ARP/ND Synching And IP Aliasing without IRB
draft-wang-bess-evpn-arp-nd-synch-without-irb-08

Document Type Active Internet-Draft (individual)
Authors Yubao Wang  , Zheng Zhang 
Last updated 2021-09-01
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BESS WG                                                          Y. Wang
Internet-Draft                                                  Z. Zhang
Intended status: Standards Track                         ZTE Corporation
Expires: 5 March 2022                                   1 September 2021

              ARP/ND Synching And IP Aliasing without IRB
            draft-wang-bess-evpn-arp-nd-synch-without-irb-08

Abstract

   This draft discusses serveral signalling modes of EVPN Signalled
   L3VPNs.  EVPN Signalled L3VPNs are used to improve L3VPNs for some
   new use cases.  Then it discusses which style of RT-5 routes can be
   selected for these new use cases, and why they are selected.

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
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   This Internet-Draft will expire on 5 March 2022.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology and Acronyms  . . . . . . . . . . . . . . . .   3
   2.  Service Interfaces of L3 EVIs . . . . . . . . . . . . . . . .   6
   3.  IP Discovery Mode . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Adjacencies Discovery . . . . . . . . . . . . . . . . . .   6
       3.1.1.  Spreadable Mode (Faraway Mode)  . . . . . . . . . . .   6
       3.1.2.  Non-Spreadable Mode (Nearby Mode) . . . . . . . . . .   7
     3.2.  CE-Prefixes Auto-Discovery Modes  . . . . . . . . . . . .   7
       3.2.1.  Distributed A-D Mode  . . . . . . . . . . . . . . . .   7
       3.2.2.  Centerlized A-D Mode  . . . . . . . . . . . . . . . .   8
   4.  Styles of RT-5 Route  . . . . . . . . . . . . . . . . . . . .   8
     4.1.  L-Style: no Overlay Index . . . . . . . . . . . . . . . .   8
       4.1.1.  RT-5L Advertisement in Distributed A-D Mode . . . . .   8
       4.1.2.  RT-5L Advertisement in Centerlized A-D Mode . . . . .   9
     4.2.  G-Style: GW-IP as Overlay Index . . . . . . . . . . . . .   9
       4.2.1.  RT-5G Advertisement in Distributed A-D Mode . . . . .   9
       4.2.2.  RT-5G Advertisement in Centerlized A-D Mode . . . . .   9
     4.3.  E-Style: ESI as Overlay Index . . . . . . . . . . . . . .  10
       4.3.1.  RT-5E in Bump-in-the-wire use case  . . . . . . . . .  10
       4.3.2.  RT-5E Advertisement on Distributed L3 GW  . . . . . .  11
       4.3.3.  RT-5E Advertisement in Centerlized A-D mode . . . . .  11
     4.4.  M-Style: MAC as Overlay Index . . . . . . . . . . . . . .  12
   5.  Centerlized RT-5G Advertisement for Distributed L3
           Forwarding  . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  CE-side Configurations  . . . . . . . . . . . . . . . . .  13
     5.2.  Why Centerlized A-D mode is used  . . . . . . . . . . . .  13
     5.3.  Basic Control Plane Procedures  . . . . . . . . . . . . .  14
       5.3.1.  Centerlized CE-BGP  . . . . . . . . . . . . . . . . .  14
       5.3.2.  RT-2E Advertisement from PE1/PE2 to DGW1  . . . . . .  14
       5.3.3.  RT-5G Advertisement from DGW1 to PE1/PE2  . . . . . .  14
       5.3.4.  RT-2E Advertisement between PE1 and PE2 . . . . . . .  15
     5.4.  Mass-Withdraw by EAD/ES Route . . . . . . . . . . . . . .  15
     5.5.  If Mutiple VLAN-based Service Inerface is Used  . . . . .  16
     5.6.  If VLAN-bundle Service Interface is Used  . . . . . . . .  17
     5.7.  On the Failure of PE3 Node  . . . . . . . . . . . . . . .  17
     5.8.  For Common CE-prefixes behind R1 and R2 . . . . . . . . .  18
   6.  Load Balancing of Unicast Packets . . . . . . . . . . . . . .  20
     6.1.  IP Aliasing using GW-IP . . . . . . . . . . . . . . . . .  20
     6.2.  IP Aliasing using ESI . . . . . . . . . . . . . . . . . .  20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Explanation for Physical Links of the Use-cases  . .  22
     A.1.  Failure Detections for P1.2 (or P2.1) . . . . . . . . . .  24

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     A.2.  Protection Approaches for N1 (or N2)  . . . . . . . . . .  24
       A.2.1.  CCC-Approaches  . . . . . . . . . . . . . . . . . . .  24
         A.2.1.1.  CCC Active-Active Protection  . . . . . . . . . .  24
         A.2.1.2.  CCC Active-Standby Protection . . . . . . . . . .  24
       A.2.2.  VSI-Approaches  . . . . . . . . . . . . . . . . . . .  25
   Appendix B.  Different Understandings on Resolve GW-IP to RT-5  .  25
     B.1.  Section 3.2 of I-D.ietf-bess-evpn-prefix-advertisement  .  25
     B.2.  How to Interpret Above Paragraphs . . . . . . . . . . . .  26
     B.3.  Special PEs . . . . . . . . . . . . . . . . . . . . . . .  26
     B.4.  GW-IP or a new TLV  . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   This draft discusses serveral signalling modes of EVPN Signalled
   L3VPNs. they are:

   *  Adjacencies Discovery modes: spreadable mode and non-spreadable
      mode.

   *  CE-Prefix Auto-discovery modes: Centerlized mode and Distributed
      mode.

   *  Styles of RT-5 routes: RT-5L (L-Sytle), RT-5G (G-Style), RT-5E
      (E-Style), RT-5M (M-Style).

   These signalling modes can help to improve L3VPNs for some new use
   cases.  Then we will discuss which style of RT-5 routes will be
   selected for each new use case, and why it is selected for that new
   use case.

1.1.  Terminology and Acronyms

   Most of the acronyms and terms used in this documents comes from
   [RFC7432], [I-D.sajassi-bess-evpn-ip-aliasing] and
   [I-D.wang-bess-evpn-ether-tag-id-usage] except for the following:

   * VRF AC -  An Attachment Circuit (AC) that attaches a CE to an
         IP-VRF but is not an IRB interface.

   * VRF Interface -  An IRB interface or a VRF-AC or an IRC
         interface.  Note that a VRF interface will be bound to the
         routing space of an IP-VRF.

   * L3 EVI -  An EVPN instance spanning the Provider Edge (PE)
         devices participating in that EVPN which contains VRF ACs and
         maybe contains IRB interfaces or IRC interfaces.

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   * IP-AD/EVI -  Ethernet Auto-Discovery route per EVI, and the EVI
         here is an IP-VRF.  Note that the Ethernet Tag ID of an IP-AD/
         EVI route may be not zero.

   * IP-AD/ES -  Ethernet Auto-Discovery route per ES, and the EVI
         for one of its route targets is an IP-VRF.

   * RMAC -  Router's MAC, which is signaled in the Router's MAC
         extended community.

   * ESI Overlay Index -  ESI as overlay index.

   * ET-ID -  Ethernet Tag ID, it is also called ETI for short in
         this document.

