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Versions: (draft-hfa-bier-pim-signaling) 00 01 02 03 04 05 06 07 08 09

Network Working Group                                    H. Bidgoli, Ed.
Internet-Draft                                                     Nokia
Intended status: Informational                                     F. Xu
Expires: January 10, 2021                                        Verizon
                                                             J. Kotalwar
                                                                   Nokia
                                                             I. Wijnands
                                                               M. Mishra
                                                            Cisco System
                                                                Z. Zhang
                                                        Juniper Networks
                                                           July 09, 2020


                    PIM Signaling Through BIER Core
                    draft-ietf-bier-pim-signaling-09

Abstract

   Consider large networks deploying traditional PIM multicast service.
   Typically, each portion of these large networks have their own
   mandates and requirements.

   It might be desirable to deploy BIER technology in some part of these
   networks to replace traditional PIM services.  In such cases
   downstream PIM states need to be signaled over BIER Domain toward the
   source.

   This draft explains the procedure to signal PIM joins and prunes
   through a BIER Domain, as such enable provisioning of traditional PIM
   services through a BIER Domain.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 10, 2021.



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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  PIM Signaling Through BIER domain . . . . . . . . . . . . . .   5
     3.1.  Ingress BBR procedure . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Determining EBBR on IBBR  . . . . . . . . . . . . . .   6
       3.1.2.  Considering ECMP in EBBR selection  . . . . . . . . .   6
       3.1.3.  PIM Signaling packet construction at IBBR . . . . . .   7
         3.1.3.1.  BIER packet construction at IBBR  . . . . . . . .   8
     3.2.  Signaling PIM through the BIER domain procedure . . . . .   8
     3.3.  EBBR procedure  . . . . . . . . . . . . . . . . . . . . .   9
   4.  Datapath Forwarding . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  BFIR tracking of (S,G)  . . . . . . . . . . . . . . . . .   9
     4.2.  Datapath traffic flow . . . . . . . . . . . . . . . . . .   9
   5.  PIM-SM behavior . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Applicability to MVPN . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  SPF . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.2.  Indirect next-hop . . . . . . . . . . . . . . . . . . . .  12
       A.2.1.  Static Route  . . . . . . . . . . . . . . . . . . . .  12
       A.2.2.  Interior Border Gateway Protocol (iBGP) . . . . . . .  12
     A.3.  Inter-area support  . . . . . . . . . . . . . . . . . . .  13
       A.3.1.  Inter-area Route summarization  . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13




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1.  Introduction

   Consider large networks deploying traditional PIM multicast service.
   Typically, each portion of these large networks have their own
   mandates and requirements.

   It might be desirable to deploy BIER technology in some part of these
   networks to replace traditional PIM services.  In such cases
   downstream PIM states need to be signaled over BIER Domain toward the
   source.

   This draft explains the procedure to signal PIM joins and prunes
   through a BIER Domain, as such enable provisioning of traditional PIM
   services through a BIER Domain.

2.  Conventions used in this document

   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 RFC 2119 [RFC2119].

2.1.  Definitions

   Some of the terminology specified in [I-D. rfc8279] is replicated
   here and extended by necessary definitions:

   BIER:

   Bit Index Explicit Replication (The overall architecture of
   forwarding multicast using a Bit Position).

   BFR:

   Bit Forwarding Router (A router that participates in Bit Index
   Multipoint Forwarding).A BFR is identified by a unique BFR-prefix in
   a BIER domain.

   BFIR:

   Bit Forwarding Ingress Router (The ingress border router that
   performs BIER encapsulation).  Each BFIR must have a valid BFR-id
   assigned.  In this draft BIER will be used for forwarding and
   tunneling of control plane packet (i.e.  PIM) and forwarding
   dataplane packets.  BFIR is term used for dataplane packet
   forwarding.

   BFER:




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   Bit Forwarding Egress Router.  A router that participates in Bit
   Index Forwarding as leaf.  Each BFER must have a valid BFR-id
   assigned.  In this draft BIER will be used for forwarding and
   tunneling of control plane packet (i.e.  PIM) and forwarding
   dataplane packets.  BFIR is term used for dataplane packet
   forwarding.

   BBR:

   BIER Boundary router.  A router between the PIM domain and BIER
   domain.  Maintains PIM adjacency for all routers attached to it on
   the PIM domain and terminates the PIM adjacency toward the BIER
   domain.

