EVPN Interworking with IPVPN
draft-ietf-bess-evpn-ipvpn-interworking-06

Document Type Active Internet-Draft (bess WG)
Authors Jorge Rabadan  , Ali Sajassi  , Eric Rosen  , John Drake  , Wen Lin  , Jim Uttaro  , Adam Simpson 
Last updated 2021-09-22 (latest revision 2021-06-22)
Replaces draft-rabadan-sajassi-bess-evpn-ipvpn-interworking
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BESS Workgroup                                           J. Rabadan, Ed.
Internet-Draft                                                     Nokia
Intended status: Standards Track                         A. Sajassi, Ed.
Expires: March 26, 2022                                            Cisco
                                                                E. Rosen
                                                              Individual
                                                                J. Drake
                                                                  W. Lin
                                                                 Juniper
                                                               J. Uttaro
                                                                    AT&T
                                                              A. Simpson
                                                                   Nokia
                                                      September 22, 2021

                      EVPN Interworking with IPVPN
               draft-ietf-bess-evpn-ipvpn-interworking-06

Abstract

   EVPN is used as a unified control plane for tenant network intra and
   inter-subnet forwarding.  When a tenant network spans not only EVPN
   domains but also domains where BGP VPN-IP or IP families provide
   inter-subnet forwarding, there is a need to specify the interworking
   aspects between BGP domains of type EVPN, VPN-IP and IP, so that the
   end to end tenant connectivity can be accomplished.  This document
   specifies how EVPN interworks with VPN-IPv4/VPN-IPv6 and IPv4/IPv6
   BGP families for inter-subnet forwarding.

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 March 26, 2022.

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Copyright Notice

   Copyright (c) 2021 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 and Problem Statement  . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Terminology and Interworking PE Components  . . . . . . . . .   3
   4.  Domain Path Attribute (D-PATH)  . . . . . . . . . . . . . . .   9
   5.  BGP Path Attribute Propagation across ISF SAFIs . . . . . . .  14
     5.1.  No-Propagation-Mode . . . . . . . . . . . . . . . . . . .  14
     5.2.  Uniform-Propagation-Mode  . . . . . . . . . . . . . . . .  14
     5.3.  Aggregation of Routes and Path Attribute Propagation  . .  16
   6.  Route Selection Process between EVPN and other ISF SAFIs  . .  16
   7.  Composite PE Procedures . . . . . . . . . . . . . . . . . . .  18
   8.  Gateway PE Procedures . . . . . . . . . . . . . . . . . . . .  20
   9.  Interworking Use-Cases  . . . . . . . . . . . . . . . . . . .  23
   10. Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  24
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  26
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  26
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     15.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction and Problem Statement

   EVPN is used as a unified control plane for tenant network intra and
   inter-subnet forwarding.  When a tenant network spans not only EVPN
   domains but also domains where BGP VPN-IP or IP families provide
   inter-subnet forwarding, there is a need to specify the interworking
   aspects between the different families, so that the end to end tenant
   connectivity can be accomplished.  This document specifies how EVPN

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   should interwork with VPN-IPv4/VPN-IPv6 and IPv4/IPv6 BGP families
   for inter-subnet forwarding.

   EVPN supports the advertisement of IPv4 or IPv6 prefixes in two
   different route types:

   o  Route Type 2 - MAC/IP route (only for /32 and /128 host routes),
      as described by [I-D.ietf-bess-evpn-inter-subnet-forwarding].

   o  Route Type 5 - IP Prefix route, as described by
      [I-D.ietf-bess-evpn-prefix-advertisement].

   When interworking with other BGP address families (AFIs/SAFIs) for
   inter-subnet forwarding, the IP prefixes in those two EVPN route
   types must be propagated to other domains using different SAFIs.
   Some aspects of that propagation must be clarified.  Examples of
   these aspects or procedures across BGP families are: route selection,
   loop prevention or BGP Path attribute propagation.  The Interworking
   PE concepts are defined in section 2, and the rest of the document
   describes the interaction between Interworking PEs and other PEs for
   end-to-end inter-subnet forwarding.

2.  Conventions used in this document

   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.  Terminology and Interworking PE Components

   This section summarizes the terminology related to the "Interworking
   PE" concept that will be used throughout the rest of the document.

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      +-------------------------------------------------------------+
      |                                                             |
      |              +------------------+           Interworking PE |
      | Attachment   | +------------------+                         |
      | Circuit(AC1) | |  +----------+    |                MPLS/NVO tnl
    ----------------------*Bridge    |    |                    +------
      |              | |  |Table(BT1)|    |    +-----------+  / \     \
   MPLS/NVO tnl +-------->|          *---------*           |<--> | Eth |
     -------+   |    | |  |Eth-Tag x +    |IRB1|           |  \ /     /
    / Eth  / \<-+    | |  +----------+    |    |           |   +------
   |      |   |      | |     ...          |    |  IP-VRF1  |        |
    \      \ /<-+    | |  +----------+    |    |  RD2/RT2  |MPLS/NVO tnl
     -------+   |    | |  |Bridge    |    |    |           |   +------
      |         +-------->|Table(BT2)|    |IRB2|           |  / \     \
      |              | |  |          *---------*           |<--> | IP  |
    ----------------------*Eth-Tag y |    |    +-----*-----+  \ /     /
      |  AC2         | |  +----------+    |       AC3|         +------
      |              | |    MAC-VRF1      |          |              |
      |              +-+    RD1/RT1       |          |              |
      |                +------------------+          |  SAFIs       |
      |                                              |  1     +---+ |
    -------------------------------------------------+  128   |BGP| |
      |                                                 EVPN  +---+ |
      |                                                             |
      +-------------------------------------------------------------+

                   Figure 1: EVPN-IPVPN Interworking PE

   o  ISF SAFI: Inter-Subnet Forwarding (ISF) SAFI is a MP-BGP Sub-
      Address Family that advertises reachability for IP prefixes and
      can be used for inter-subnet forwarding within a given tenant
      network.  The ISF SAFIs are 1 (including IPv4 and IPv6 AFIs), 128
      (including IPv4 and IPv6 AFIs) and 70 (EVPN, including only AFI
      25).  This document uses the following terms interchangeably: ISF
      SAFI 1 or BGP IP, ISF SAFI 128 or IPVPN, ISF SAFI 70 or EVPN.

   o  ISF route: a route for a given prefix whose ISF SAFI may change as
      it transits different domains.

   o  IP-VRF: an IP Virtual Routing and Forwarding table, as defined in
      [RFC4364].  It is also the instantiation of an IPVPN in a PE.
      Route Distinguisher and Route Target(s) are required properties of
      an IP-VRF.

   o  MAC-VRF: a MAC Virtual Routing and Forwarding table, as defined in
      [RFC7432].  It is also the instantiation of an EVI (EVPN Instance)
      in a PE.  Route Distinguisher and Route Target(s) are required

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      properties and they are normally different than the ones defined
      in the associated IP-VRF.

   o  BT: a Bridge Table, as defined in [RFC7432].  A BT is the
      instantiation of a Broadcast Domain in a PE.  When there is a
      single Broadcast Domain in a given EVI, the MAC-VRF in each PE
      will contain a single BT.  When there are multiple BTs within the
      same MAC-VRF, each BT is associated to a different Ethernet Tag.
      The EVPN routes specific to a BT, will indicate which Ethernet Tag
      the route corresponds to.

