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Versions: 00 01 02 03                                                   
Network Working Group                                    A. Sajassi, Ed.
Internet-Draft                                                 G. Badoni
Intended status: Standards Track                               P. Warade
Expires: April 28, 2022                                      S. Pasupula
                                                           Cisco Systems
                                                           J. Drake, Ed.
                                                                 Juniper
                                                         J. Rabadan, Ed.
                                                                   Nokia
                                                        October 25, 2021


     EVPN Support for L3 Fast Convergence and Aliasing/Backup Path
                 draft-sajassi-bess-evpn-ip-aliasing-03

Abstract

   This document proposes an EVPN extension to allow several of its
   multihoming functions, fast convergence and aliasing/backup path, to
   be used in conjunction with 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 April 28, 2022.

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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Ethernet Segments for Host Routes in Symmetric IRB  . . .   3
     1.2.  Inter-subnet Forwarding for Prefix Routes in the
           Interface-less IP-VRF-to-IP-VRF Model . . . . . . . . . .   4
     1.3.  Ethernet Segments for Prefix routes in IP-VRF-to-IP-VRF
           use-cases . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Terminology and Conventions . . . . . . . . . . . . . . .   6
   2.  Ethernet Segments for L3 Aliasing/Backup Path and Fast
       Convergence . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  IP Aliasing and Backup Path . . . . . . . . . . . . . . . . .   9
     3.1.  Constructing the IP A-D per EVI Route . . . . . . . . . .  10
   4.  Fast Convergence for Routed Traffic . . . . . . . . . . . . .  10
     4.1.  Constructing IP A-D per Ethernet Segment Route  . . . . .  11
       4.1.1.  IP A-D per ES Route Targets . . . . . . . . . . . . .  11
     4.2.  Avoiding convergence issues by synchronizing IP prefixes   11
     4.3.  Handling Silent Host MAC/IP route for IP Aliasing . . . .  12
     4.4.  MAC Aging . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Determining Reachability to Unicast IP Addresses  . . . . . .  13
     5.1.  Local Learning  . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Remote Learning . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Constructing the EVPN IP Routes . . . . . . . . . . . . .  13
       5.3.1.  Route Resolution  . . . . . . . . . . . . . . . . . .  13
   6.  Forwarding Unicast Packets  . . . . . . . . . . . . . . . . .  14
   7.  Load Balancing of Unicast Packets . . . . . . . . . . . . . .  14
   8.  IP Aliasing and Unequal ECMP for IP Prefix Routes . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     13.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This document proposes an EVPN extension to allow several of its
   multihoming functions, fast convergence and aliasing/backup path, to
   be used in conjunction with inter-subnet forwarding.  It re-uses the
   existing EVPN routes, the Ethernet A-D per ES and the Ethernet A-D
   per EVI routes, which are used for these multihoming functions.  In



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   particular, there are three use-cases that could benefit from the use
   of these multihoming functions:

   a.  Inter-subnet forwarding for host routes in symmetric IRB
       [I-D.ietf-bess-evpn-inter-subnet-forwarding].

   b.  Inter-subnet forwarding for prefix routes in the interface-less
       IP-VRF-to-IP-VRF model [I-D.ietf-bess-evpn-prefix-advertisement].

   c.  Inter-subnet forwarding for prefix routes when the ESI is used
       exclusively as an L3 construct
       [I-D.ietf-bess-evpn-prefix-advertisement].

1.1.  Ethernet Segments for Host Routes in Symmetric IRB

   Consider a pair of multi-homing PEs, PE1 and PE2, as illustrated in
   Figure 1.  Let there be a host H1 attached to them.  Consider PE3 and
   a host H3 attached to it.

                                  +----------------+
                                  |     EVPN       |
                               +------+            |
                               | PE1  | +--->      |
                        +------+      | RT-2       |
                        |      |      | IP1     +--+---+
                 +---+  | ES1  +------+ ESI1    | PE3  |
            H1+--+CE1+--+         |             |      +-+H3
                 +---+  |      +------+         |      |
                        |      | PE2  |         +--+---+
                        +------+      |            |
                               |      |            |
                               +------+            |
                                  |                |
                                  +----------------+



   Figure 1: Inter-subnet traffic between Multihoming PEs and Remote PE

   With Asymmetric IRB [I-D.ietf-bess-evpn-inter-subnet-forwarding], if
   H3 sends inter-subnet traffic to H1, routing will happen at PE3.  PE3
   will be attached to the destination IRB interface and will trigger
   ARP/ND requests if it does not have an ARP/ND adjacency to H1.  A
   subsequent routing lookup will resolve the destination MAC to H1's
   MAC address.  Furthermore, H1's MAC will point to an ECMP EVPN
   destination on PE1 and PE2, either due to host route advertisement
   from both PE1 and PE2, or due to Ethernet Segment MAC Aliasing as
   detailed in [RFC7432].