   * RT-2R -  When a MAC/IP Advertisement Route whose ESI is not
         zero is used for IP-VRF forwarding, it is called as a RT-2R in
         this draft.  When it is used for MAC-VRF forwarding, it is not
         called as a RT-2R in this draft.

   * RT-5E -  An EVPN Prefix Advertisement Route with a non-reserved
         ESI as its overlay index (the E-style RT-5) .

   * IRC -  Integrated Routing and Cross-connecting, thus a IRC
         interface is the virtual interface connecting an IP-VRF and an
         EVPN VPWS.

   * CE-BGP -  The BGP session between PE and CE.  Note that CE-BGP
         route doesn't have a RD or Route-Target.

   * CE-Prefix -  An IP Prefixes behind a CE is called as that CE's
         CE-Prefix.

   * RT-5G -  An EVPN Prefix Advertisement Route with a zero ESI and
         a non-zero GW-IP (the G-style RT-5).

   * RT-5L -  An EVPN Prefix Advertisement Route with both zero ESI
         and zero GW-IP, but a non-zero EVPN Label (the L-style RT-5).

   * RT-5M -  An EVPN Prefix Advertisement Route with zero ESI, zero
         GW-IP and zero EVPN Label, but a non-zero Router's MAC (the
         M-style RT-5).

   * EVC -  Ethernet Virtual Connection, which is typically
         constructed per <Port, VLAN> basis.

   * Internal Remote PE:  When PEx is called as an EVPN route ERy's

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     internal remote PE, that is saying that, PEx is on the ES which is
     identified by ERy's ESI field.  When ERy's SOI is not zero, that is
     aslo saying that PEx has been attached to the ethernet tag which is
     identified by the <ESI, SOI>.

   * External Remote PE:  When PEx is called as an EVPN route ERy's
     external remote PE, that is saying that, PEx is not on the ES which
     is identified by ERy's ESI field.  When ERy's SOI is not zero, PEx
     may aslo be a PE which has not been attached to the ethernet tag
     which is identified by the <ESI, SOI>.

   * CE-Prefix:  When an IP prefix can be reached through CEx from PEy,
     that IP prefix is called as PEy's CE-prefix behind CEx in this
     draft.  PEy's CE-prefix behind CEx is also called as PEy's CE-
     prefix for short in this draft.

   * Common CE-Prefix:  When an CE-Prefix can be reached through either
     CEy or CEz from PEy, in this draft, it is called as a common CE-
     Prefix of CEy and CEz,from the viewpoint of PEy.

   * Exclusive CE-Prefix:  When an CE-Prefix of PEy can be reached
     through CEy, and it can't be reached through other CEs of PEy, it
     is called as an exlusive CE-Prefix of CEy, from the viewpoint of
     PEy.

   * SNGW:  Sub-Net-specific Gate Way IP address, the SNGW of a subnet
     is an IP address which is used by the hosts of that subnet to be
     the nexthop of the default route of these host.

   * Intermediate subnet:  The subnet that connects a PE and a CE of a
     L3 EVI.

   * Intermediate SNGW :  The SNGW of a intermediate subnet.  It will be
     the IP address of a IRC interface in this draft.

   * Intermediate nexthop :  The CE's IP address in the intermediate
     subnet.

   * Overlay nexthop :  The CE-Prefix's nexthop IP address which is in
     the address-space of the L3 EVI.

   * Original Overlay nexthop :  The overlay nexthop which is advertised
     by the CE through a PE-CE route protocol.

   * L3EVI-Specific EADR -  When the <ESI, L3EVI> uses L3EVI-
         Specific Ethernet Auto-discovery mode, the only Ethernet A-D
         per EVI route (which will be <ESI, ET-ID=0>) of that <ESI,
         L3EVI> is called as a L3EVI-Specific EADR in this draft.

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   * ACI-Specific EADR -  When the <ESI, SOI> uses ACI-Specific
         Ethernet Auto-discovery mode, the Ethernet A-D per EVI routes
         of that <ESI, SOI> are called as ACI-Specific EADRs in this
         draft.

2.  Service Interfaces of L3 EVIs

   Service interface describes how an ES is attached to a L3EVI.
   [I-D.wang-bess-evpn-ether-tag-id-usage] discussed the following
   service interfaces:

   *  Mono VLAN-based Service Interface.
   *  Multiple VLAN-based Service Interface.
   *  Separated Risk VLAN-bundle Service Interface.
   *  Shared Risk VLAN-bundle Service Interface.
   *  IRC Service Interface.

   Different service interface will require different control-plane
   procedures, then this draft discusses the behavior of RT5 routes
   advertisement per each service interface, especially when they are
   RT5 routes with ESI as overlay index or GW-IP as overlay index.

   Note that an ES may be attached to different L3EVIs via different
   VLANs, and mutiple ESes can be attached to the same L3EVI Instance.
   So service interface is ESI-specific and EVI-specific.  When ES1 is
   of VLAN-bundle Service Interface to EVI1, it may be of Mono VLAN-
   based Service Interface for EVI2.  Thus service interfaces of L3EVIs
   are <ESI, EVI>-specific in this draft.

3.  IP Discovery Mode

   IP discovery in L3EVIs include ARP/ND Adjacencies discovery and CE-
   prefixes discovery.  The adjacencies discovery is done distributively
   by each VRF-AC using ARP/ND.  But the CE-prefixes can be discovered
   by two ways.

3.1.  Adjacencies Discovery

3.1.1.  Spreadable Mode (Faraway Mode)

   In Spreadable Mode, an adjacent MAC/IP can be imported into IP-VRF by
   both external remote PEs and internal remote PEs.

   In this mode, the RT-2 route of the MAC/IP is used to synchronize
   adjacency information (<MAC, IP> mapping) to internal remote PEs.
   and it is also used to advertise host route to external remote PEs.

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   When CEs are hosts, this mode will make the amount of EVPN routes
   increase greatly.

   The spreadable mode is also called as Faraway Mode because that the
   external remote PEs of the MAC/IP entries can imported the RT-2
   routes of these MAC/IP entries into IP-VRF.

   The spreadable mode can be used to avoid making a detour when there
   is a straightforward path.  This mode is typically used in EVPN IRB
   scenarios, where different hosts of the same BD may be reached
   through different ESes.  Another example of spreadable mode can be
   found in Section 2.1.2 of [I-D.wz-bess-evpn-vpws-as-vrf-ac], where
   the CEs are a few routers.

3.1.2.  Non-Spreadable Mode (Nearby Mode)

   In Spreadable Mode, an adjacent MAC/IP can only be imported into IP-
   VRF by internal remote PEs.  In other words, the MAC/IP can not be
   imported into IP-VRF by the external remote PEs of it.

   In this mode, the RT-2 route of the MAC/IP is just used to
   synchronize adjacency information (<MAC, IP> mapping) to internal
   remote PEs.

   In non-spreadable mode, it should be insured that only the internal
   remote PEs of the MAC/IP entry can imported the RT-2 route of the
   MAC/IP entry into IP-VRF.  Thus that RT-2 route should carry EVI-RT
   and ES-Import RT only, and that's why non-spreadable mode is also
   called as nearby mode.

   An example of non-spreadable mode can be found in Section 2.1.1 of
   [I-D.wz-bess-evpn-vpws-as-vrf-ac], where the CEs are lots of hosts,
   and all CEs can be reached through the same VPWS service instance.