      Example: In Figure 1, MAC-VRF1 has two BTs: BT1 and BT2.  Ethernet
      Tag x is defined in BT1 and Ethernet Tag y in BT2.

   o  AC: Attachment Circuit or logical interface associated to a given
      BT or IP-VRF.  To determine the AC on which a packet arrived, the
      PE will examine the combination of a physical port and VLAN tags
      (where the VLAN tags can be individual c-tags, s-tags or ranges of
      both).

      Example: In Figure 1, AC1 is associated to BT1, AC2 to BT2 and AC3
      to IP-VRF1.

   o  IRB: Integrated Routing and Bridging interface.  It refers to the
      logical interface that connects a BT to an IP-VRF and allows to
      forward packets with destination in a different subnet.

   o  MPLS/NVO tnl: It refers to a tunnel that can be MPLS or NVO-based
      (Network Virtualization Overlays) and it is used by MAC-VRFs and
      IP-VRFs.  Irrespective of the type, the tunnel may carry an
      Ethernet or an IP payload.  MAC-VRFs can only use tunnels with
      Ethernet payloads (setup by EVPN), whereas IP-VRFs can use tunnels
      with Ethernet (setup by EVPN) or IP payloads (setup by EVPN or
      IPVPN).  IPVPN-only PEs have IP-VRFs but they cannot send or
      receive traffic on tunnels with Ethernet payloads.

      Example: Figure 1 shows an MPLS/NVO tunnel that is used to
      transport Ethernet frames to/from MAC-VRF1.  The PE determines the
      MAC-VRF and BT the packets belong to based on the EVPN label (MPLS
      or VNI).  Figure 1 also shows two MPLS/NVO tunnels being used by
      IP-VRF1, one carrying Ethernet frames and the other one carrying
      IP packets.

   o  RT-2: Route Type 2 or MAC/IP route, as per [RFC7432].

   o  RT-5: Route Type 5 or IP Prefix route, as per
      [I-D.ietf-bess-evpn-prefix-advertisement].

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   o  Domain: Two PEs are in the same domain if they are attached to the
      same tenant and the packets between them do not require a data
      path IP lookup (in the tenant space) in any intermediate router.
      A gateway PE is always configured with multiple DOMAIN-IDs.

      Example 1: Figure 2 depicts an example where TS1 and TS2 belong to
      the same tenant, and they are located in different Data Centers
      that are connected by gateway PEs (see the gateway PE definition
      later).  These gateway PEs use IPVPN in the WAN.  When TS1 sends
      traffic to TS2, the intermediate routers between PE1 and PE2
      require a tenant IP lookup in their IP-VRFs so that the packets
      can be forwarded.  In this example there are three different
      domains.  The gateway PEs connect the EVPN domains to the IPVPN
      domain.

                           GW1------------GW3
                         +------+       +------+
           +-------------|IP-VRF|       |IP-VRF|-------------+
          PE1            +------+       +------+            PE2
        +------+   DC1      |     WAN      |     DC2     +------+
    TS1-|IP-VRF|   EVPN     |    IPVPN     |     EVPN    |IP-VRF|-TS2
        +------+           GW2            GW4            +---+--+
           |             +------+       +------+             |
           +-------------|IP-VRF|       |IP-VRF|-------------+
                         +------+       +------+
                            +--------------+
               DOMAIN 1         DOMAIN 2       DOMAIN 3
           <---------------> <------------> <---------------->

                   Figure 2: Multiple domain DCI example

     Example 2: Figure 3 illustrates a similar example, but PE1 and PE2
     are now connected by a BGP-LU (BGP Labeled Unicast) tunnel, and
     they have a BGP peer relationship for EVPN.  Contrary to Example 1,
     there is no need for tenant IP lookups on the intermediate routers
     in order to forward packets between PE1 and PE2.  Therefore, there
     is only one domain in the network and PE1/PE2 belong to it.

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                                EVPN
           <------------------------------------------------->
                                BGP-LU
           <------------------------------------------------->

                          ASBR------------ASBR
                         +------+       +------+
           +-------------|      |       |      |-------------+
          PE1            +------+       +--+---+            PE2
        +------+   DC1      |     WAN      |     DC2     +------+
    TS1-|IP-VRF|   EVPN     |              |     EVPN    |IP-VRF|-TS2
        +------+          ASBR            ASBR           +---+--+
           |             +------+       +------+             |
           +-------------|      |       |      |-------------+
                         +------+       +------+
                            +--------------+

           <--------------------DOMAIN-1--------------------->

                    Figure 3: Single domain DCI example

   o  Regular Domain: a domain in which a single control plane, BGP IP,
      IPVPN or EVPN, is used and which is composed of regular PEs, see
      below.  In Figure 2 and Figure 3, above, all domains are regular
      domains.

   o  Composite Domain: a domain in which multiple control planes, BGP
      IP, IPVPN and EVPN, are used and which is composed of regular PEs,
      see below, and composite PEs, see below.

   o  Regular PE: a PE that is attached to a domain, either regular or
      composite, and which uses one of the control plane protocols (BGP
      IP, IPVPN or EVPN) operating in the domain.

   o  Interworking PE: a PE that may advertise a given prefix with an
      EVPN ISF route (RT-2 or RT-5) and/or an IPVPN ISF route and/or a
      BGP IP ISF route.  An interworking PE has one IP-VRF per tenant,
      and zero, one or multiple MAC-VRFs per tenant.  Each MAC-VRF may
      contain one or more BTs, where each BT may be attached to that IP-
      VRF via IRB.  There are two types of Interworking PEs: composite
      PEs and gateway PEs.  Both PE functions can be independently
      implemented per tenant and they may both be implemented for the
      same tenant.