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   With Symmetric IRB [I-D.ietf-bess-evpn-inter-subnet-forwarding], if
   H3 sends inter-subnet traffic to H1, a routing lookup will happen at
   PE3's IP-VRF and this routing lookup will not yield the destination
   IRB interface and therefore MAC Aliasing is not possible.  In order
   to have per-flow load balancing for H3's routed traffic to H1, an IP
   ECMP list (to PE1/PE2) needs to be associated to H1's host route in
   the IP-VRF route-table.  If H1 is locally learned only at one of the
   multi-homing PEs, PE1 or PE2, due to LAG hashing, PE3 will not be
   able to build an IP ECMP list for the H1 host route.

   With the extension described in this document, PE3's IP-VRF becomes
   Ethernet-Segment-aware and builds an IP ECMP list for H1 based on the
   advertisement of ES1 along with H1 in a MAC/IP route and the
   availability of ES1 on PE1 and PE2.

1.2.  Inter-subnet Forwarding for Prefix Routes in the Interface-less
      IP-VRF-to-IP-VRF Model

   In the Interface-less IP-VRF-to-IP-VRF model described in
   [I-D.ietf-bess-evpn-prefix-advertisement] there is no Overlay Index
   and hence no recursive resolution of the IP Prefix route to either a
   MAC/IP Advertisement or an Ethernet A-D per ES/EVI route, which means
   that the fast convergence and aliasing/backup path functions are
   disabled.  Although the use-case is different, in a sense the
   recursive resolution of an IP Prefix route to an Ethernet A-D per ES/
   EVI route is already described in section 4.3 of
   [I-D.ietf-bess-evpn-prefix-advertisement], Bump-in-the-Wire Use-Case,
   but that section does not describe aliasing.

   Although this document can be considered to be adding the aliasing/
   backup path function to the Bump-in-the-Wire Use-Case, the scenario
   illustrated in Figure 2 will be used to explain the procedures.



















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                                  +----------------+
                                  |     EVPN       |
                               +------+            |
                               | PE1  | +--->      |
                        +------+      | RT-5       |
                        |      |      | IP1/32  +--+---+
                 +---+  | ES1  +------+ ESI1    | PE3  |
            H1+--+CE1+--+         |             |      +-+H3
                 +---+  |      +------+         |      |
                        |      | PE2  |         +--+---+
                        +------+      |            |
                               |      |            |
                               +------+            |
                                  |                |
                                  +----------------+


           Figure 2: Inter-subnet example with IP Prefix routes

   Consider PE1 and PE2 are multi-homed to CE1 (in an All-Active
   Ethernet Segment ES1), and PE1, PE2 and PE3 are attached to an IP-VRF
   of the same tenant.  Suppose H1's host route is learned (via ARP or
   ND snooping) on PE1 only, and PE1 advertises an EVPN IP Prefix route
   for H1's host route.  If H3 sends inter-subnet traffic to H1, a
   routing lookup on PE3 would normally yield a single next-hop, i.e.,
   PE1.

   This document proposes the use of the ESI in the IP Prefix route and
   the recursive resolution to A-D per ES/EVI routes advertised from PE1
   and PE2, so that H1's host route in PE3 can be associated to an IP
   ECMP list (to PE1/PE2) for aliasing purposes.

1.3.  Ethernet Segments for Prefix routes in IP-VRF-to-IP-VRF use-cases

   This document also enables fast convergence and aliasing/backup path
   to be used even when the ESI is used exclusively as an L3 construct.

   As an example, consider the scenario in Figure 3 in which PE1 and PE2
   are multi-homed to CE1.  However, and contrary to CE1 in Figure 2, in
   this case the links between CE1 and PE1/PE2 are used exclusively for
   L3 protocols and L3 forwarding in different BDs, and a BGP session
   established between CE1's loopback address and PE1's IRB address.