3.2.  CE-Prefixes Auto-Discovery Modes

   There are two ways to discover the IP prefixes behind a CE (that's
   why these prefixes are called CE-prefixes for short), they are
   distributed AD-Mode and centerlized AD-mode.

3.2.1.  Distributed A-D Mode

   The CE-Prefixes inside a DC are discovered by each NVE separately.
   Then these NVEs advertise their CE-prefixes to DC Gateways and other
   NVEs of that DC.

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   Note that the external-prefixes (which are received from other DCs)
   will be discovered by DC gateways even in distributed AD-mode.
   Distributed A-D mode and Centerlized A-D mode just talks about how
   CE-prefixes inside the DC will be discovered.

3.2.2.  Centerlized A-D Mode

   The CE-Prefixes (behind each NVE) are discovered by the same group of
   DC Gateways.  Then these DC Gateways advertise these CE-prefixes to
   NVEs.

   No matter what the A-D mode is, the distributed forwarding behavior
   should be expected in this draft.  That is, the communication between
   two subnets behind two NVEs inside the same DC should not be required
   to pass through the DC Gateway.

4.  Styles of RT-5 Route

   When a RT-5 route is used to forward a data packet, the label/VNI/SID
   of that data packet's EVPN header may be obtained relying on four
   different fields of that RT-5.

   In other words, we can say that RT-5 routes can be classified into
   four styles, which are called L-style, G-style, E-style, M-style
   respectively.

   These styles have different usages and they are suitable for
   different secenarios.

4.1.  L-Style: no Overlay Index

   When a L-style RT-5 is used to forward a data packet, the label/VNI/
   SID of that data packet's EVPN header is obtained from the RT-5's own
   MPLS Label (that's why it is called L-Style) field (of its NLRI), and
   the forwarding path is determined by its own underlay next-hop (BGP
   next hop).

   A L-style RT-5 route is also called as a RT-5L in this draft.

   Note that the ESI and GW IP fields are both zero at the same time,
   otherwise it will be considered to be another style.

4.1.1.  RT-5L Advertisement in Distributed A-D Mode

   When N1/N2 establish CE-BGP sessions with both PE1 and PE2, it is
   enough for PE1/PE2 to advertise RT-5L routes to DGW1.  There is no
   need for RT-5G or RT-5E advertisement on PE1/PE2 in that usecase.

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   Note that when N1/N2 establish CE-BGP sessions with both PE1 and PE2,
   the downlink VRF-interface addresses on PE1 and PE2 may be different
   IP addresses of the same subnet.  Otherwise we may use loopback
   interfaces to establish the CE-BGP sessions.

4.1.2.  RT-5L Advertisement in Centerlized A-D Mode

   When a PE advertises RT-5Ls just for its own direct subnets, it can
   be used in both distributed A-D mode and centerlized A-D mode.  When
   a PE advertises RT-5Ls for CE-prefixes, it can not be used in
   centerlized A-D mode, otherwise the data forwarding will be
   centerlized too.  When a PE (which is a DC Gateway) advertises RT-5Ls
   for external-prefixes (which are received from other DCs or non-EVPN
   neighbors), it can be used in either centerlized A-D mode or
   distributed A-D mode.

4.2.  G-Style: GW-IP as Overlay Index

   When a G-style RT-5 is used to forward a data packet, the label/VNI/
   SID of that data packet's EVPN header is obtained using another EVPN
   route whose IP field (of its NLRI) matches this RT-5 route's own GW-
   IP (that's why it is called G-Style) field (of its NLRI), and the
   forwarding path is determined by that EVPN route.

   A G-style RT-5 route is also called as a RT-5G in this draft.

   RT-5G can be used wether the CE-prefex AD-mode is centerlized mode or
   distributed mode.  and RT-5G can be used wether the Service Interface
   is Mono VLAN-based mode or Mutiple VLAN-based mode.  It can be a
   uniform approach to advertise CE-prefixes no matter what the EVPN
   mode is.

   Note that the ESI field of a RT-5G route MUST be zero as per
   [I-D.ietf-bess-evpn-prefix-advertisement].

4.2.1.  RT-5G Advertisement in Distributed A-D Mode

   It follows [I-D.wz-bess-evpn-vpws-as-vrf-ac] section 2.3.2 and
   section 6.2.  Note that these procedures can be used in every L3EVI
   Service Interface, not just in IRC Service Interface.

4.2.2.  RT-5G Advertisement in Centerlized A-D Mode

   When a PE (which may be a DC gateway) learns that CE-prefix prefix1's
   overlay next hop is IP1, then the PE advertise a RT-5G for prefix1,
   the GW-IP of that RT-5G is set to IP1.

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   An example of RT-5G advertisement in centeralized A-D mode can be
   found in Section 5;

4.3.  E-Style: ESI as Overlay Index

   When a E-style RT-5 is used to forward a data packet, the label/VNI/
   SID of that data packet's EVPN header is obtained from another RT-1
   route whose ESI and Ethernet Tag ID matches this RT-5 route's ESI
   (that's why it is called L-Style) and Supplementary Overlay Index
   (Section 3.3 of [I-D.wang-bess-evpn-ether-tag-id-usage] and
   Section 6.3.3 of [I-D.wz-bess-evpn-vpws-as-vrf-ac]), and the
   forwarding path is determined by that RT-1 route.

   A E-style RT-5 route is also called as a RT-5E in this draft.

   RT-5E can only be used when the CE-prefex AD-mode is distributed
   mode.  RT-5E can be used in Mono VLAN-based Service Interface.  But
   when RT-5E is used in Multiple VLAN-based Service interface or
   Separated Risk VLAN-bundle service interface, the ACI-specific
   ethernet auto-discovery per [I-D.wang-bess-evpn-ether-tag-id-usage]
   should be followed.

   Note that the GW-IP field of a RT-5E route MUST be zero as per
   [I-D.ietf-bess-evpn-prefix-advertisement].

4.3.1.  RT-5E in Bump-in-the-wire use case

   The RT-5 route that specifies an ESI as overlay index is first
   defined in Section 4.3 of [I-D.ietf-bess-evpn-prefix-advertisement],
   where the Bump-in-the-wire use case (the former RT-5E usage) is also
   defined there.

   Then it is discussed in Section 2.4 and Section 3.6.4 of
   [I-D.wang-bess-evpn-ether-tag-id-usage].  The RT-5E routes (the
   latter RT-5E usage) of Section 6 of revision-02
   [I-D.wang-bess-evpn-arp-nd-synch-without-irb-02] and Section 1.3 of
   [I-D.sajassi-bess-evpn-ip-aliasing] are different from these RT-5E
   routes of Bump-in-the-wire use case in the following factors:

   *  Source MAC - The ethernet header can not be absent in the former
      usage even if the data plane is MPLS.  The source MAC MUST be set
      to the MAC address of the IRB interface of BD-10 in Bump-in-the-
      wire usecase.  But in the latter usage the ethernet header can be
      absent if the data plane is MPLS.

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   *  Recursive Resolution - The recursive resolution of the former
      usage are done in the context of a BD, But the recursive
      resolution of the latter usage are done in the context of a IP-
      VRF.