      Example: Figure 1 shows an interworking PE of type gateway, where
      ISF SAFIs 1, 128 and 70 are enabled.  IP-VRF1 and MAC-VRF1 are
      instantiated on the PE, and together provide inter-subnet
      forwarding for the tenant.

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   o  Composite PE: an interworking PE that is attached to a composite
      domain and advertises a given prefix to an IPVPN peer with an
      IPVPN ISF route, to an EVPN peer with an EVPN ISF route, and to a
      route reflector with both an IPVPN and EVPN ISF route.  A
      composite PE performs the procedures of Sections 5 and 6.

      Example: Figure 4 shows an example where PE1 is a composite PE
      since PE1 has EVPN and another ISF SAFI enabled to the same route-
      reflector, and PE1 advertises a given IP prefix IPn/x twice, one
      using EVPN and another one using ISF SAFI 128.  PE2 and PE3 are
      not composite PEs.

                                   +---+
                                   |PE2|
                                   +---+
                                    ^
                                    |EVPN
                       IW    EVPN   v
                      +---+  IPVPN ++-+       +---+
                      |PE1| <----> |RR| <---> |PE3|
                      +---+        +--+ IPVPN +---+
                    Composite

                Figure 4: Interworking composite PE example

   o  Gateway PE: an interworking PE that is attached to two domains,
      each either regular or composite, and which, based on
      configuration, does one of the following:

      -  Propagates the same control plane protocol, BGP IP, IPVPN or
         EVPN, between the two domains.

      -  Propagates an ISF route with different ISF SAFIs between the
         two domains.  E.g., propagate an EVPN ISF route in one domain
         as an IPVPN ISF route in the other domain and vice versa.  A
         gateway PE performs the procedures of Sections Section 4,
         Section 5, Section 6 and Section 8.

      A gateway PE is always configured with multiple DOMAIN-IDs.  The
      DOMAIN-ID is encoded in the Domain Path Attribute (D-PATH), and
      advertised along with ISF SAFI routes.  Section 4 describes the
      D-PATH attribute.

      Example: Figure 5 illustrates an example where PE1 is a gateway PE
      since the EVPN and IPVPN SAFIs are enabled on different BGP peers,
      and a given local IP prefix IPn/x is sent to both BGP peers for
      the same tenant.  PE2 and PE1 are in one domain and PE3 and PE1
      are in another domain.

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                                     IW
                       +---+ EVPN   +---+ IPVPN  +---+
                       |PE2| <----> |PE1| <----> |PE3|
                       +---+        +---+        +---+
                                   Gateway

                 Figure 5: Interworking gateway PE example

   o  Composite/Gateway PE: an interworking PE that is both a composite
      PE and a gateway PE that is attached to two domains, one regular
      and one composite, and which does the following:

      -  Propagates an ISF route from the regular domain into the
         composite domain.  Within the composite domain it acts as a
         composite PE.

      -  Propagates an ISF route from the composite domain into the
         regular domain.  Within the regular domain it is propagated as
         an ISF route using the ISF SAFI for that domain.

      This is particularly useful when a tenant network is attached to
      multiple ISF SAFIs (BGP IP, IPVPN and EVPN domains) and any-to-any
      connectivity is required, and also end-to-end control plane
      consistency, when possible, is desired.

      It would be instantiated by attaching the disparate, regular BGP
      IP, IPVPN and EVPN domains via these PEs to a central composite
      domain.

4.  Domain Path Attribute (D-PATH)

   The BGP Domain Path (D-PATH) attribute is an optional and transitive
   BGP path attribute.

   Similar to AS_PATH, D-PATH is composed of a sequence of Domain
   segments.  Each Domain segment is comprised of <domain segment
   length, domain segment value>, where the domain segment value is a
   sequence of one or more Domains, as illustrated in Figure 6.  Each
   domain is represented by <DOMAIN-ID:ISF_SAFI_TYPE>.

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   Octets
   0               1                    8                         n
   +---------------+----------------//--+----//-------------------+
   |Domain Segment |   Last Domain      |        Domain of Origin |
   |    Length     |                    |                         |
   +---------------+----------------//--+----//-------------------+
                    \__________________/
                                |
              Octets            v
              0                         6                7
              +------------------//-----+----------------+
              |    DOMAIN-ID            | ISF_SAFI_TYPE  |
              +------------------//-----+----------------+
              \________________________/
                           |
           Octets          v
           0     1     2     3     4     5     6
           +-----------------------+-----------+
           |        Global         |  Local    |
           |        Admin          |  Admin    |
           +-----------------------+-----------+

                      Figure 6: D-PATH Domain Segment

   o  The domain segment length field is a 1-octet field, containing the
      number of domains in the segment.

   o  DOMAIN-ID is a 6-octet field that represents a domain.  It is
      composed of a 4-octet Global Administrator sub-field and a 2-octet
      Local Administrator sub-field.  The Global Administrator sub-field
      MAY be filled with an Autonomous System Number (ASN), an IPv4
      address, or any value that guarantees the uniqueness of the
      DOMAIN-ID when the tenant network is connected to multiple
      Operators.

   o  ISF_SAFI_TYPE is a 1-octet field that indicates the Inter-Subnet
      Forwarding SAFI type in which a route was received, before the
      route is re-exported into a different domain.  The following types
      are valid in this document:

                     Value Type
                     ----- --------------------------
                     0     Gateway PE local ISF route
                     1     SAFI 1
                     70    EVPN
                     128   SAFI 128

   About the BGP D-PATH attribute:

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   a.  Identifies the sequence of domains, each identified by a <DOMAIN-
       ID:ISF_SAFI_TYPE> through which a given ISF route has passed.

       -  This attribute list MAY contain one or more segments.

       -  The first entry in the list (leftmost) is the <DOMAIN-
          ID:ISF_SAFI_TYPE> from which a gateway PE is propagating an
          ISF route.  The last entry in the list (rightmost) is the
          <DOMAIN-ID:ISF_SAFI_TYPE> from which a gateway PE received an
          ISF route without a D-PATH attribute (the Domain of Origin).
          Intermediate entries in the list are domains that the ISF
          route has transited.

