BGP for BIER: BGP Extensions for Bit Index Explicit Replication (BIER)

Internet Engineering Task Force (IETF)                             X. Xu
Request for Comments: 9793                                  China Mobile
Category: Standards Track                                        M. Chen
ISSN: 2070-1721                                                   Huawei
                                                                K. Patel
                                                            Arrcus, Inc.
                                                            IJ. Wijnands
                                                              Individual
                                                           T. Przygienda
                                                           Z. Zhang, Ed.
                                                                 Juniper
                                                               June 2025


        BGP Extensions for Bit Index Explicit Replication (BIER)

Abstract

   Bit Index Explicit Replication (BIER) is a multicast forwarding
   architecture that doesn't require an explicit tree-building protocol
   and doesn't require intermediate routers to maintain per-tree
   multicast states.  Some BIER-specific information and states, which
   are only in proportion to the number of BIER routers but not per-
   tree, do need to be advertised, calculated, and maintained.  This
   document describes BGP extensions for advertising the BIER
   information and methods for calculating BIER states based on the
   advertisements.

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/rfc9793.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Requirements Language
   3.  BIER Path Attribute
     3.1.  BIER MPLS Encapsulation Sub-TLV
     3.2.  BIER Non-MPLS Encapsulation Sub-TLV
     3.3.  BIER Nexthop Sub-TLV
   4.  Originating/Propagating/Updating the BIER Attribute
   5.  BIFT Calculation with BGP Signaling
   6.  Example of BIER Nexthop Usage and Handling
   7.  Operational Considerations
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Bit Index Explicit Replication (BIER) [RFC8279] is a multicast
   forwarding architecture that doesn't require an explicit tree-
   building protocol and doesn't require intermediate routers to
   maintain per-tree multicast states.  It supports both direct and
   tunneled BIER forwarding.  This document describes BGP extensions for
   advertising the BIER information and methods for calculating BIER
   states based on the advertisements.  More specifically, in this
   document, we define a new optional transitive BGP attribute, referred
   to as the "BIER attribute", to convey the BIER-specific information
   such as Bit-Forwarding Router Identifier (BFR-ID), BitStringLength
   (BSL), and so on.  The signaling is to be used in a single
   Administrative Domain (AD), and Section 7 specifies procedures to
   prevent the BIER attribute from "leaking out" of the domain.

2.  Terminology

   This document makes use of the terminology defined in [RFC4271] and
   [RFC8279].  Some terms are listed below for convenience.

   BIER:  Bit Indexed Explicit Replication

   BFR:  Bit-Forwarding Router

   BFR-ID:  BFR Identifier

   BSL:  BitStringLength

   BIFT:  Bit Index Forwarding Table

   BIFT-id:  Bit Index Forwarding Table Identifier

   BFER:  Bit-Forwarding Egress Router

   BFR-prefix:  Each BFR is assigned a single "BFR-prefix" for each sub-
      domain to which it belongs.  It is recommended that the BFR-prefix
      be a loopback address of the BFR.

   NLRI:  Network Layer Reachability Information [RFC4271]

   AFI:  Address Family Identifier [RFC4760]

   SAFI:  Subsequent Address Family Identifier [RFC4760]

2.1.  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.

3.  BIER Path Attribute

   This specification defines an optional, transitive BGP path
   attribute, referred to as the "BIER attribute".  This attribute can
   be attached to a BGP UPDATE message by the originator for NLRIs of
   AFI 1 or 2 and SAFI 1, 2, or 4 to indicate the BIER-specific
   information of a particular BFR identified by the /32 (for IPv4) or
   /128 (for IPv6) host address prefix contained in the NLRI.  The
   attachment of the BIER attribute to non-host address prefixes is not
   defined by this document.  It may be specified in the future, for
   example, by [BIER-Prefix-Redistribute].

   If the BIER path attribute is present, the NLRI is referred to as a
   "BFR-prefix".  Use of the attribute with other AFIs/SAFIs is outside
   the scope of this document.

   The BIER path attribute is an optional, transitive BGP path attribute
   with type code 41 and of variable length.  The attribute value
   portion carries BIER TLVs, which are encoded as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                        Value (variable)                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Length field defines the length of the value portion in octets
   (thus, a TLV with no value portion would have a length of zero).  The
   TLV is not padded to 4-octet alignment.  Unknown and unsupported
   types MUST be preserved and propagated within the BIER attribute.
   The presence of unknown or unexpected TLVs MUST NOT result in the
   NLRI or the BIER attribute being considered malformed.