   *  EVPN label - The EVPN label of the corresponding RT-1 per EVI
      route of the former usage is a MPLS label which identifies a BD,
      But the EVPN label of the corresponding RT-1 per EVI route of the
      latter usage is a MPLS label which identifies an IP-VRF.

   *  ESI - The ESI of the former usage is attached to a BD, But ESIs of
      the latter usage are attached to IP-VRFs.

   The Bump-in-the-wire use case is a special form of EVPN IRB use case,
   that's why it is different from the non-IRB use cases.

4.3.2.  RT-5E Advertisement on Distributed L3 GW

   Given that PE1/PE2 (see Figure 1) can install a synced ARP entry to
   its proper VRF-interface benefitting from the RT-2 route of
   Section 3.1.  So it is not necessary for PE1/PE2 to advertise per-
   host IP prefixes to remote PEs (e.g.  PE3) by RT-2 routes.  It is
   recommended that PE1/PE2 advertise an RT-5E route per subnet to PE3
   instead.  The ESI of these RT-5E routes can be set to the ESI of the
   corresponding VRF interface.  If the VRF interface fails, these
   subnets will achieve more faster convergency on PE3 by the withdraw
   of the corresponding IP-AD/EVI route.

   Note that N1/N2 may be a host or a router, when it is a router, those
   subnets (which are advertised by RT-5E routes) will be the CE-
   prefixes behind it.  When N1 and N2 are hosts, those subnets will be
   the intermediate subnets (the subnet of N1/N2's own IP address).

   When RT-5E routes are used to advertise direct-subnets, the details
   can be found in Section 4.3.3.  When RT-5E routes are used to
   advertise CE-prefixes, there are two approaches, the details can be
   found in Section 1.3 of [I-D.sajassi-bess-evpn-ip-aliasing] and
   Section 6.3 of [I-D.wz-bess-evpn-vpws-as-vrf-ac].

4.3.3.  RT-5E Advertisement in Centerlized A-D mode

   When the CE-prefixes are discovered by centerlized auto-discovery
   approaches, the RT-5E can be used to advertise the direct-subnets of
   NVE1/NVE2, but these RT-5E routes are not used to advertise the CE-
   Prefixes.

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   When the direct-subnets are advertised by RT-5E routes, when the
   main-interface of the corresponding ESI fails, mass-withdraw
   procedures can be triggered for these prefiexes.  This is the
   advantage of advertising direct-subnets through RT-5E routes instead
   of RT-5L routes.

   Note that the example of the mass-withdraw use-case of RT-5E routes
   can be found in Section 5.4.  and it can be used in Dstributed A-D
   mode too.

4.4.  M-Style: MAC as Overlay Index

   When a M-style RT-5 is used to forward a data packet, the label/VNI/
   SID of that data packet's EVPN header is obtained using another RT-2
   route whose MAC field (of its NLRI) matches this RT-5 route's own
   RMAC, and the forwarding path is determined by that RT-2 route.

   A M-style RT-5 route is also called as a RT-5M in this draft.

   RT-5M is used in Interfaceful IP-VRF-to-IP-VRF mode and Bump-in-the-
   wire use case as per [I-D.ietf-bess-evpn-prefix-advertisement].

5.  Centerlized RT-5G Advertisement for Distributed L3 Forwarding

   When N1/N2/N3 is a router, it is called R1/R2/R3 in the following
   figure.  Note that Figure 6 only illustrates the physical ethernet
   links, but Figure 1 illustrates the logical L3 adjacencies between PE
   and CE as the following.  We assume that ESI21 are attched to L3EVI
   VPNx of Section 1.1.2 of [I-D.wang-bess-evpn-ether-tag-id-usage].

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                      PE1
                     +----------+
                     | +------+ | ------>
    R1               | |      | | RT-1
   +-------+         | | VPNx | | ESI21
   |       |  P1.1   | |      | | ETI1
   | ...................(10.9)| |
   | .     |  ESI21  | +------+ |                          DGW1
   | .     |    +    +----------+                    +-------------+
   | .     |    |       ^                <---------- |             |
   | .     |    |       | RT-2            RT-5G      | +---------+ |
   |(10.2) |    |       | 10.2            CE-Prefix1 | |  VPNx   | |
   | .     |    |       | ESI21           GW-IP=10.2 | |         |....R3
   | .     |    |       |                            | |(3.3.3.3)| |
   | .     |    +    +----------+ ------>            | +---------+ |
   | .     |  ESI21  | +------+ | RT-2R              |   ^         |
   | ...................(10.9)| | 10.2               |   |         |
   |       |  P2.1   | |      | | ESI21              +---|---------+
   +-------+         | | VPNx | |                        |
       |             | |      | | ------>                | CE-BGP
       |             | +------+ | RT-1                   | Prefix1
       |             +----------+ ESI21                  | NH=10.2
       |              PE2         ETI1                   |
       |                   CE-BGP                        |
       +--------------------->---------------------------+

                 Figure 1: Centerlized RT-5G Advertisement

   If R1 prefers to establish a single CE-BGP session, it can establish
   the CE-BGP session with DC GW (e.g.  PE3 of Section 1.1.2 of
   [I-D.wang-bess-evpn-ether-tag-id-usage]) instead.  This CE-BGP
   session can be called the centerlized CE-BGP session.  But when we
   use centerlized CE-BGP session, we should use RT-5G route instead.

   Note that we just use centerlized CE-BGP session to discover CE-
   prefixes, but we still expect a distributed Layer 3 forwarding
   framework.

5.1.  CE-side Configurations

   Let us assume that CCC Active-Active Protection are used inside
   PNEC1, that's to say, when R1 send packets to 10.9, these packets
   will be load-balanced between PE1 and PE2.

5.2.  Why Centerlized A-D mode is used

   Because of the factors discussed in Section 5.1, perhaps the CE-BGP
   session can be established between 10.2 and 10.9.

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   There may be other reasons that prevent the routing protocols to be
   established between 10.2 and 10.9.

5.3.  Basic Control Plane Procedures

5.3.1.  Centerlized CE-BGP

   The CE-BGP session between R1 and DGW1 (when PE3 is a DC GW, it is
   called DGW1) is established between 10.2 and 3.3.3.3.  The IP address
   10.2 is called the uplink interface address of R1 in this document.
   The IP address 3.3.3.3 is called the centerlized loopback address of
   VPNx in this document.  The IP address 10.9 is called the downlink
   VRF-interface address of PE1/PE2 in this document.

   R1 advertises a BGP route for a prefix (say "Prefix1") behind it to
   DGW1 via that CE-BGP session.  The nexthop for Prefix1 is R1's uplink
   interface address (say 10.2).

   Note that the data packets from R1 to the centerlized loopback
   address may be routed following the default route on R1.  Thus DGW1
   don't need to use the CE-BGP session to advertise prefixes of VPNx to
   R1.

5.3.2.  RT-2E Advertisement from PE1/PE2 to DGW1

   When PE1 learns the ARP entry of 10.2, it advertises a RT-2R route to
   DGW1.  The ESI value of the RT-2R route is ESI21, which is the ESI of
   PE1's downlink VRF-interface for R1.  The RT-2R route is constructed
   following Section 3.1.  This is a mono VLAN-based service interface,
   thus the ETI1 (Ethernet Tag ID 1) of that RT-2R route can be 0.