   IBBR:

   Ingress BIER Boundary Router.  An ingress router from signaling point
   of view.  It maintains PIM adjacency toward the PIM domain and
   determines if PIM joins and prunes arriving from PIM domain need to
   be signaled across the BIER domain.  If so it terminates the PIM
   adjacency toward the BIER domain and signals the PIM joins/prunes
   through the BIER core.

   EBBR:

   Egress BIER Boundary Router.  An egress router in BIER domain from
   signaling point of view.  It terminates the BIER packet and forwards
   the signaled joins and prunes into PIM Domain.

   BFT:

   Bit Forwarding Tree used to reach all BFERs in a domain.

   BIFT:

   Bit Index Forwarding Table.

   BIER sub-domain:

   A further distinction within a BIER domain identified by its unique
   sub-domain identifier.  A BIER sub-domain can support multiple
   BitString Lengths.

   BFR-id:

   An optional, unique identifier for a BFR within a BIER sub-domain.





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3.  PIM Signaling Through BIER domain

                         BBR                   BBR
           |--PIM Domain--|-----BIER domain-----|--PIM domain--|
      S--( A )----------( B ) ---- ( C ) ---- ( D )----------( E )--h

                        EBBR                  IBBR
      Sig   <-----PIM-----|<--BIER Tunneling----|<----PIM------
      (new)
                        BFIR                  BFER
            ------------->|--------BIER-------->|-------------> Datapath
                                                               (no change)



                       Figure 1: BIER boundry router

   As per figure 1, the procedures of PIM signaling is done at the BIER
   boundary router.  The BIER boundary routers (BBR) are connected to
   PIM capable routers toward the PIM domain and BIER routers toward the
   BIER domain.  PIM routers in PIM domain continue to send PIM state
   messages to the BBR.  The BBR will create PIM adjacency between all
   the PIM routers attached to it on the PIM domain.  That said the BBR
   does not propagate all PIM packets natively into the BIER domain.
   Instead when it determines that the PIM join or prune messages needs
   to be signaled through the BIER domain it will tunnel the PIM packet
   through the BIER network.  This tunneling is only done for signaling
   purposes and not for creating a PIM adjacency between the two
   disjoint PIM domains through the BIER domain.

   The terminology ingress BBR (IBBR) and egress BBR (EBBR) are relative
   from signaling point of view.

   The ingress BBR will determine if an arriving PIM join or prune needs
   to be signaled across the BIER domain.  While the egress BBR will
   determine if the arriving BIER packet is a signaling packet and if so
   it will generate a PIM signaling packet toward its attached PIM
   domain.

   The BFER and BFIR are BBR from datapath point of view.  It should be
   noted the new procedures in this draft are only applicable to
   signaling and there are no changes from datapath point of view.

3.1.  Ingress BBR procedure

   IBBR will create PIM adjacency to all PIM routers attached to it
   toward the PIM domain.




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   When a PIM join or prune for certain (S,G) arrives, the IBBR first
   determines whether the join or prune is meant for a source that is
   reachable through the BIER domain.  As an example, this source is
   located in a disjoint PIM domain that is reachable through the BIER
   domain.  If so the IBBR will try to resolve the source via an EBBR
   closest to the source.

   The procedure to find the EBBR (BFIR from datapath point of view) can
   be via many mechanisms explained in more detail in upcoming section.

   After discovering the EBBR and its BFR-ID, the IBBR will include a
   new PIM Join Attribute in the Join/prune message as per [RFC5384].
   Two new "BIER IBBR" attributes are defined and explained in upcoming
   section.  The PIM Join Attribute is used on EBBR to obtain necessary
   BIER information to build its multicast states.  In addition the IBBR
   will change the PIM signaling packet source IP address to its BIER
   prefix address (standard PIM procedure).  It will also keep the
   destination address as the well known multicast IP address.  It then
   will construct the BIER header.  The signaling packet, in this case
   the PIM join/prune packet, is encapsulated in the BIER header and
   transported through BIER domain to EBBR.

   The IBBR will track all the PIM interfaces on the attached PIM domain
   which are interested in a certain (S,G).  It creates multicast states
   for arriving (S,G)s from PIM domain, with incoming interface as BIER
   "tunnel" interface and outgoing interface as the PIM domain
   interface(s) on which PIM Join(s) were received on.

3.1.1.  Determining EBBR on IBBR

   As it was explained in previous section, IBBR needs to determine the
   EBBR closest to the source.  This is needed to encode the BIER header
   BitString field to forward the signaling packet through the BIER
   domain.