   In these use-cases, sometimes the CE supports a single BGP session to
   one of the PEs (through which it advertises a number of IP Prefixes
   seating behind itself) and yet, it is desired that remote PEs can
   build an IP ECMP list or backup IP list including all the PEs multi-
   homed to the same CE.  For example, in Figure 3, CE1 has a single



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   eBGP neighbor, i.e., PE1.  Load-balancing for traffic from CE1 to H4
   can be accomplished by a default route with next-hops PE1 and PE2,
   however, load-balancing from H4 to any of the prefixes attached to
   CE1 would not be possible since only PE1 would advertise EVPN IP
   Prefix routes for CE1's prefixes.  This document provides a solution
   so that PE3 considers PE2 as a next-hop in the IP ECMP list for CE1's
   prefixes, even if PE2 did not advertise the IP Prefix routes for
   those prefixes in the first place.

                                         +-----------------------+
                                         |        EVPN           |
                           PE1           |                       |
                          +-------------------+                  |
                          |       IRB1        |                  |
                          |  +---+   +------+ | ------->         |
                 +-----------|BD1|---|IPVRF1| | RT-5             |
         eBGP    |        |  +---+   |      | | 50.0/24          | PE3
      +------------------------>10.1 +------+ | ESI1  +----------------+
      |          |        +-------------------+       | +------+       |
     +-----+10.2 |                       |   ^        | |IPVRF1| +---+ |
     | CE1 |-----+    ES1                |   |        | |      |-|BD3| |
     |     |-----+                       |   +--------| +------+ +---+ |
     +-----+20.2 |         PE2           |        +---|            |   |
     lo1         |        +--------------+----+   |   +------------|---+
     1.1.1.1     |        |       IRB2        |   |              | |
     Prefixes:   |        |  +---+   +------+ |   |              | H4
     50.0/24     +-----------|BD2|---|IPVRF1| |<--+              |
     60.0/24              |  +---+   |      | |                  |
                          |     20.1 +------+ |                  |
                          +-------------------+                  |
                                         |                       |
                                         +-----------------------+

     Note:
       IP addresses expanded by adding 0s
       E.g., 50.0 expands to 50.0.0.0

                     Figure 3: Layer-3 Multihoming PEs

1.4.  Terminology and Conventions

   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.

   -  IRB: Integrated Routing and Bridging



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   -  IRB Interface: Integrated Bridging and Routing Interface.  A
      virtual interface that connects the Bridge Table and the IP-VRF on
      an NVE.

   -  BD: Broadcast Domain.  An EVI may be comprised of one BD (VLAN-
      based or VLAN Bundle services) or multiple BDs (VLAN-aware Bundle
      services).

   -  Bridge Table: An instantiation of a broadcast domain on a MAC-VRF.

   -  CE: Customer Edge device, e.g., a host, router, or switch.s

   -  EVI: An EVPN instance spanning the Provider Edge (PE) devices
      participating in that EVPN.

   -  MAC-VRF: A Virtual Routing and Forwarding table for Media Access
      Control (MAC) addresses on a PE.

   -  Ethernet Segment (ES): When a customer site (device or network) is
      connected to one or more PEs via a set of Ethernet links, then
      that set of links is referred to as an 'Ethernet segment'.

   -  Ethernet Segment Identifier (ESI): A unique non-zero identifier
      that identifies an Ethernet segment is called an 'Ethernet Segment
      Identifier'.

   -  IP-VRF: A VPN Routing and Forwarding table for IP routes on an
      NVE/PE.  The IP routes could be populated by any routing protocol,
      E.g., EVPN, IP-VPN and BGP PE-CE IP address families.  An IP-VRF
      is also an instantiation of a layer 3 VPN in an NVE/PE.

   -  EVPN IP route: An EVPN IP Prefix route or an EVPN MAC/IP
      Advertisement route.

   -  LACP: Link Aggregation Control Protocol.

   -  PE: Provider Edge device.

   -  Single-Active Redundancy Mode: When only a single PE, among all
      the PEs attached to an Ethernet segment, is allowed to forward
      traffic to/from that Ethernet segment for a given VLAN, then the
      Ethernet segment is defined to be operating in Single-Active
      redundancy mode.

   -  All-Active Redundancy Mode: When all PEs attached to an Ethernet
      segment are allowed to forward known unicast traffic to/from that
      Ethernet segment for a given VLAN, then the Ethernet segment is
      defined to be operating in All-Active redundancy mode.



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   -  RT-2: EVPN MAC/IP Advertisement route, as specified in [RFC7432].