   Note that in [RFC7432], when the ESI is single-active, the MAC
   forwarding only use the label and the BGP nexthop of the RT-2R route
   as long as they are valid for forwarding status.  But in RT-5E routes
   we assume that the ESI is always preferred even if the ESI is single-
   active.  This is follows [I-D.ietf-bess-evpn-prefix-advertisement]
   section 3.2 Table 1.

5.3.3.  RT-5G Advertisement from DGW1 to PE1/PE2

   When DGW1 receives the prefix1 from the CE-BGP session.  The nexthop
   for Prefix1 is 10.2.  So DGW1 advertises a RT-5G route to PE1/PE2 for
   Prefix1.  The GW-IP value of the RT-5G route for Prefix1 is 10.2.

   Note that DGW1 can load-balance packets for Prefix1 via the IP-AD/EVI
   routes (of ESI21) from PE1/PE2.  Because ESI21 (which is advertised
   along with RT-2R of 10.2) is the ESI for Prefix1's GW-IP.

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   Note that the centerlized loopback address is advertised to PE1/PE2
   by DGW1 via RT-5L route.  The nexthop of the RT-5L route is DGW1.
   The label of the RT-5L route is VPNx's label on DGW1.  The RMAC of
   the RT-5L route is DGW1's MAC when the encapsulation is VXLAN.

5.3.4.  RT-2E Advertisement between PE1 and PE2

   The RT-2R routes advertisement between PE1 and PE2 is used to sync
   their ARP entries to each other in order to avoid ARP missing.  The
   ESI Value of these two RT-2R routes is ESI21.

5.4.  Mass-Withdraw by EAD/ES Route

   In the figure of Section 1.1.2 of
   [I-D.wang-bess-evpn-ether-tag-id-usage], there are two L3EVIs, VPNx
   and VPNy.  We just take VPNx for example in Section 5.3, now we
   consider these two L3EVIs together.

                                      +-----------------------+
   PNEC1                      PE1     |                       |
  +-------------+          +----------+--------+              |
  |             |          |  __(20.9)__(VPNy) | Withdraw     |
  | Prefix1  "  |   P1     | /                 | IP-AD/ES     |
  |  /       #===========X==<                  | ----X--->    | DGW1
  | R1_______"  |  ESI21   | \__      __       |         +----+----+
  |    10.2  "  |    +     |    (10.9)  (VPNx) |         |         |
  |          "  |    |     +-----------+-------+         |(3.3.3.3)|
  |          "  |    |                 |                 |    |    |
  | Prefix2  "  |    |                 |                 |  (VPNx)---+N3
  |  /       "  |    |        PE2      |                 |         |
  | R2_______"  |    |     +-----------+-------+         |  (VPNy)---+N5
  |    20.2  "  |    +     |  __(20.9)__(VPNy) |         |         |
  |          "  |  ESI21   | /                 |         +----+----+
  |          #==============<                  |              |
  |          "  |   P2     | \__      __       |              |
  |             |          |    (10.9)  (VPNx) |              |
  +-------------+          +----------+--------+              |
                                      |                       |
                                      +-----------------------+

                  Figure 2: Mono VLAN-based S-I Use Case

   When the physical interface of the downlink VRF-interface (P1) on PE1
   fails (illustrated by the 'X' on P1), PE1 will withdraw the IP-AD/ES
   route of ESI21, so DGW1 will re-route 10.2 for VPNx's CE-prefiex1.
   and re-route 20.2 for VPNy's CE-prefix2 at the same time.  Then data
   packets for CE-Prefix1 and CE-Prefix2 will be sent to PE2 instead.

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5.5.  If Mutiple VLAN-based Service Inerface is Used

   Now we assume that ESI21 are attached to L3EVI VPN1 according to
   Section 1.1.3 of [I-D.wang-bess-evpn-ether-tag-id-usage].  And we
   assume that CCC Active-Active Protection are used inside PNEC1.

                                   +--------------------------+
   PNEC1                      PE1  |                          |
  +-------------+          +-------+------+                   | DGW1'
  |             |          | X__(20.9)    | ----X---->   +----+----+
  |          "  |   P1     | /        \   | Withdraw     |         |
  |          #==============<      (VPN1) | IP-AD/EVI    |  (VPN1)---+N6
  | R1_______"  |  ESI21   | \__      /   | ET-ID=2      |         |
  |    10.2  "  |    +     |    (10.9)    |              +----+----+
  |          "  |    |     +--------+-----+                   |
  |          "  |    |              |                         |
  |          "  |    |              |                         | DGW1
  |          "  |    |        PE2   |                    +----+----+
  | R2_______"  |    |     +--------+-----+              |         |
  |    20.2  "  |    +     |  __(20.9)    |              |(3.3.3.3)|
  |          "  |  ESI21   | /        \   | Withdraw     |    |    |
  |          #==============<      (VPN1) | IP-AD/EVI    |  (VPN1)---+N3
  |          "  |   P2     | \__      /   | ET-ID=1      |         |
  |             |          | X  (10.9)    | ----X---->   +----+----+
  +-------------+          +-------+------+                   |
                                   |                          |
                                   +--------------------------+

                Figure 3: Mutiple VLAN-based S-I Use Case

   When physical port P3 (see Figure 6, which illustrates the physical
   links of Figure 3) fails, the CFM session of P2.1 (10.9 of PE2) goes
   down (illustrated by the 'X' inside PE2), while the CFM session of
   P2.2 (20.9 of PE2) continues to be UP.  thus only the IP-AD/EVI route
   (whose ET-ID=1) of P2.1 should be withdrawn by PE2.  the IP-AD/EVI
   route (where ET-ID=2) of P2.2 and the IP-AD/ES route should not be
   withdrawn by PE2.

   Note that if the ET-IDs of these two IP-AD/EVI routes are the same,
   when P2.1 fails, DGW1 will continue to load-balance traffics whose
   DA=20.2 to PE2, because that there is still another IP-AD/EVI route
   (of VPN1) whose ESI and ET-ID are the same.  That's why ACI-specifice
   Ethernet auto-discovery mode [I-D.wang-bess-evpn-ether-tag-id-usage]
   should be followed in this case.

   Note that we assume that the ARP entry for 10.2 will be learnt on PE1
   only, and 20.2 will be learnt on PE2 only.  Note that the two
   downlink VRF-interfaces P2.1 (to R1) and P2.2 (to R2) on PE2 are sub-

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   interfaces of the same physical interface P2.  So they have the same
   ESI.  ESI21 are attached to L3EVI VPN1 using multiple VLAN-based
   service interface, thus the mass-withdraw procedures of Section 5.4
   can be used in this case too.

5.6.  If VLAN-bundle Service Interface is Used

   If R1 and R2 can share the same gateway IP address, P2.1 and P2.2 can
   be aggregated into the same subinterface (where the shared gateway IP
   is configured to).  Although they are aggregated, this can't change
   the fact that they don't share the same risks.  When that physical
   interface P3 (see Figure 6) fails, one of them will fail, while the
   other will continue to work well.

   Thus different (in ET-ID) IP-AD/EVI routes for P2.1 and P2.2 should
   be advertised separately.  That's why
   [I-D.wang-bess-evpn-ether-tag-id-usage] should be followed in this
   case.