       -  As an example, an ISF route received with a D-PATH attribute
          containing a domain segment of {length=2,
          <6500:2:IPVPN>,<6500:1:EVPN>} indicates that the ISF route was
          originated in EVPN domain 6500:1, and propagated into IPVPN
          domain 6500:2.

   b.  It is added/modified by a gateway PE when propagating an update
       to a different domain:

       -  A gateway PE's IP-VRF, that connects two domains, belongs to
          two DOMAIN-IDs, e.g. 6500:1 for EVPN and 6500:2 for IPVPN.

       -  Whenever a prefix arrives at a gateway PE in a particular ISF
          SAFI route, if the gateway PE needs to export that prefix to a
          BGP peer, the gateway PE MUST prepend a <DOMAIN-
          ID:ISF_SAFI_TYPE> to the list of domains in the received
          D-PATH, as long as the gateway PE works in Uniform-
          Propagation-Mode, as explain in Section 5.2 .

       -  For instance, in an IP-VRF configured with DOMAIN-IDs 6500:1
          for EVPN and 6500:2 for IPVPN, if an EVPN route for prefix P
          is received and P installed in the IP-VRF, the IPVPN route for
          P that is exported to an IPVPN peer will prepend the domain
          <6500:1:EVPN> to the previously received D-PATH attribute.
          Likewise, IP-VRF prefixes that are received from IP-VPN, will
          be exported to EVPN peers with the domain <6500:2:IPVPN> added
          to the segment.

       -  In the above example, if the EVPN route is received without
          D-PATH, the gateway PE will add the D-PATH attribute with one
          segment {length=1, <6500:1:EVPN>} when re-advertising to
          domain 6500:2.

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       -  Within the Domain of Origin, the update does not contain a
          D-PATH attribute because the update has not passed through a
          gateway PE yet.

   c.  For a local ISF route, i.e., a configured route or a route
       learned from a local attachment circuit, a gateway PE has three
       choices:

       1.  It MAY advertise that ISF route without a D-PATH attribute
           into one or more of its configured domains, in which case the
           D-PATH attribute will be added by the other gateway PEs in
           each of those domains.

       2.  It MAY advertise that ISF route with a D-PATH attribute into
           one or more of its configured domains, in which case the
           D-PATH attribute in each copy of the ISF route is initialized
           with an ISF_SAFI_TYPE of 0 and the DOMAIN-ID of the domain
           with which the ISF route is associated.

       3.  It MAY advertise that ISF route with a D-PATH attribute that
           contains a configured domain specific to its local ISF routes
           into one or more of its configured domains, in which case the
           D-PATH attribute in each copy of the ISF route is initialized
           with a ISF_SAFI_TYPE of 0 and the DOMAIN-ID for the local ISF
           routes.  This DOMAIN-ID MUST be globally unique and MAY be
           shared by two or more gateway PEs.

   d.  An ISF route received by a gateway PE with a D-PATH attribute
       that contains one or more of its locally associated domains for
       the IP-VRF is considered to be a looped ISF route and MUST NOT be
       installed in that IP-VRF.  The ISF route in this case MUST be
       flagged as "looped".

       For instance, in the example of Figure 2, gateway GW1 receives
       TS1 prefix in two different ISF routes:

       -  In an EVPN RT-5 with next-hop PE1 and no D-PATH attribute.

       -  In a SAFI 128 route with next-hop GW2 and D-PATH = {length=1,
          <6500:1:EVPN>}, assuming that DOMAIN-ID for domain 1 is
          6500:1.

       Gateway GW1 flags the SAFI 128 route as "looped" and it will not
       install it in the tenant IP-VRF, since the route includes one of
       the GW1's local domains.

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   e.  A DOMAIN-ID value on a GW-PE (gateway PE) MAY be globally
       assigned for a peering domain or MAY be scoped for an individual
       tenant IP-VRF.

       -  If globally allocated for a peering domain, the DOMAIN-ID
          applies to all tenant IP-VRFs for that domain.

       -  If allocated for a specific tenant IP-VRF, the processing of
          the received D-PATH and its propagation will be in the context
          of the IP-VRF DOMAIN-ID.  Route leaking is a use-case where a
          per-IP-VRF DOMAIN-ID assignment is necessary.  Suppose
          gateways PE1 and PE2 are attached to two different tenant IP-
          VRFs, IP-VRF-1 and IP-VRF-2.  ISF SAFI routes advertised by
          gateway PE1 for IP-VRF-1 are received on gateway PE2 with
          DOMAIN-ID 6500:1.  If the routes are leaked from IP-VRF-1 into
          IP-VRF-2 on PE2, and re-advertised back to PE1 in the context
          of IP-VRF-2, PE1 will not treat the route as a looped route.
          This is because PE1 processes the route in the context of IP-
          VRF-2, where DOMAIN-ID 6500:1 is not a local DOMAIN-ID.

   f.  The number of domains in the D-PATH attribute indicates the
       number of gateway PEs that the ISF route update has transited.
       If one of the transit gateway PEs leaks a given ISF route between
       two local IP-VRFs, it MAY prepend a domain with a ISF_SAFI_TYPE
       of 0 for the leaked route when the route is exported into an ISF
       SAFI.  In that case, the number of domains in the D-PATH
       attribute indicates the number of tenant IP-VRFs that the ISF
       route update has transited.

   g.  The following error-handling rules apply to the D-PATH attribute:

       1.  A received D-PATH attribute is considered malformed if it
           contains a malformed Domain Segment.

       2.  A Domain Segment is considered malformed in any of the
           following cases:

           *  The Domain Segment length of the last Domain Segment
              causes the D-PATH attribute length to be exceeded.

           *  After the last successfully parsed Domain Segment there
              are less than eight octets remaining.

           *  The Domain Segment has a Domain Segment Length of zero.

       3.  A PE receiving an UPDATE message with a malformed D-PATH
           attribute SHALL apply "treat-as-withdraw" [RFC7606].

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       4.  Domains in the D-PATH attribute with unknown ISF_SAFI_TYPE
           values are accepted and not considered an error.

5.  BGP Path Attribute Propagation across ISF SAFIs

   Based on its configuration, a gateway PE is required to propagate an
   ISF route with different ISF SAFIs between two domains.  This
   requires a definition of what a gateway PE has to do with Path
   attributes attached to the ISF route that it is propagating.

5.1.  No-Propagation-Mode

   This is the default mode of operation for gateway PEs that re-export
   ISF routes from any ISF SAFI into EVPN, and from EVPN into any other
   SAFI.  In this mode, the gateway PE will simply re-initialize the
   Path Attributes when propagating an ISF route, as though it would for
   direct or local IP prefixes.  This model may be enough in those use-
   cases where the EVPN domain is considered an "abstracted" CE and
   remote IPVPN/IP PEs don't need to consider the original EVPN
   Attributes for path calculations.