   When creating a BIER attribute, a BFR MUST include one BIER TLV for
   every sub-domain that the prefix belongs to.  The attribute type code
   for the BIER attribute is 41.  The value field of the BIER attribute
   contains one or more BIER TLVs as shown below:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Type = 1            |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Sub-domain   |            BFR-ID             |   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      |                           Sub-TLVs                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+..........................

   Type:  1

   Length:  2 octets encoding the length in octets of the Value part.

   Sub-domain:  A 1-octet field encoding the sub-domain ID corresponding
      to the BFR-ID (see [RFC8279]).

   BFR-ID:  A 2-octet field encoding the BFR-ID (see [RFC8279]).

   Reserved:  SHOULD be set to 0 on transmission and MUST be ignored on
      reception.

   Sub-TLVs:  Contains one or more sub-TLVs.

   The BIER TLV MAY appear multiple times in the BIER path attribute,
   one for each sub-domain.  There MUST be no more than one BIER TLV
   with the same Sub-domain value; if there is, the entire BIER path
   attribute MUST be ignored.

   A BIER TLV may have sub-TLVs, which may have their own sub-TLVs.  All
   those are referred to as sub-TLVs and share the same Type space,
   regardless of the level.

3.1.  BIER MPLS Encapsulation Sub-TLV

   The BIER MPLS Encapsulation sub-TLV has the following format.  It MAY
   appear multiple times in the BIER TLV.

   The BIER MPLS Encapsulation sub-TLV has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Type = 2            |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Max SI    |BS Len |             Label                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                        sub-TLVs                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  2

   Length:  2 octets encoding the length in octets of the Value part.
      The value is 4 or other (depending on sub-TLVs).

   Max SI:  A 1-octet field encoding the Maximum Set Identifier (see
      Section 1 of [RFC8279]) used in the encapsulation for this BIER
      sub-domain for this BitString length.

   BS Len:  BitString Length.  A 4-bit field encoding the supported
      BitString length associated with this BFR-prefix.  The values
      allowed in this field are specified in Section 2 of [RFC8296].

   Label:  A 20-bit value representing the first label in the label
      range.

   The "label range" is the set of labels beginning with the Label and
   ending with (Label + (Max SI)).  A unique label range is allocated
   for each BitString length and sub-domain-id.  These labels are used
   for BIER forwarding, as described in [RFC8279] and [RFC8296].

   The size of the label range is determined by the number of SIs
   (Section 1 of [RFC8279]) that are used in the network.  Each SI maps
   to a single label in the label range: the first label is for SI=0,
   the second label is for SI=1, etc.

   If the label associated with the Maximum SI exceeds the 20-bit range,
   the BIER MPLS Encapsulation sub-TLV containing the error MUST be
   ignored.

   If the same BitString length is repeated in multiple BIER MPLS
   Encapsulation sub-TLVs inside the same BIER TLV, all BIER MPLS
   Encapsulation sub-TLVs in the BIER TLV MUST be ignored.

   Label ranges within all BIER MPLS Encapsulation sub-TLVs advertised
   by the same BFR MUST NOT overlap.  If an overlap is detected, all
   BIER MPLS Encapsulation sub-TLVs advertised by the BFR MUST be
   ignored.

3.2.  BIER Non-MPLS Encapsulation Sub-TLV

   The BIER non-MPLS Encapsulation sub-TLV is used for non-MPLS
   encapsulation and has the following format.  It MAY appear multiple
   times within a single BIER TLV.  If the same BitString length is
   repeated in multiple BIER non-MPLS Encapsulation sub-TLVs inside the
   same BIER TLV, the BIER TLV MUST be ignored.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Type = 3            |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Max SI    |BS Len |                  BIFT-id              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                        sub-TLVs                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  3

   Length:  2 octets encoding the length in octets of the Value part.
      The value is 4 or other (depending on sub-TLVs).

   Max SI:  A 1-octet field encoding the Maximum Set Identifier
      (Section 1 of [RFC8279]) used in the encapsulation for this BIER
      sub-domain for this BitString length.  The first BIFT-id is for
      SI=0, the second BIFT-id is for SI=1, etc.  If the BIFT-id
      associated with the Maximum SI exceeds the 20-bit range, the sub-
      TLV MUST be ignored.

   BS Len:  BitString Length.  A 4-bit field encoding the BitString
      length (as per [RFC8296]) supported for the encapsulation.