5.7.  On the Failure of PE3 Node

   Take the Figure 3 for example, on the failure of DGW1, PE1/PE2 should
   delay the deletion of the RT-5G route from DGW1.  DGW1 can use a new
   BGP attribute to indicate the delayed-deletion requirement to PE1/
   PE2.  Otherwise the L3 traffic between R1 and R2 will be interrupted.
   Fortunately, DGW1 will typically have a redundant node (DGW1' in
   Figure 3), and DGW1' can be used to take DGW1's place when DGW1
   fails.

   Note that from the viewpoint of R1 and R2, the total of PE1, PE2,
   DGW1, DGW1' and the underlay network between them is regarded as the
   following VNF:

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   +---------------------------------+
   |                                 |
   |    +----------------------+     |
   |    |  MPU1 (DGW1)         |     |
   |    +----------------------+     |
   |                                 |
   |    +----------------------+     |
   |    |  MPU2 (DGW1')        |     |
   |    +----------------------+     |
   |                                 |
   |    +----------------------+     |
   |    |  LPU1 (PE1)          |----------------R1
   |    +----------------------+     |
   |                                 |
   |    +----------------------+     |
   |    |  LPU2 (PE2)          |----------------R2
   |    +----------------------+     |
   |                                 |
   +---------------------------------+

                      Figure 4: EVPN Instance as a VNF

   R1 and R2 connect to the LPUs of the VNF. and the data packets
   between R1 and R2 just pass through the LPUs, not through the MPUs.
   But R1/R2 establish the BGP session with the MPUs, not the LPUs.
   When the MPU1(or actually DGW1) fails, the LPUs(or actually PE1/PE2)
   will keep the forwarding state unchanged untill the MPU1 or MPU2
   comes up.  So the delayed deletion on PE1/PE2 for DGW1's sake is
   apprehensible for the same reason.

   Note that for the north-bound traffics, the DC GWs also plays a LPU
   role of this VNF.

5.8.  For Common CE-prefixes behind R1 and R2

   We can assume that there is a common prefix (say Prefix3) behind both
   R1 and R2, That's saying that DGW1 can reach Prefix3 through either
   R1 or R2.  When R1 advertise Prefix3 to DGW1 over that CE-BGP
   session, 10.2 may not be the best choice for Prefix3's BGP next hop.

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         EVPN Instance as a VNF
   +---------------------------------+
   |                                 |
   |    +----------------------+     |
   |    |  MPU1 (DGW1)         |<---------<-----+
   |    +----------------------+     |          |
   |                                 |          ^
   |    +----------------------+     |          | CE-BGP
   |    |  MPU2 (DGW1')        |     |          | Prefix3
   |    +----------------------+     |          | NH=7.7.7.7
   |                                 |          |
   |    +----------------------+     |     10.2 |
   |    |  LPU1 (PE1)          |---------------[R1(7.7.7.7)]---+
   |    +----------------------+     |                         |
   |                                 |                       Prefix3
   |    +----------------------+     |     20.2                |
   |    |  LPU2 (PE2)          |---------------[R2(7.7.7.7)]---+
   |    +----------------------+     |
   |                                 |
   +---------------------------------+

                Figure 5: IP Aliasing of Common CE-Prefixes

   In such case, we can configure a common anycast loopback address (say
   7.7.7.7) on R1 and R2.  Then, when R1 advertise Prefix3 to DGW1, R1
   choose 7.7.7.7 to be the BGP next-hop of the advertisement.  Thus the
   RT-5G of Prefix3 from DGW1 will be advertised along with GW-
   IP=7.7.7.7.

   In addition to the common prefixes behind R1 and R2, there will be
   exclusive prefixes particular to R1 or R2, and maybe R1/R2 can't
   distinguish the common prefixes from the exclusive prefixes, so R1/R2
   just advertise all prefixes behind it to PEs by CE-BGP using the
   common nexthop (e.g. 10.2).  then the PEs can not distinguish the
   common prefixes from the exclusive prefixes either.  Thus RT-5E
   routes can not be used even if distributed CE-prefix auto-discovery
   mode is used, because that PE1/PE2 can't advertise different ESIs for
   the common prefixes and the exclusive prefixes.

   The ECMP-Merging approaches of Section 6.2.1 and Section 6.2.2 of
   [I-D.wz-bess-evpn-vpws-as-vrf-ac] can also be used in such cases in
   order to simplify the required recursive resolution.

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   If one of the PEs can't resolve the GW-IP (e.g. 7.7.7.7) of a RT-5G
   route to another RT-5 route (e.g. the RT-5L route of 7.7.7.7), DGW1
   can proxy the recursive resolution for other PEs.  When 7.7.7.7 can
   be resolved to two RT-2 routes of 10.2 and 20.2, DGW1 can advertise
   the RT-5G route of the CE-prefix along with GW-IP=10.2 or GW-IP=20.2.
   Further, DGW1 may advertise two RT-5G routes for that CE-prefix, 10.2
   is the GW-IP of one of them, 20.2 is the GW-IP of the other.

6.  Load Balancing of Unicast Packets

6.1.  IP Aliasing using GW-IP

   When a RT-5G's GW-IP can be resolved to an ECMP-list of RT-5L (e.g.
   Section 6.2.1 of [I-D.wz-bess-evpn-vpws-as-vrf-ac]) routes, we can
   say that the IP aliasing is implemented using GW-IP.

   Note that when the encapsulation is VXLAN, in this case, PE3 will
   encapsulate the RMAC per each path of that ECMP-list.

6.2.  IP Aliasing using ESI

   When a RT-5G's GW-IP can only be resolved to a single RT-2R (e.g.
   Section 5.3.3, where the RT-5G is a local-discovered RT-5G) route,
   but the <ESI,SOI> of that RT-2R route can be resolved to an ECMP-list
   of RT-1 routes, we can say that the IP aliasing is implemented using
   ESI.

   It is similar to [I-D.sajassi-bess-evpn-ip-aliasing] except for a few
   notable exceptions as explained in the following.

   o How to encapsulate Destination MAC ?
     * The IP-AD/EVI routes don't have their own RMAC -  Note that when
       the encapsulation is VXLAN, PE3 will encapsulate the RMAC of the
       RT-2R route for corresponding GW-IP address.  And the RMAC of PE1
       MUST have the same value with the RMAC of PE2.  This can be
       achieved by configuration.

     * The IP-AD/EVI routes have their own RMAC -  Note that when the
       encapsulation is VXLAN, PE3 will encapsulate the RMAC of an IP-
       AD/EVI route in that ECMP-list.  When an IP packet is
       encapsulated with a VNI label according to an IP-AD/EVI route,
       the packet SHOULD be encapsulated with a Destination-MAC
       according to the RMAC of that IP-AD/EVI route, if and only if the
       IP-AD/EVI route have a RMAC of its own.

   o How to select the IP-AD/EVI routes?

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     When selecting corresponding IP-AD/EVI routes for a RT-5E route,
     the procedures discussed in Section 3.2 of
     [I-D.wang-bess-evpn-ether-tag-id-usage] should be followed.

7.  IANA Considerations

   no IANA Considerations.

8.  Security Considerations

   TBD.

9.  References

9.1.  Normative References

   [I-D.wang-bess-evpn-ether-tag-id-usage]
              Wang, Y., "Ethernet Tag ID Usage Update for Ethernet A-D
              per EVI Route", Work in Progress, Internet-Draft, draft-
              wang-bess-evpn-ether-tag-id-usage-03, 26 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-wang-bess-
              evpn-ether-tag-id-usage-03>.