   Since this mode of operation does not propagate the D-PATH attribute
   either, redundant gateway PEs are exposed to routing loops.  Those
   loops may be resolved by policies and the use of other attributes,
   such as the Route Origin extended community [RFC4360], however not
   all the loop situations may be solved.

5.2.  Uniform-Propagation-Mode

   In this mode, the gateway PE simply keeps accumulating or mapping
   certain key commonly used Path Attributes when propagating an ISF
   route.  This mode is typically used in networks where EVPN and IPVPN
   SAFIs are used seamlessly to distribute IP prefixes.

   The following rules MUST be observed by the gateway PE when
   propagating Path Attributes:

   1.  The gateway PE imports an ISF route in the IP-VRF and stores the
       original Path Attributes.  The following set of Path Attributes
       SHOULD be propagated by the gateway PE to other ISF SAFIs (other
       Path Attributes SHOULD NOT be propagated):

       -  AS_PATH

       -  D-PATH

       -  IBGP-only Path Attributes: LOCAL_PREF, ORIGINATOR_ID,
          CLUSTER_ID

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       -  MED

       -  AIGP

       -  Communities, Extended Communities and Large Communities,
          except for the EVPN extended communities, Route Target
          extended communities and BGP Encapsulation extended
          communities.

   2.  When propagating an ISF route to a different ISF SAFI and IBGP
       peer, the gateway PE SHOULD keep the AS_PATH of the originating
       family and add it to the destination family without any
       modification.  When re-advertising to a different ISF SAFI and
       EBGP peer, the gateway PE SHOULD keep the AS_PATH of the
       originating family and prepend the IP-VRF's AS before sending the
       route.

   3.  When propagating an ISF route to IBGP peers, the gateway PE
       SHOULD keep the IBGP-only Path Attributes from the originating
       SAFI to the re-advertised route.

   4.  As discussed, Communities, Extended Communities and Large
       Communities SHOULD be kept by the gateway PE from the originating
       SAFI route.  Exceptions of Extended Communities that SHOULD NOT
       be kept are:

       A.  BGP Encapsulation extended communities
           [I-D.ietf-idr-tunnel-encaps].

       B.  Route Target extended communities.  Route Targets are always
           initialized when readvertising an ISF route into a different
           domain, i.e., they are not propagated.  The initialized Route
           Target in the re-advertised ISF route may or may not have the
           same value as the Route Target of the originating ISF route.

       C.  All the extended communities of type EVPN.

       The gateway PE SHOULD NOT copy the above extended communities
       from the originating ISF route to the re-advertised ISF route.

   5.  For a given ISF route, only the Path Attributes of the best path
       can be propagated to another ISF route.  If multiple paths are
       received for the same route in an ISF SAFI, the BGP best path
       selection will determine what the best path is, and therefore the
       set of Path Attributes to be propagated.  Even if Equal Cost
       Multi-Path (ECMP) is enabled on the IP-VRF by policy, only the
       Path Attributes of the selected best path are propagated.

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5.3.  Aggregation of Routes and Path Attribute Propagation

   Instead of propagating a high number of (host) ISF routes between ISF
   SAFIs, a gateway PE that receives multiple ISF routes of one ISF SAFI
   MAY choose to propagate a single ISF aggregate route into a different
   domain.  In this document, aggregation is used to combine the
   characteristics of multiple ISF routes of the same ISF SAFI in such
   way that a single aggregate ISF route of a different ISF SAFI can be
   propagated.  Aggregation of multiple ISF routes of one ISF SAFI into
   an aggregate ISF route is only done by a gateway PE.

   Aggregation on gateway PEs may use either the No-Propagation-Mode or
   the Uniform-Propagation-Mode explained in Section 5.1 and
   Section 5.2, respectively.

   When using Uniform-Propagation-Mode, Path Attributes of the same type
   code MAY be aggregated according to the following rules:

   o  AS_PATH is aggregated based on the rules in [RFC4271].  The
      gateway PEs SHOULD NOT receive AS_PATH attributes with path
      segments of type AS_SET [RFC6472].  Routes received with AS_PATH
      attributes including AS_SET path segments MUST NOT be aggregated.

   o  ISF routes that have different attributes of the following type
      codes MUST NOT be aggregated: D-PATH, LOCAL_PREF, ORIGINATOR_ID,
      CLUSTER_ID, MED or AIGP.

   o  The Community, Extended Community and Large Community attributes
      of the aggregate ISF route MUST contain all the Communities/
      Extended Communities/Large Communities from all of the aggregated
      ISF routes, with the exceptions of the extended communities listed
      in Section 5.2 that are not propagated.

   Assuming the aggregation can be performed (the above rules are
   applied), the operator should consider aggregation to deal with
   scaled tenant networks where a significant number of host routes
   exists.  For example, large Data Centers.

6.  Route Selection Process between EVPN and other ISF SAFIs

   A PE may receive an IP prefix in ISF routes with different ISF SAFIs,
   from the same or different BGP peer.  It may also receive the same IP
   prefix (host route) in an EVPN RT-2 and RT-5.  A route selection
   algorithm across all ISF SAFIs is needed so that:

   o  Different gateway and composite PEs have a consistent and
      deterministic view on how to reach a given prefix.

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   o  Prefixes advertised in EVPN and other ISF SAFIs can be compared
      based on path attributes commonly used by operators across
      networks.

   o  Equal Cost Multi-Path (ECMP) is allowed across EVPN and other ISF
      SAFI routes.

   For a given prefix advertised in one or more non-EVPN ISF routes, the
   BGP best path selection procedure will produce a set of "non-EVPN
   best paths".  For a given prefix advertised in one or more EVPN ISF
   routes, the BGP best path selection procedure will produce a set of
   "EVPN best paths".  To support EVPN/non-EVPN ISF interworking in the
   context of the same IP-VRF receiving non-EVPN and EVPN ISF routes for
   the same prefix, it is then necessary to run a tie-breaking selection
   algorithm on the union of these two sets.  This tie-breaking
   algorithm begins by considering all EVPN and other ISF SAFI routes,
   equally preferable routes to the same destination, and then selects
   routes to be removed from consideration.  The process terminates as
   soon as only one route remains in consideration.