   BIFT-id:  A 20-bit field representing the first BIFT-id in the BIFT-
      id range.

   The "BIFT-id range" is the set of 20-bit values beginning with the
   BIFT-id and ending with (BIFT-id + (Max SI)).  These BIFT-ids are
   used for BIER forwarding, as described in [RFC8279] and [RFC8296].

   The size of the BIFT-id range is determined by the number of SIs
   (Section 1 of [RFC8279]) that are used in the network.  Each SI maps
   to a single BIFT-id in the BIFT-id range: the first BIFT-id is for
   SI=0, the second BIFT-id is for SI=1, etc.

   If the BIFT-id associated with the Maximum SI exceeds the 20-bit
   range, the BIER non-MPLS Encapsulation sub-TLV containing the error
   MUST be ignored.

   BIFT-id ranges within all the BIER non-MPLS Encapsulation sub-TLVs
   advertised by the same BFR MUST NOT overlap.  If an overlap is
   detected, all the BIER non-MPLS Encapsulation sub-TLVs advertised by
   the BFR MUST be ignored.  However, the BIFT-id ranges may overlap
   across different encapsulation types and that is allowed.  As an
   example, the BIFT-id value in the non-MPLS Encapsulation sub-TLV may
   overlap with the Label value in the Label range in the BIER MPLS
   Encapsulation sub-TLV.

3.3.  BIER Nexthop Sub-TLV

   The BIER Nexthop sub-TLV MAY be included, and it MUST NOT be included
   more than once in each of the MPLS or non-MPLS Encapsulation sub-TLVs
   or in the top-level BIER TLV.  It is used when calculating BIFT
   entries, as described in Section 5 and illustrated in Section 6.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Type = 4           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Nexthop                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  4

   Length:  2 octets.  The value is 4 if the Nexthop is an IPv4 address
      and 16 if the Nexthop is an IPv6 address.

   Nexthop:  4 or 16 octets of an IPv4/IPv6 address.

4.  Originating/Propagating/Updating the BIER Attribute

   A Bit-Forwarding Egress Router (BFER) MUST attach a BIER attribute to
   its own /32 (for IPv4) or /128 (for IPv6) host BFR-prefix NLRI.  The
   BIER attribute MUST include one BIER TLV for each BIER sub-domain
   that it supports.  Each BIER TLV MUST include an MPLS and/or non-MPLS
   Encapsulation sub-TLV and MAY include a BIER Nexthop sub-TLV with the
   Nexthop set to the BIER prefix.  If the BIER Nexthop sub-TLV is not
   included, the BIER prefix will be used by receiving BFRs as the BIER
   next hop when calculating BIFT.

   When a BFR receives an update with the BIER path attribute, the
   attribute is parsed with the following validations:

   *  Syntactic checking based on the Length field of TLVs and sub-TLVs:

      -  The total length of BIER TLVs (including the Type and Length
         fields) MUST be equal to the BIER path attribute length.

      -  The total length of sub-TLVs (including the Type and Length
         fields) of a TLV MUST be equal to the length of the TLV.

   *  Semantic checking as per Section 3.

   If the syntactic checking fails, the attribute is considered
   malformed and the "attribute discard" action [RFC7606] for the BIER
   attribute MUST be taken.  If the semantic checking passes, BIFT
   entries are calculated as described in Section 5.  Otherwise (i.e.,
   if semantic checking fails), some or all BIER TLVs are ignored, per
   the rules given in Section 3, and if the remaining data permits, BIFT
   entries are calculated per Section 5.

   When a BFR re-advertises a BGP NLRI with a BIER attribute, for the
   sub-domains that this BFR supports, in the corresponding BIER TLV, it
   SHOULD set/update the BIER Nexthop sub-TLV to use its own BIER
   prefix; in which case, it MUST replace the MPLS or non-MPLS
   Encapsulation sub-TLV with its own, i.e., as if the BFR is attaching
   the encapsulation sub-TLV for its own BIER prefix.  If it does not
   update the BIER Nexthop sub-TLVs, it MUST NOT update the MPLS or non-
   MPLS Encapsulation sub-TLV.  If it does not support a sub-domain, it
   MUST NOT update the corresponding BIER TLV.

   It's possible that the BFR supports some but not all BitStringLengths
   (BSLs) in the received MPLS or non-MPLS Encapsulation sub-TLVs.
   After setting/updating the BIER Nexthop sub-TLV in the top BIER TLV
   to itself, for the BSLs that it does support, the BFR MUST remove the
   BIER Nexthop sub-TLV (if present) in the corresponding Encapsulation
   sub-TLVs.  For the BSLs that it does not support:

   *  If a BIER Nexthop sub-TLV is included in the Encapsulation sub-
      TLV, it MUST NOT be updated.