   [I-D.sajassi-bess-evpn-ip-aliasing]
              Sajassi, A., Badoni, G., Warade, P., Pasupula, S., Drake,
              J., and J. Rabadan, "EVPN Support for L3 Fast Convergence
              and Aliasing/Backup Path", Work in Progress, Internet-
              Draft, draft-sajassi-bess-evpn-ip-aliasing-02, 8 June
              2021, <https://datatracker.ietf.org/doc/html/draft-
              sajassi-bess-evpn-ip-aliasing-02>.

   [I-D.ietf-bess-evpn-prefix-advertisement]
              Rabadan, J., Henderickx, W., Drake, J., Lin, W., and A.
              Sajassi, "IP Prefix Advertisement in EVPN", Work in
              Progress, Internet-Draft, draft-ietf-bess-evpn-prefix-
              advertisement-11, 18 May 2018,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bess-
              evpn-prefix-advertisement-11>.

   [I-D.ietf-bess-evpn-inter-subnet-forwarding]
              Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", Work
              in Progress, Internet-Draft, draft-ietf-bess-evpn-inter-
              subnet-forwarding-15, 26 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bess-
              evpn-inter-subnet-forwarding-15>.

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   [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>.

   [RFC8679]  Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
              Michel, C., and H. Chen, "MPLS Egress Protection
              Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
              <https://www.rfc-editor.org/info/rfc8679>.

   [I-D.wz-bess-evpn-vpws-as-vrf-ac]
              Wang, Y. and Z. Zhang, "EVPN VPWS as VRF Attachment
              Circuit", Work in Progress, Internet-Draft, draft-wz-bess-
              evpn-vpws-as-vrf-ac-02, 28 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-wz-bess-evpn-
              vpws-as-vrf-ac-02>.

   [I-D.sajassi-bess-evpn-ac-aware-bundling]
              Sajassi, A., Brissette, P., Mishra, M. P., Thoria, S.,
              Rabadan, J., and J. Drake, "AC-Aware Bundling Service
              Interface in EVPN", Work in Progress, Internet-Draft,
              draft-sajassi-bess-evpn-ac-aware-bundling-04, 11 July
              2021, <https://datatracker.ietf.org/doc/html/draft-
              sajassi-bess-evpn-ac-aware-bundling-04>.

9.2.  Informative References

   [I-D.ietf-idr-tunnel-encaps]
              Patel, K., Velde, G., Sangli, S., and J. Scudder, "The BGP
              Tunnel Encapsulation Attribute", Work in Progress,
              Internet-Draft, draft-ietf-idr-tunnel-encaps-22, 7 January
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              idr-tunnel-encaps-22>.

   [I-D.wang-bess-evpn-arp-nd-synch-without-irb-02]
              Wang, Y. and Z. Zhang, "ARP/ND Synching And IP Aliasing
              without IRB", Work in Progress, Internet-Draft, draft-
              wang-bess-evpn-arp-nd-synch-without-irb-02, 28 November
              2019, <https://datatracker.ietf.org/doc/html/draft-wang-
              bess-evpn-arp-nd-synch-without-irb-02>.

Appendix A.  Explanation for Physical Links of the Use-cases

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                                       +------------------+
                             PE1       | P6               |
               L2NE1        +----------+---------+        |
            +----------+    |  __(P1.1)__(VPNx)  |        |
   +---+ P4 |          | P1 | /            \     |        |
   |N1 |-----O==------=======<             (NIz) |     P6 |    PE3
   +---+    |   \    / |    | \__      __  /     |   +----+-------+
            |    |  |  |    |    (P1.2)  (VPNy)  |   |            |
            +----|P3|--+    +-----------+--------+   |      (VPNx)--+N3
                 |  |                   |            |      /     |
            P3.1 |  | P3.2              | P7         | (NIz)--------+N4
                 |  |        PE2        |            |      \     |
            +----|P3|--+    +-----------+--------+   |      (VPNy)--+N5
            |     \/   |    |  __(P2.2)__(VPNy)  |   |            |
   +---+    |     /\   |    | /            \     |   +----+-------+
   |N2 |-----O====--=========<            (NIz)  |     P8 |
   +---+ P5 |          | P2 | \__      __  /     |        |
            +----------+    |    (P2.1)  (VPNx)  |        |
               L2NE2        +----------+---------+        |
                                       | P8               |
                                       +------------------+

                    Figure 6: Physical Links Illustrated

   There are three PEs, two L2NEs (Layer 2 Network Elements) and five
   L3NEs (Layer 3 Network Elements) in abobe network.  The PEs are PE1,
   PE2 and PE3.  The L2NEs are L2NE1 and L2NE2.  The L3NEs are
   N1/N2/N3/N4/N5.  They are all illustrated in Figure 6.

   There are 9 physical links among these 10 physical devices as
   illustrated in Figure 6.  These physical links are called as PLi
   (i=1,2...8).  The two physical ports of the same physical link PLi
   are both called as Pi (i=1,2...8).

   As illustrated in Figure 6, some of these physical ports may have
   subinterfaces.  When a subinterface's VLAN ID is j and it is physical
   port Pi's subinterface, that subinterface is called as Pi.j.  For
   example, P1.2 is a subinterface of physical port P1 and its VLAN ID
   is 2.

   There are three NIs (Network Instances) among PE1, PE2 and PE3.  They
   are VPNx, VPNy and NIz.  Two subinterfaces are attached to VPNx, they
   are P1.1 and P2.1.  Other two subinterfaces are attached to VPNy,
   they are P1.2 and P2.2.  N3 is also attched to VPNx, while N5 is also
   attached to VPNy.

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   There are two EVCs (Ethernet Virtual Connections) between L2NE1 and
   L2NE2, they are EVC1 and EVC2.  The L2NE1's EVC1 instance (which is
   illustrated as the "O" on L2NE1) have three member interfaces, they
   are P4, P1.1 and P3.1, where P3.1 and P1.1 are of the same
   protection-group.  The L2NE2's EVC1 instance have two member
   interfaces, they are P3.1 and P2.1.  The L2NE2's EVC2 instance (which
   is illustrated as the "O" on L2NE2) have three member interfaces,
   they are P5, P2.2 and P3.2, where P3.1 and P1.1 are of the same
   protection-group.  The L2NE1's EVC2 instance have two member
   interfaces, they are P3.2 and P1.2.  The L2NE2's EVC1 instance and
   L2NE1's EVC2 instance are both CCC (Circuit Cross Connection) local
   connections.

   VPNx and VPNy are associated to NIz on each PE.

A.1.  Failure Detections for P1.2 (or P2.1)

   There is a CFM session CFM1 between P1.2 of PE1 and L2NE2's P3.2,
   when physical port P3 fails, the CFM session CFM1 will go down.
   There is a CFM session CFM2 between P2.1 of PE2 and L2NE1's P3.1,
   when physical port P3 fails, the CFM session CFM2 will go down.