   The route selection algorithm must remove from consideration the
   routes following the rules and the order defined in [RFC4271], with
   the following exceptions and in the following order:

   1.  Immediately after removing from consideration all routes that are
       not tied for having the highest Local Preference, any routes that
       do not have the shortest D-PATH are also removed from
       consideration.  Routes with no D-PATH are considered to have a
       zero-length D-PATH.

   2.  Then regular [RFC4271] selection criteria is followed.

   3.  At the end of the selection algorithm, if at least one route
       still under consideration is an RT-2 route, remove from
       consideration any RT-5 routes.

   4.  If Steps 1-3 leave Equal Cost Multi-Paths (ECMP) between non-EVPN
       and EVPN paths, the EVPN path MUST be considered (and the non-
       EVPN path removed from consideration).  However, if ECMP across
       ISF SAFIs is enabled by policy, and one EVPN path and one non-
       EVPN path remain at the end of step 3, both path types MUST be
       used. .

   The above process modifies the [RFC4271] selection criteria to
   include the shortest D-PATH so that operators minimize the number of
   Gateways and domains through which packets need to be routed.

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   Example 1 - PE1 receives the following routes for IP1/32, that are
   candidate to be imported in IP-VRF-1:

      {SAFI=EVPN, RT-2, Local-Pref=100, AS-Path=(100,200)}
      {SAFI=EVPN, RT-5, Local-Pref=100, AS-Path=(100,200)}
      {SAFI=128, Local-Pref=100, AS-Path=(100,200)}

      Selected route: {SAFI=EVPN, RT-2, Local-Pref=100, AS-Path=100,200]
      (due to step 3, and no ECMP)

   Example 2 - PE1 receives the following routes for IP2/24, that are
   candidate to be imported in IP-VRF-1:

      {SAFI=EVPN, RT-5, D-PATH=(6500:3:IPVPN), AS-Path=(100,200),
      MED=10}
      {SAFI=128, D-PATH=(6500:1:EVPN,6500:2:IPVPN), AS-Path=(200),
      MED=200}

      Selected route: {SAFI=EVPN, RT-5, D-PATH=(6500:3:IPVPN), AS-
      Path=(100,200), MED=10} (due to step 1)

7.  Composite PE Procedures

   As described in Section 3, composite PEs are typically used in tenant
   networks where EVPN and IPVPN are both used to provide inter-subnet
   forwarding within the same composite domain.

   Figure 7 depicts an example of a composite domain, where PE1/PE2/PE4
   are composite PEs (they support EVPN and IPVPN ISF SAFIs on their
   peering to the Route Reflector), and PE3 is a regular IPVPN PE.

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                +-----------------------------------+
                |                                   |
                |        MPLS                 IPVPN PE3
                |        Network              +----------+ IP3/24
                |                     IPVPN   |+------+  |   +---+
                |                      +----->||IP-VRF|------|CE3|
           Composite PE1               |      |+------+  |   +---+
          +---------------+            |      +----------+
          |      +------+ |  EVPN      v             |
          |      |IP-VRF| |  IPVPN   +--+            |
          | +----|      | | <------> |RR|            |
   +---+  | |    +------+ |          +--+         Composite PE4
   |CE2|----|MAC-VRF|     |          ^  ^         +---------+ IP4/24
   +---+  | +-------+     |    EVPN  |  | EVPN    |+------+ |   +---+
          +---|-----------+    IPVPN |  | IPVPN   ||IP-VRF|-----|CE4|
              |  |              +----+  +-------->|+------+ |   +---+
       IP1/24 |  |              v                 +---------+
       +---+  |  |    +---------------+              |
       |CE1|--+  +----|      +------+ +--------------+
       +---+          |      |IP-VRF| |
         |            | +----|      | |
         |            | |    +------+ |
         +--------------|MAC-VRF|     |
                      | +-------+     |
                      +---------------+
                         Composite PE2

                      Figure 7: Composite PE example

   In a composite domain with composite and regular PEs:

   1.  The composite PEs MUST advertise the same IP prefixes in each ISF
       SAFI to the RR.  For example, in Figure 7, the prefix IP1/24 is
       advertised by PE1 and PE2 to the RR in two separate NLRIs, one
       for AFI/SAFI 1/128 and another one for EVPN.

   2.  As an informative note, the RR does not forward EVPN routes to
       neighbors on which the EVPN SAFI is not enabled.  For example,
       the RR does not forward EVPN routes to PE3 (since the RR does not
       have the EVPN SAFI enabled on its BGP session to PE3), whereas
       the IPVPN routes are forwarded to all the PEs.

   3.  IPVPN PEs process and import IPVPN routes.  As an example, PE3
       receives only the IPVPN route for IP1/24 and resolves the BGP
       next-hop to an MPLS tunnel (with IP payload) to PE1 and/or PE2.

   4.  Composite PEs process routes for the same prefix coming from
       different ISF SAFI routes, and perform route selection.

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       -  As an example, PE4 receives IP1/24 encoded in EVPN and another
          ISF SAFI route (EVPN RT-5 and IPVPN).  The route selection
          follows the procedures in Section 6.

       -  Assuming an EVPN route is selected, PE4 resolves the BGP next-
          hop to an MPLS tunnel (with Ethernet or IP payload) to PE1
          and/or PE2.  As described in Section 3, two EVPN PEs may use
          tunnels with Ethernet or IP payloads to connect their IP-VRFs,
          depending on the [I-D.ietf-bess-evpn-prefix-advertisement]
          model implemented.

       -  The other composite PEs (PE1 and PE2) receive also the same IP
          prefix via EVPN and IPVPN SAFIs and they also follow the route
          selection in Section 6.

   5.  When a given route has been selected as the route for a
       particular packet, the transmission of the packet is done
       according to the rules for that route's AFI/SAFI.

   6.  As an informative note, in composite domains, such as the one in
       Figure 7, the EVPN advanced forwarding features will only be
       available to composite and EVPN PEs (assuming they select an RT-5
       to forward packets for a given IP prefix), and not to IPVPN PEs.
       For example, assuming PE1 sends IP1/24 in an EVPN and an IPVPN
       route and the EVPN route is the best one in the selection, the
       recursive resolution of the EVPN RT-5s can only be used in PE2
       and PE4 (composite PEs), and not in PE3 (IPVPN PE).  As a
       consequence of this, the indirection provided by the RT5's
       recursive resolution and its benefits in a scaled network, will
       not be available in all the PEs in the network.