   *  Otherwise, if a BIER Nexthop sub-TLV is included in the received
      BIER TLV, its original value (before changed for supported BSLs by
      this BFR) MUST be copied into the Encapsulation sub-TLV.

   *  Otherwise, a BIER Nexthop sub-TLV MUST be added to the
      Encapsulation sub-TLV with its value set to the BFR-prefix.

   All impacted Length fields (e.g., the Encapsulation sub-TLV Length
   and the top-level BIER TLV Length) MUST be updated accordingly.

   Since the BIER attribute is an optional, transitive BGP path
   attribute, a non-BFR BGP speaker could still re-advertise the
   received route with a BIER attribute.

   Two different BFR-prefixes MUST NOT have the same non-zero BFR-ID in
   the same sub-domain.  If a duplication is detected, the receiving BFR
   MUST NOT use the BFR-prefixes with the same BFR-ID for BIFT
   calculation for the sub-domain and an error SHOULD be logged.

5.  BIFT Calculation with BGP Signaling

   As pointed out in [RFC8279], BIFTs are derived from the unicast FIB
   by adding BIER-specific information.

   For each sub-domain, a BFR calculates the corresponding BIFTs by
   going through the BIER prefixes whose BIER attribute includes a BIER
   TLV for the sub-domain.  For a non-zero BFR-id in the BIER TLV, a
   BIFT entry is created or updated.  The entry's BFR Neighbor (BFR-NBR)
   [RFC8279] is the Nexthop in the BIER Nexthop sub-TLV in the
   corresponding Encapsulation sub-TLV or in the top-level BIER TLV if
   the Encapsulation sub-TLV does not have a BIER Nexthop sub-TLV.  If
   there is no BIER Nexthop sub-TLV at all, the entry's BFR-NBR is the
   BIER prefix itself.  The BIER label or BIFT-id for the entry is
   derived from the label range in the MPLS Encapsulation sub-TLV or
   from the BIFT-id range in the non-MPLS Encapsulation sub-TLV.

   BIER traffic is sent to the BFR-NBR either directly (BIER header
   directly follows a Layer 2 header) if the BFR-NBR is directly
   connected or via a tunnel.  Notice that, if a non-BFR BGP speaker re-
   advertises a BIER prefix (in this case, it cannot update the BIER
   attribute since it is not capable), or if a BFR BGP speaker re-
   advertises a BIER prefix without updating the BIER Nexthop sub-TLV,
   the BFR receiving the prefix will tunnel BIER traffic -- the BGP
   speaker re-advertising the BIER prefix will not see the BIER traffic
   for the BIER prefix.

   How the tunnel is set up and chosen is outside the scope of this
   document.  It can be any kind of tunnel, e.g., MPLS Label Switched
   Path or IP/GRE, as long as the tunnel header can indicate that the
   payload is BIER.

6.  Example of BIER Nexthop Usage and Handling

   Consider a simple topology as follows:

                                         ----- BFER1
                                        /
              BFR1 --- non-BFR --- BFR2 ------ BFER2
                                        \
                                         ----- BFER3

   The BFER1/2/3 each advertises a route for its loopback address with a
   BIER path attribute, listing one BIER TLV for each sub-domain that it
   is in, with a non-zero BFR-ID and an MPLS Encapsulation sub-TLV.  A
   BIER Nexthop sub-TLV is not included in the one from BFER1 but is
   included in the ones from BFER2/3.  The BIER Nexthop sub-TLV encodes
   the BFR-prefix of BFER2 and BFER3, respectively.

   When BFR2 receives the route, it calculates its BIFT entries.
   Because the route from BFER1 does not include a BIER Nexthop, BFR2
   uses BFR1's BFR-prefix as the next hop.

   When BFR2 re-advertises the routes to the non-BFR, it adds a BIER
   Nexthop sub-TLV to the BFER1 route and updates the BIER Nexthop sub-
   TLV in the BFER2/3 routes, all encoding BFR2's own address.  It also
   updates the MPLS Encapsulation sub-TLV to encode its own labels.

   When the non-BFR receives the routes, since it does not support BIER,
   no BIER-specific action is taken and the routes are re-advertised to
   BFR1 with the BIER path attribute unchanged.