   It should be noted, the PIM domains can be either part of the same
   IGP area as BIER domain(single area) or are stitched to the BIER
   domain via an ABR or ASBR routers.  As such on IBBR, there can be
   many different procedures to determine the EBBR.  Some examples of
   these procedures have been provided in Appendix A.

3.1.2.  Considering ECMP in EBBR selection

   If the lookup for source results into multiple EBBRs, then the EBBR
   selection algorithm should ensure that all signaling for a particular
   (C-S, C-G) is forward to a single EBBR.  How the this selection is
   done is vendor specific and beyond this draft.  As an example it can




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   be round robin per (C-S, C-G) or smallest EBBR IP for all (C-S,
   C-G)s.

3.1.3.  PIM Signaling packet construction at IBBR

   To ensure all necessary BIER information needed by EBBR is present in
   the BIER signaling message, a new PIM Join Attribute [RFC5384] is
   used.  EBBR can use this attribute to build its multicast states, as
   described in EBBR procedure section.  This new PIM join Attribute is
   added to PIM signaling message on the IBBR.  Its format is as follow:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |F|E|    Type=tbd |     Length  |  Addr Family    | BIER info
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

                       Figure 2: PIM Join Attribute

   F bit: The Transitive bit.  Specifies whether this attribute is
   transitive or non-transitive.  MUST be set to zero.  This attribute
   is ALWAYS non-transitive.

   E bit: End-of-Attributes bit.  Specifies whether this attribute is
   the last.  Set to zero if there are more attributes.  Set to 1 if
   this is the last attribute.

   Type: TBD assign by IANA

   Length: The length in octets of the attribute value.  MUST be set to
   the length in octets of the BIER info +1 octet to account for the
   Address Family field.  For IPv4 AF Length = 7+1 For IPv6 AF Length =
   19+1

   Addr Family: Signaled PIM Join/Prune address family as defined in
   [RFC7761]

   BIER Info: IBBR Prefix (IPv4 or IPv6), SD, bfr-id as per below figure

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                 IBBR Prefix IPv4 or IPv6                      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | subdomain-id  |        BFR-ID                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 3: PIM Join Attribute detail



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3.1.3.1.  BIER packet construction at IBBR

   The BIER header will be encoded with the BFR-id of the IBBR(with
   appropriate bit set in the bitstring) and the PIM signaling packet is
   then encapsulated in the packet.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              BIFT-id                  | TC  |S|     TTL       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Nibble |  Ver  |  BSL  |              Entropy                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |OAM|Rsv|    DSCP   |   Proto   |            BFIR-id            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                BitString  (first 32 bits)                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                                                               ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                BitString  (last 32 bits)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 4: BIER header

   BIERHeader.Proto = IPv4 or IPv6

   BIERHeader.BitString= Bit corresponding to the BFR-ID of the EBBR

   BIERHeader.BFIR-id = BFR-Id of the BER originating the encapsulated
   PIM packet, i.e. the IBBR.

   Rest of the values in the BIER header are determined based on the
   network (MPLS/non-MPLS), capabilities (BSL), and network
   configuration.

3.2.  Signaling PIM through the BIER domain procedure

   Throughout the BIER domain the BIER forwarding procedure is on par
   with RFC 8279.  No BIER router will examine the BIER packet
   encapsulating the PIM signaling packet.  As such there is no
   multicast state built in the BIER domain.

   The packet will be forwarded through the BIER domain until it reaches
   the BER with matching BFR-ID as in the BIERHeader.Bitstring.  EBBR
   will remove the BIER header and examine the PIM IPv4 or IPv6
   signaling packet farther as per EBBR Procedure section.






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3.3.  EBBR procedure

   EBBR will remove the BIER header and determine this is a signaling
   packet.  The Received PIM join/prune Signaling packet is processed as
   if it were received from neighbors on a virtual interface, (i.e. as
   if the pim adjacency was presents, regardless of the fact there is no
   adjacency)

   The EBBR will build a forwarding table for the arriving (S,G) using
   the obtained BFIR-id and the Sub-Domain information from BIER Header
   and/or the PIM join Attributes added to the PIM Signaling packet.  In
   short it tracks all IBBRs interested in this (S,G).  This is
   explained in section 4.1.

   The multicast state on EBBR will contain PIM domain incoming
   interfaces, according to PIM specification and outgoing interfaces
   based on the above build forwarding table.

   It should be noted EBBR will maintain PIM adjacency toward the PIM
   domain and all PIM routers which are connected to it.  At this point
   the end-to-end multicast traffic flow setup is complete.