A.2.  Protection Approaches for N1 (or N2)

A.2.1.  CCC-Approaches

   The L2NE1's EVC1 instance and L2NE2's EVC2 instance are both CCC
   local connections too.  In L2NE1's EVC1 instance, P1.1 and P3.1 are
   of the same protection-group PG1.  In L2NE2's EVC2 instance, P2.2 and
   P3.2 are of the same protection-group PG2.  In PG1, both P1.1 and
   P3.1 will receive data packets.  In PG2, both P2.2 and P3.2 will
   receive data packets.

A.2.1.1.  CCC Active-Active Protection

   L2NE1 (or L2NE2) will load-balance N1's (N2's) data packets between
   P1.1 and P3.1 (or P2.2 and P3.2).

A.2.1.2.  CCC Active-Standby Protection

   In PG1, P1.1 is the active path, P3.1 is the backup path.  In PG2,
   P2.2 is the active path, P3.2 is the backup path.

   That's saying that L2NE1 (or L2NE2) will not send N1's (or N2's) data
   packets over P3.1 (or P3.2), unless P1.1 (or P2.2) or P1 (or P2) has
   been in failure before that data forwarding.

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A.2.2.  VSI-Approaches

   L2NE1's EVC2 instance and L2NE2's EVC1 instance are both VSI
   instances in this case.  P1.1, P3.1, P2.2 and P3.2 are all individual
   ACs in these VSIs.

   Note that L2NE2's EVC1 instance and L2NE1's EVC2 instance are still
   both CCC local connections in this case, and there is no PG1 or PG2
   in this case, and there are no PWs in this case.

Appendix B.  Different Understandings on Resolve GW-IP to RT-5

B.1.  Section 3.2 of I-D.ietf-bess-evpn-prefix-advertisement

   The following bullets in Section 3.2 of
   [I-D.ietf-bess-evpn-prefix-advertisement]:

   "RT-5 routes support recursive lookup resolution through the use of
   Overlay Indexes as follows:

   o ... It is important to note that recursive
     resolution of the Overlay Index applies upon installation into an
     IP-VRF, and not upon BGP propagation (for instance, on an ASBR).

   ...

   o In order to enable the recursive lookup resolution at the ingress
     NVE, an NVE that is a possible egress NVE for a given Overlay Index
     must originate a route advertising itself as the BGP next hop on
     the path to the system denoted by the Overlay Index. For instance:

     . ...
     . If the RT-5 specifies an ESI as the Overlay Index, recursive
       resolution can only be done if the NVE has received and installed
       an RT-1 (Auto-Discovery per-EVI) route specifying that ESI.
     . If the RT-5 specifies a GW IP address as the Overlay Index,
       recursive resolution can only be done if the NVE has received and
       installed an RT-2 (MAC/IP route) specifying that IP address in
       the IP address field of its NLRI.
     . ...

     Note that the RT-1 or RT-2 routes needed for the recursive
     resolution may arrive before or after the given RT-5 route.

   o ..."

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B.2.  How to Interpret Above Paragraphs

   We should note that above section can be interpreted that it was
   written based on the following principles:

   *  The following paragraph (say Praragraph 1) sepecifies how the
      recursive lookup resolution will be done:

         "In order to enable the recursive lookup resolution at the
         ingress NVE, an NVE that is a possible egress NVE for a given
         Overlay Index must originate a route advertising itself as the
         BGP next hop on the path to the system denoted by the Overlay
         Index.  For instance:"

   *  The examples that is constrained by the phrase "For instance:"
      described some use-cases that followed above paragraph, with the
      understanding that new use-cases were possible in the future with
      new documents, as long as the rules of above Paragraph 1 were
      respected.

B.3.  Special PEs

   If there are devices that have interpreted above Paragraph 2 as the
   following:

      "if the recursive resolution can't find out a RT-2 for that RT-5's
      GW-IP, that RT-5 should not be installed."

   Such behavior of that PE might not be considered as according to
   Section 3.2 of [I-D.ietf-bess-evpn-prefix-advertisement].  It is just
   not included in [I-D.ietf-bess-evpn-prefix-advertisement].

B.4.  GW-IP or a new TLV

   No matter how to understand Section 3.2 of
   [I-D.ietf-bess-evpn-prefix-advertisement], now we can assume that the
   function of the GW-IP field is replaced with a new TLV (e.g. the IP-
   mapping SOI extended community, similar to what have been done in
   Section 6.3 of [I-D.wz-bess-evpn-vpws-as-vrf-ac]), then we can
   compare these two implementations and see whether a new TLV will
   bring us some benefits or not.

   Now assume that the Figure 5 of this draft is changed to distributed
   CE-prefixes auto-discovery mode (which is similar to Section 6.3 of
   [I-D.wz-bess-evpn-vpws-as-vrf-ac]).  The comparisons are illustrated
   as the following:

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   +=====+=================================+=======+=========+=========+
   | No. | Compared Points                 | GW-IP | New TLV | RT-5E   |
   +=====+=================================+=======+=========+=========+
   | 1   | Can non-upgraded                | yes   | yes     | yes     |
   |     | RRs accept it?                  |       |         |         |
   +-----+---------------------------------+-------+---------+---------+
   | 2   | Can non-upgraded                | maybe | no      | no      |
   |     | DGWs* install it?               |       |         |         |
   +-----+---------------------------------+-------+---------+---------+
   | 3   | Should PE1/PE2 be               | yes   | yes     | yes     |
   |     | upgraded?                       |       |         |         |
   +-----+---------------------------------+-------+---------+---------+
   | 4   | Will it confuse                 | no    | no      | no      |
   |     | non-upgraded RRs?               |       |         |         |
   +-----+---------------------------------+-------+---------+---------+
   | 5   | Will it confuse                 | no    | no      | maybe** |
   |     | non-upgraded DGWs?              |       |         |         |
   +-----+---------------------------------+-------+---------+---------+

                      Table 1: GW-IP vs IP-mapping SOI

       Notes:

   *   We also can take the Figure 4 of Section 6.3 of
       [I-D.wz-bess-evpn-vpws-as-vrf-ac] for example, in such case, its
       PE3 may be a DGW.

   **  If the RT-5E routes of the original Bump-in-the-wire usecase are
       advertised along with the route-target of the IP-VRF (thus no RTs
       of the BD-10), when DGW1 receives a RT-5E route and there is a
       SBD IRB in the IP-VRF instance, it may select RT-1 per EVI routes
       for the RT-5E route in the context of that SBD.  This is
       discussed in section Section 3.6.4 of
       [I-D.wang-bess-evpn-ether-tag-id-usage].

   We can found in above table that a new TLV will be no better than the
   original GW-IP field.

   Note that when PEs can not distinguish the common prefixes from the
   exclusive prefixes, only CE-BGP nexthop based Overlay Index can be
   used for IP aliasing (independent CE-BGP sessions and RT-5L routes
   can also be used as per Section 6.1 of
   [I-D.wz-bess-evpn-vpws-as-vrf-ac], but this is not IP aliasing),
   because that the PEs can't advertise different ESIs for the common
   prefixes and the exclusive prefixes.

Authors' Addresses

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   Yubao Wang
   ZTE Corporation
   No.68 of Zijinghua Road, Yuhuatai Distinct
   Nanjing
   China

   Email: wang.yubao2@zte.com.cn

   Zheng(Sandy) Zhang
   ZTE Corporation
   No. 50 Software Ave, Yuhuatai Distinct
   Nanjing
   China

   Email: zhang.zheng@zte.com.cn

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