8.  Gateway PE Procedures

   Section 3 defines a gateway PE as an Interworking PE that advertises
   IP prefixes to different BGP peers, using EVPN to one BGP peer and
   another ISF SAFI to another BGP peer.  Examples of gateway PEs are
   Data Center gateways connecting domains that make use of EVPN and
   other ISF SAFIs for a given tenant.  Figure 8 illustrates this use-
   case, in which PE1 and PE2 (and PE3/PE4) are gateway PEs
   interconnecting domains for the same tenant.

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     <----EVPN---->    <----------IPVPN--------->   <----EVPN---->
       6500:1:EVPN             6500:2:IPVPN           6500:3:EVPN
  <DOMAIN-ID:ISF_SAFI_TYPE>
                        +-----------------------+
                 Gateway PE1              Gateway PE3
                 +----------+             +----------+
     +-----------|+------+  |  MPLS tnls  |+------+  |-------------+
     |           ||IP-VRF|  |             ||IP-VRF|  |             |
   PE5           |+------+  |             |+------+  |           PE6
  +------+       +----------+             +----------+          +------+
  |IP-VRF| NVO tnls |   |                       |  |  NVO tnls  |IP-VRF|
  |      |          |   |                       |  |            |      |
  +------+       +----------+             +----------+          +------+
  IP1/24-->      |+------+  |             |+------+  |             |
     |           ||IP-VRF|  |             ||IP-VRF|  |             |
     +-----------|+------+  |             |+------+  |-------------+
                 +----------+             +----------+
                 Gateway PE2   +------+   Gateway PE4
                       +-------|IP-VRF|---------+
                               |      |
                               +------+
                                 PE7

                       Figure 8: Gateway PE example

   The procedures for a gateway PE enabled for ISF SAFI-x and ISF SAFI-y
   on the same IP-VRF follow:

   1.  A gateway PE that imports an ISF SAFI-x route to prefix P in an
       IP-VRF, MUST export P in ISF SAFI-y if:

       A.  P is installed in the IP-VRF - which means the SAFI-x route
           is well-formed, valid and the best one for P - and

       B.  PE has a BGP peer for SAFI-y (enabled for the same IP-VRF)
           and

       C.  The advertisement is allowed by policy and

       D.  ISF SAFI-x and ISAF SAFI-y are any of the types defined in
           Section 3.

       In the example of Figure 8, gateway PE1 and PE2 receive an EVPN
       RT-5 with IP1/24, install the prefix in the IP-VRF and re-
       advertise it using SAFI 128.

   2.  A gateway PE that receives an ISF SAFI-x route to prefix P in an
       IP-VRF MUST NOT export P in ISF SAFI-y if:

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       A.  The ISF SAFI-x route is not well-formed or valid.  Rules to
           determine if a route is well-formed or valid for a given ISF
           SAFI are out of the scope of this document, and are defined
           by the specification of each ISF SAFI.

       B.  The ISF SAFI-x route contains a D-PATH attribute with one or
           more of the gateway PE's locally associated domains for the
           IP-VRF.  In this case the route is considered to be a looped
           ISF route and is not even installed in the IP-VRF, as
           described in Section 4.

   Once the gateway PE determines that P must be exported, P will be
   advertised using ISF SAFI-y as follows:

   a.  The D-PATH attribute MUST be included, so that loops can be
       detected in remote gateway PEs.  When a gateway PE propagates an
       IP prefix between EVPN and another ISF SAFI, it MUST prepend a
       <DOMAIN-ID:ISF_SAFI_TYPE> to the received D-PATH attribute.  The
       DOMAIN-ID and ISF_SAFI_TYPE fields refer to the domain over which
       the gateway PE received the IP prefix and the ISF SAFI of the
       route, respectively.  If the received IP prefix route did not
       include any D-PATH attribute, the gateway IP MUST add the D-PATH
       when readvertising.  The D-PATH in this case will have only one
       segment on the list, the <DOMAIN-ID:ISF_SAFI_TYPE> of the
       received route.

       In the example of Figure 8, gateway PE1/PE2 receive the EVPN RT-5
       with no D-PATH attribute since the route is originated at PE5.
       Therefore PE1 and PE2 will add the D-PATH attribute including
       <DOMAIN-ID:ISF_SAFI_TYPE> = <6500:1:EVPN>.  Gateways PE3/PE4 will
       propagate the route again, now prepending their <DOMAIN-
       ID:ISF_SAFI_TYPE> = <6500:2:IPVPN>.  PE6 receives the EVPN RT-5
       routes with D-PATH = {<6500:2:IPVPN>,<6500:1:EVPN>} and can use
       that information to make BGP path decisions.

   b.  The gateway PE MAY use the Route Distinguisher of the IP-VRF to
       readvertise P in the ISF SAFI-y.

   c.  The label allocation used by each gateway PE is a local
       implementation matter.  The IP-VRF advertising IP prefixes for
       EVPN and another ISF SAFI may use a label per-VRF, per-prefix,
       etc.

   d.  The gateway PE MUST be able to use the same or different set of
       Route Targets per ISF SAFI on the same IP-VRF.  In particular, if
       different domains use different set of Route Targets for the same
       tenant, the gateway PE MUST be able to import and export routes
       with the different sets.

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   e.  Even though Figure 8 only shows two domains per gateway PE, the
       gateway PEs may be connected to more than two domains.

   f.  There is no limitation of gateway PEs that a given IP prefix P
       can pass through until it reaches a given PE.

   g.  As an informative note, if P was originated in an EVPN domain but
       traversed a different ISF SAFI domain (or domains), it will lose
       EVPN-specific attributes that are used in advanced EVPN
       procedures.  For example, even if PE1 advertises IP1/24 along
       with a given non-zero ESI (for recursive resolution to that ESI),
       when PE6 receives the IP prefix in an EVPN route, the ESI value
       will be zero.  This is because the route traverses an ISF SAFI
       domain that is different than EVPN.

9.  Interworking Use-Cases

   While Interworking PE networks may well be similar to the examples
   described in Section 7 and Section 8, in some cases a combination of
   both functions may be required.  Figure 9 illustrates an example
   where the gateway PEs are also composite PEs, since not only they
   need to re-advertise IP prefixes from EVPN routes to another ISF SAFI
   routes, but they also need to interwork with IPVPN-only PEs in a
   domain with a mix of composite and IPVPN-only PEs.