4.  Datapath Forwarding

4.1.  BFIR tracking of (S,G)

   For a specific Source and Group, BFIR (EBBR)should track all the
   interested BFERs (IBBRs) via arriving PIM signaling from BIER Domain.
   BFIR builds its (s,g)forwarding state with incoming interface (IIF)
   as the RPF interface (in PIM domain) towards the source and one of
   the outgoing interfaces as for sending to tracked BFERs in the SD.

4.2.  Datapath traffic flow

   When the multicast data traffic arrives on the BFIR (EBBR) the router
   will find all the interested BFERs for that specific (S,G).  The
   router then constructs the BIERHeader.BitString with all the BFER
   interested in the group and will forward the packet to the BIER
   domain.  The BFER(s) will accept the packets and remove the BIER
   header and forward the multicast packet as per pre-build multicast
   state for (S,G) and its outgoing interfaces.

5.  PIM-SM behavior

   The procedures described in this document can work with ASM as long
   as static RP or embedded RP for IPv6 is used.  Future drafts would
   cover BSR and more complicated SM discovery mechanisms.




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   It should be noted that this draft only signals PIM Joins and Prunes
   through the BIER domain and not any other PIM message types including
   PIM Hellos or Asserts.  As such functionality related to these other
   type of massages will not be possible through a BIER domain with this
   draft and future drafts might cover these scenarios.  As an example
   DR selection should be done in the PIM domain or if the PIM routers
   attached to IBBRs are performing DR selection there needs to be a
   dedicated PIM interface between these routers.

   In case of PIM ASM Static RP or embedded RP for IPv6 the procedure
   for leaves joining RP is same as above.  It should be noted that for
   ASM, the EBBRs are determined with respect to the RP instead of the
   source.

6.  Applicability to MVPN

   With just minor changes, the above procedures apply to MVPN as well,
   with BFIR/BFER/EBBR/IBBR being VPN PEs.  All the PIM related
   procedures, and the determination of EBBR happens in the context of a
   VRF, following procedures for PIM-MVPN.

   When a PIM packet arrives from PIM domain attached to the VRF (IBBR),
   and it is determined that the source is reachable via the VRF through
   the BIER domain, a PIM signaling message is sent via BIER to the
   EBBR.  In this case usually the PE terminating the PIM-MVPN is the
   EBBR.  A label is imposed before the BIER header is imposed, and the
   "proto" field in the BIER header is set to 1 (for "MPLS packet with
   downstream-assigned label at top of stack").  The label is advertised
   by the EBBR/BFIR to associate incoming packets to its correct VRF.
   In many scenarios a label is already bound to the VRF loopback
   address on the EBBR/BFIR and it can be used.

   When a multicast data packet is sent via BIER by an EBBR/BFIR, a
   label is imposed before the BIER packet is imposed, and the "proto"
   field in the BIER header is set to 1 (for "MPLS packet with
   downstream-assigned label at top of stack").  The label is assigned
   to the VPN consistently on all VRFs [draft-zzhang-bess-mvpn-evpn-
   aggregation-label].

   If the more complicated label allocation scheme is needed for the
   data packets as specified in [draft-zzhang-bess-mvpn-evpn-
   aggregation-label], then additional PMSI signaling is needed as
   specified in [RFC6513].

   To support per-area subdomain in this case, the ABRs would need to
   become VPN PEs and maintain per-VPN state so it is unlikely
   practical.




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7.  IANA Considerations

   This document contains no actions for IANA.

8.  Security Considerations

   The procedures of this document do not, in themselves, provide
   privacy, integrity, or authentication for the control plane or the
   data plane.  For a discussion of the security considerations
   regarding the use of BIER, please see RFC8279 and RFC8296.  Security
   considerations regarding PIM protocol is based on RFC 7761.

9.  Acknowledgments

   The authors would like to thank Eric Rosen, Stig Venaas for thier
   reviews and comments.

10.  References

10.1.  Normative References

   [RFC8279]  "Wijnands, IJ., Rosen, E., Dolganow, A., Przygienda, T.
              and S. Aldrin, "Multicast using Bit Index Explicit
              Replication"", October 2016.

10.2.  Informative References

   [BIER-MVPN]
              "Rosen, E., Ed., Sivakumar, M., Wijnands, IJ., Aldrin,
              S.,Dolganow, A., and T. Przygienda, "Multicast VPN Using
              BIER", internet-draft draft-ietf-bier-mvpn-11", March
              2018.

