EC,

   *  None of them have the DCB-flag, or

   *  None of them carry the Context-Specific Label Space ID EC.

   Otherwise, a receiving PE MUST treat the routes as if they were
   withdrawn.

5.  Security Considerations

   This document allows three methods (Section 3.3) of label allocation
   for MVPN PEs [RFC6514] or EVPN PEs [RFC7432] and specifies
   corresponding signaling and procedures.  The first method (Option 1)
   is the equivalent of using common SRGBs [RFC8402] from the regular
   per-platform label space.  The second method (Option 2) is the
   equivalent of using common SRGBs from a third-party label space
   [RFC5331].  The third method (Option 3) is a variation of the second
   in that the third-party label space is divided into disjoint blocks
   for use by different PEs, where each PE will use labels from its
   respective block to send traffic.  In all cases, a receiving PE is
   able to identify one of the few label forwarding tables to forward
   incoming labeled traffic.

   [RFC6514], [RFC7432], [RFC8402], and [RFC5331] do not list any
   security concerns related to label allocation methods, and this
   document does not introduce any new security concerns in this regard.

6.  IANA Considerations

   IANA has made the following assignments:

   *  Bit 47 (DCB) in the "Additional PMSI Tunnel Attribute Flags"
      registry:

      +==========+======+===========+
      | Bit Flag | Name | Reference |
      +==========+======+===========+
      | 47       | DCB  | RFC 9573  |
      +----------+------+-----------+

                  Table 1

   *  Sub-type 0x08 for "Context-Specific Label Space ID Extended
      Community" in the "Transitive Opaque Extended Community Sub-Types"
      registry:

      +================+=============================+===========+
      | Sub-Type Value | Name                        | Reference |
      +================+=============================+===========+
      | 0x08           | Context-Specific Label      | RFC 9573  |
      |                | Space ID Extended Community |           |
      +----------------+-----------------------------+-----------+

                                Table 2

   IANA has created the "Context-Specific Label Space ID Type" registry
   within the "Border Gateway Protocol (BGP) Extended Communities" group
   of registries.  The registration procedure is First Come First Served
   [RFC8126].  The initial assignment is as follows:

   +============+============+===========+
   | Type Value | Name       | Reference |
   +============+============+===========+
   | 0          | MPLS Label | RFC 9573  |
   +------------+------------+-----------+
   | 1-254      | Unassigned |           |
   +------------+------------+-----------+
   | 255        | Reserved   |           |
   +------------+------------+-----------+

                   Table 3

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <https://www.rfc-editor.org/info/rfc6514>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
              <https://www.rfc-editor.org/info/rfc7524>.

   [RFC7582]  Rosen, E., Wijnands, IJ., Cai, Y., and A. Boers,
              "Multicast Virtual Private Network (MVPN): Using
              Bidirectional P-Tunnels", RFC 7582, DOI 10.17487/RFC7582,
              July 2015, <https://www.rfc-editor.org/info/rfc7582>.

   [RFC7902]  Rosen, E. and T. Morin, "Registry and Extensions for
              P-Multicast Service Interface Tunnel Attribute Flags",
              RFC 7902, DOI 10.17487/RFC7902, June 2016,
              <https://www.rfc-editor.org/info/rfc7902>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [BIER-EVPN]
              Zhang, Z., Przygienda, A., Sajassi, A., and J. Rabadan,
              "EVPN BUM Using BIER", Work in Progress, Internet-Draft,
              draft-ietf-bier-evpn-14, 2 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              evpn-14>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8556]  Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
              and A. Dolganow, "Multicast VPN Using Bit Index Explicit
              Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
              2019, <https://www.rfc-editor.org/info/rfc8556>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC9572]  Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
              Sajassi, "Updates to EVPN Broadcast, Unknown Unicast, or
              Multicast (BUM) Procedures", RFC 9572,
              DOI 10.17487/RFC9572, May 2024,
              <https://www.rfc-editor.org/info/rfc9572>.

Acknowledgements

   The authors thank Stephane Litkowski, Ali Sajassi, and Jingrong Xie
   for their reviews of, comments on, and suggestions for this document.

Contributors

   The following individual also contributed to this document:

   Selvakumar Sivaraj
   Juniper Networks
   Email: ssivaraj@juniper.net

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Eric Rosen
   Individual
   Email: erosen52@gmail.com

   Wen Lin
   Juniper Networks
   Email: wlin@juniper.net

   Zhenbin Li
   Huawei Technologies
   Email: lizhenbin@huawei.com

   IJsbrand Wijnands
   Individual
   Email: ice@braindump.be