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                       +-----------------------------------+
                       |                                   |
                       |        MPLS                 IPVPN PE3
                       |        Network              +---------+
                       |                     IPVPN   |+------+ |
                       |                      +----->||IP-VRF|---TS3
                (GW+composite) PE1            |      |+------+ |
                 +---------------+            |      +---------+
                 |      +------+ |  EVPN      v            |
                 |      |IP+VRF| |  IPVPN   +-++           |
                 | +----|      | | <------> |RR|           |
        +--------| |    +------+ |          +--+        Composite PE4
        |        | |MAC+VRF|     |          ^  ^         +---------+
        |        | +-------+     |    EVPN  |  | EVPN    |+------+ |
     +----+      +---------------+    IPVPN |  | IPVPN   ||IP-VRF|---TS4
 TS1-|NVE1|             |              +----+  +-------->|+------+ |
     +----+             |              v                 +---------+
        |    EVPN DC    |    +---------------+             |
        |    NVO tnls   +----|      +------+ |-------------+
        |                    |      |IP+VRF| |
        |                    | +----|      | |
        |                    | |    +------+ |
        |     +----+         | |MAC+VRF|     |
        +-----|NVE2|---------| +-------+     |
              +----+         +---------------+
                |           (GW+composite) PE2
               TS2

       Figure 9: Gateway and composite combined functions - example

   In the example above, PE1 and PE2 MUST follow the procedures
   described in Section 7 and Section 8.  Compared to Section 8, PE1 and
   PE2 now need to also propagate prefixes from EVPN to EVPN, in
   addition to propagating prefixes from EVPN to IPVPN.

   It is worth noting that PE1 and PE2 will receive TS4's IP prefix via
   IPVPN and RT-5 routes.  When readvertising to NVE1 and NVE2, PE1 and
   PE2 will consider the D-PATH rules and attributes of the selected
   route for TS4 (Section 6 describes the Route Selection Process).

10.  Conclusion

   This document describes the procedures required in PEs that use EVPN
   and another Inter-Subnet Forwarding SAFI to import and export IP
   prefixes for a given tenant.  In particular, this document defines:

   o  A route selection algorithm so that a PE can determine what path
      to choose between EVPN paths and other ISF SAFI paths.

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   o  A new BGP Path attribute called D-PATH that provides loop
      protection and visibility on the domains a particular route has
      traversed.

   o  The way Path attributes should be propagated between EVPN and
      another ISF SAFI.

   o  The procedures that must be followed on Interworking PEs that
      behave as composite PEs, gateway PEs or a combination of both.

   The above procedures provide an operator with the required tools to
   build large tenant networks that may span multiple domains, use
   different ISF SAFIs to handle IP prefixes, in a deterministic way and
   with routing loop protection.

11.  Security Considerations

   In general, the security considerations described in
   [I-D.ietf-bess-evpn-prefix-advertisement] and [RFC4364] apply to this
   document.

   Section 4 introduces the use of the D-PATH attribute, which provides
   a security tool against control plane loops that may be introduced by
   the use of gateway PEs that export ISF routes between domains.  A
   correct use of the D-PATH will prevent control plane and data plane
   loops in the network, however an incorrect configuration of the
   DOMAIN-IDs on the gateway PEs may lead to the detection of false
   route loops and the blackholing of the traffic.  An attacker may
   benefit of this transitive attribute to propagate the wrong domain
   information across multiple domains.

   In addition, Section 5.2 introduces the propagation of attributes
   between ISF SAFIs on gateway PEs.  Without this mode of propagation,
   Path Attributes are re-initialized when re-exporting ISF routes into
   a different ISF SAFI, however this mode introduces the capability of
   propagating Path Attributes beyond the ISF SAFI scope.  While this is
   a useful tool to provide end to end visibility across multiple
   domains, it can also be used by an attacker to propagate wrong
   (although correctly formed) Path Attributes that can influence the
   BGP path selection in remote domains.  An implementation can also
   choose Section 5.1 (No-propagation mode) to minimize the risks
   derived from propagating incorrect attributes, however this mode of
   operation will prevent the receiver PE from seeing the attributes
   that the originator of the route intended to convey in the first
   place.

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12.  IANA Considerations

   This document defines a new BGP path attribute known as the BGP
   Domain Path (D-PATH) attribute.

   IANA has assigned a new attribute code type from the "BGP Path
   Attributes" subregistry under the "Border Gateway Protocol (BGP)
   Parameters" registry:

   Path Attribute Value    Code                       Reference
   --------------------    ------------------------   ---------------
   36                      BGP Domain Path (D-PATH)   [This document]

13.  Acknowledgments

   The authors want to thank Russell Kelly, Dhananjaya Rao, Suresh
   Basavarajappa, Mallika Gautam, Senthil Sathappan, Arul Mohan Jovel,
   Naveen Tubugere, Mathanraj Petchimuthu, Amit Kumar and Mohit Kumar
   for their review and suggestions.

14.  Contributors

15.  References

15.1.  Normative References

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

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   [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>.

   [I-D.ietf-bess-evpn-prefix-advertisement]
              Rabadan, J., Henderickx, W., Drake, J. E., Lin, W., and A.
              Sajassi, "IP Prefix Advertisement in EVPN", draft-ietf-
              bess-evpn-prefix-advertisement-11 (work in progress), May
              2018.

   [I-D.ietf-bess-evpn-inter-subnet-forwarding]
              Sajassi, A., Salam, S., Thoria, S., Drake, J. E., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", draft-
              ietf-bess-evpn-inter-subnet-forwarding-15 (work in
              progress), July 2021.

15.2.  Informative References

   [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>.

   [I-D.ietf-idr-tunnel-encaps]
              Patel, K., Velde, G. V. D., Sangli, S. R., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", draft-ietf-idr-
              tunnel-encaps-22 (work in progress), January 2021.

   [RFC6472]  Kumari, W. and K. Sriram, "Recommendation for Not Using
              AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
              DOI 10.17487/RFC6472, December 2011,
              <https://www.rfc-editor.org/info/rfc6472>.

Authors' Addresses

   J. Rabadan (editor)
   Nokia
   777 Middlefield Road
   Mountain View, CA  94043
   USA

   Email: jorge.rabadan@nokia.com

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   A. Sajassi (editor)
   Cisco
   225 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: sajassi@cisco.com

   E. Rosen
   Individual

   Email: erosen52@gmail.com

   J. Drake
   Juniper

   Email: jdrake@juniper.net

   W. Lin
   Juniper

   Email: wlin@juniper.net

   J. Uttaro
   AT&T

   Email: ju1738@att.com

   A. Simpson
   Nokia

   Email: adam.1.simpson@nokia.com

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