   [RFC8401]  "Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang,
              "BIER Support via ISIS"", June 2018.

   [RFC8444]  "Psenak, P., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions
              for Bit Index Explicit Replication"", June 2018.

Appendix A.

   This appendix provides some examples and routing procedures that can
   be used to determine the EBBR on IBBR.  It should be noted, the PIM
   domains can be either part of the same IGP area as BIER domain(single
   area) or are stitched to the BIER domain via an ABR or ASBR routers.
   As such on IBBR, there can be many different procedures to determine
   the EBBR.  Not all procedures are listed below.



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A.1.  SPF

   On IBBR SPF procedures can be used to find the EBBR closest to the
   source.

   Assuming the BIER domain consists of all BIER forwarding routers, SPF
   calculation can identify the router advertising the prefix for the
   source.  A post process can find the EBBR by walking from the
   advertising router back to the IBBR in the reverse direction of
   shortest path tree branch until the first BFR is encountered.

A.2.  Indirect next-hop

   Alternatively, the route to the source could have an indirect next-
   hop that identifies the EBBR.  These methods are explained in the
   following sections.

A.2.1.  Static Route

   On IBBR there can be a static route configured for the source, with
   source next-hop set as EBBR BIER prefix.

A.2.2.  Interior Border Gateway Protocol (iBGP)

   Consider the following topology:

                          BBR                   BBR
                          EBBR                  IBBR
            |--PIM Domain--|-----BIER domain-----|--PIM domain--|
       S--( A )----------( B ) ---- ( C ) ---- ( D )----------( E )--h

                                                    Figure 2

                          Figure 5: Static Route

   Suppose BGP is enable between EBBR (B) and IBBR (D) and the PIM
   Domain routes are redistributed to the BIER domain via BGP.  This
   would include the Multicast Source IP address (S), which resides in
   the PIM Domain.  In such case BGP should use the same loopback
   interface as its next-hop as the BBR prefix.  This will ensure that
   all PIM domain routes, including the Multicast Source IP address (S)
   are resolve via BBR's BIER prefix id as their next-hop.  When the
   host (h) triggers a PIM join message to IBBR (D), IBBR tries to
   resolve (S).  It resolves (S) via BGP installed route and realizes
   its next-hop is EBBR (B).  IBBR will use this next-hop (B) to find
   its corresponding BIER bit index.

   This procedure is inline with RFC6826 mLDP in-band signaling section



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A.3.  Inter-area support

   If each area has its own BIER sub-domain, the above procedure for
   post-SPF could identify one of the ABRs and the EBBR.  If a sub-
   domain spans multiple areas, then additional procedures as described
   in A.2 is needed.

A.3.1.  Inter-area Route summarization

   In a multi-area topology, a BIER sub-domain can span a single area.
   Suppose this single area is constructed entirely of BIER capable
   routers and the ABRs are the BIER Boundary Routers attaching the BIER
   sub-domain in this area to PIM domains in adjacent areas.  These BBRs
   can summarize the PIM domain routes via summary routes, as an example
   for OSPF, a type 3 summary LSAs can be used to advertise summary
   routes from a PIM domain area to the BIER area.  In such scenarios
   the IBBR can be configured to look up the Source via IGP database and
   use the summary routes and its Advertising Router field to resolve
   the EBBR.  The IBBR needs to ensure that the IGP summary route is
   generated by a BFR.  This can be achieved by ensuring that BIER Sub-
   TLV exists for this route.  If multiple BBRs (ABRs) have generated
   the same summary route the lowest Advertising Router IP can be
   selected or a vendor specific hashing algorithm can select the
   summary route from one of the BBRs.

Authors' Addresses

   Hooman Bidgoli (editor)
   Nokia
   Ottawa
   Canada

   Email: hooman.bidgoli@nokia.com


   Fengman Xu
   Verizon
   Richardson
   US

   Email: fengman.xu@verizon.com










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Internet-Draft       PIM Signaling Through BIER Core           July 2020


   Jayant Kotalwar
   Nokia
   Montain View
   US

   Email: jayant.kotalwar@nokia.com


   IJsbrand Wijnands
   Cisco System
   Diegem
   Belgium

   Email: ice@cisco.com


   Mankamana Mishra
   Cisco System
   Milpitas
   USA

   Email: mankamis@cisco.com


   Zhaohui Zhang
   Juniper Networks
   Boston
   USA

   Email: zzhang@juniper.com





















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