   When BFR1 receives the routes, it calculates the BIFT entries, using
   BFR2's address encoded in the BIER Nexthop sub-TLV as the next hop.
   Because BFR2 is not directly connected, a tunnel must be used.

7.  Operational Considerations

   In this document, it is assumed that the BIER domain [RFC8279] is
   aligned with an Administrative Domain (AD), which may be composed of
   multiple Autonomous Systems.  Use of the BIER attribute in other
   scenarios is outside the scope of this document.

   BFR-prefixes are typically loopback addresses on the BFRs.  They are
   distributed throughout the AD, but they do not need to be distributed
   outside the AD for the BIER's purposes.  This is analogous to the
   Provider Edge router's loopback addresses that are distributed inside
   the AD, but they do not need to be distributed outside the AD.

   If prefixes are distributed outside of the AD with the BIER attribute
   attached and the neighboring AD also deploying BIER, then the two
   BIER domains, which should be independent of each other, may be
   incorrectly joined together and most likely have conflicting
   configurations, causing security risks and operational troubles.

   To prevent that, a boundary router of the AD that supports the BIER
   attribute MUST support a policy based on an External BGP (EBGP)
   session/group that indicates whether the attribute is allowed; by
   default, it is NOT allowed.  If it is not allowed, the BIER attribute
   MUST NOT be sent to any EBGP peer of the session/group.  If a BIER
   attribute is received from the peer, it MUST be treated exactly as if
   it were an unrecognized non-transitive attribute.  That is, it MUST
   be quietly ignored and not passed along to other BGP peers.

8.  IANA Considerations

   IANA has assigned codepoint 41 to the BIER attribute in the "BGP Path
   Attributes" registry <https://www.iana.org/assignments/bgp-
   parameters> as follows:

                       +=======+======+===========+
                       | Value | Code | Reference |
                       +=======+======+===========+
                       | 41    | BIER | RFC 9793  |
                       +-------+------+-----------+

                                 Table 1

   IANA has created the "BGP BIER TLV and Sub-TLV Types" registry within
   the "Border Gateway Protocol (BGP) Parameters" registry group.  The
   type field for the registry consists of 2 octets, with possible
   values from 0 to 65535 (the value 0 is reserved).  The allocation
   policy for this field is First Come First Served [RFC8126].

   The five initial values have been allocated as follows:

         +=========+================================+===========+
         | Value   | Name                           | Reference |
         +=========+================================+===========+
         | 0       | Reserved                       | RFC 9793  |
         +---------+--------------------------------+-----------+
         | 1       | BIER TLV                       | RFC 9793  |
         +---------+--------------------------------+-----------+
         | 2       | MPLS Encapsulation sub-TLV     | RFC 9793  |
         +---------+--------------------------------+-----------+
         | 3       | non-MPLS Encapsulation sub-TLV | RFC 9793  |
         +---------+--------------------------------+-----------+
         | 4       | BIER Nexthop sub-TLV           | RFC 9793  |
         +---------+--------------------------------+-----------+
         | 5-65535 | Unassigned                                 |
         +---------+--------------------------------------------+

                                 Table 2

9.  Security Considerations

   This document introduces no new security considerations beyond those
   already discussed in [RFC4271], [RFC8279], and the operational
   considerations (Section 7) of this document.

10.  References

10.1.  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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [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>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

10.2.  Informative References

   [BIER-Prefix-Redistribute]
              Zhang, Z., Wu, B., Zhang, Z. J., Wijnands, I., Liu, Y.,
              and H. Bidgoli, "BIER Prefix Redistribute", Work in
              Progress, Internet-Draft, draft-ietf-bier-prefix-
              redistribute-08, 23 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              prefix-redistribute-08>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [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>.

Acknowledgements

   Thanks to Eric Rosen and Peter Psenak for their valuable comments on
   this document.

Contributors

   This document has the following contributor:

   Zheng (Sandy) Zhang
   ZTE
   Email: zhang.zheng@zte.com.cn


Authors' Addresses

   Xiaohu Xu
   China Mobile
   Email: xuxiaohu@cmss.chinamobile.com


   Mach(Guoyi) Chen
   Huawei
   Email: mach.chen@huawei.com


   Keyur Patel
   Arrcus, Inc.
   Email: keyur@arrcus.com


   IJsbrand Wijnands
   Individual
   Email: ice@braindump.be


   Tony Przygienda
   Juniper
   Email: prz@juniper.net


   Zhaohui Zhang (editor)
   Juniper
   Email: zzhang@juniper.net