   -  RT-4: EVPN Ethernet Segment route, as specified in [RFC7432].

   -  RT-5: EVPN IP Prefix route, as specified in
      [I-D.ietf-bess-evpn-prefix-advertisement].

2.  Ethernet Segments for L3 Aliasing/Backup Path and Fast Convergence

   The first two use cases described in Section 1 do not require any
   extensions to the Ethernet Segment definition and both cases support
   Ethernet Segments as a set of Ethernet links and specified in
   [RFC7432], or virtual Ethernet Segments as a set of logical links
   specified in [I-D.ietf-bess-evpn-virtual-eth-segment].

   The third use case in Section 1 requires an extension to the way
   Ethernet Segments are defined and associated.  In this case, the
   Ethernet Segment is a Layer-3 construct characterized as follows:

   -  The ES is defined as a set of Layer-3 links to the multi-homed CE
      and its state MUST be linked to the layer-3 reachability from each
      multi-homed PE to the CE's loopback address via a non-EVPN route
      in the PE's IP-VRF.

   -  The ESI SHOULD be of type 4 [RFC7432] and set to the router ID of
      the multi-homed CE.

   -  All-active or single-active multi-homing redundancy modes are
      supported, however, the redundancy mode only affects the
      procedures in Section 3.

   -  PEs attached to the same Layer-3 ES discover each other through
      the exchange of RT4 routes (Ethernet Segment routes).  DF Election
      procedures [RFC8584] MAY be used for single-active multi-homing
      mode.

   -  The routes advertised from the multi-homed CE's and installed in
      the PE's IP-VRF table with the CE's loopback as the next-hop
      SHOULD be re-advertised by the PE in EVPN IP Prefix routes with
      the ESI of the CE.  The rest of the EVPN IP Prefix routes fields
      are set as per the Interface-less model in
      [I-D.ietf-bess-evpn-prefix-advertisement].

   In the example depicted in Figure 3, ES1 is defined as the set of
   layer-3 links that connects PE1 and PE2 to CE1.  Its ESI, e.g., ESI-
   1, is derived as a type 4 ESI using the CE's router ID.  ES-1 will be
   operationally up in the PE as long as CE1's loopback route is
   installed in the PE's IP-VRF and learned via any routing protocol



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   except for an EVPN route.  E.g., an active static route to 1.1.1.1
   via next-hop 10.0.0.2 would make the ES operationally up in PE1, and
   the eBGP routes received from CE1 with next-hop 1.1.1.1 will be re-
   advertised as RT-5 routes with ESI-1.

3.  IP Aliasing and Backup Path

   In order to address the use-cases described in Section 1, above, this
   document proposes that:

   1.  A PE that is attached to a given ES will advertise a set of one
       or more Ethernet A-D per ES routes for that ES.  Each is termed
       an 'IP A-D per ES' route and is tagged with the route targets
       (RTs) for one or more of the IP-VRFs defined on it for that ES;
       the complete set of IP A-D per ES routes contains the RTs for all
       of the IP-VRFs defined on it for that ES.

       A remote PE imports an IP A-D per ES route into the IP-VRFs
       corresponding to the RTs with which the route is tagged.  When
       the complete set of IP A-D per ES routes has been processed, a
       remote PE will have imported an IP A-D per ES route into each of
       the IP-VRFs defined on it for that ES; this enables fast
       convergence for each of these IP-VRFs.

   2.  A PE advertises for this ES, an Ethernet A-D Per EVI route for
       each of the IP-VRFs defined on it.  Each is termed an 'IP A-D per
       EVI' route and is tagged with the RT for a given IP-VRF.

       A remote PE imports an IP A-D per EVI route into the IP-VRF
       corresponding to the RT with which the route is tagged.  The
       label contained in the route enables aliasing/backup path for the
       routes in that IP-VRF.

   To address the third use-case described in Section 1, where the links
   between a CE and its multihomed PEs are used exclusively for L3
   protocols and L3 forwarding, a PE uses the procedures described in 1)
   and 2), above.

   The processing of the IP A-D per ES and the IP A-D per EVI routes is
   as defined in [RFC7432] and [RFC8365] except that the fast
   convergence and aliasing/backup path functions apply to the routes
   contained in an IP-VRF.  In particular, a remote PE that receives an
   EVPN MAC/IP Advertisement route or an IP Prefix route with a non-
   reserved ESI and the RT of a particular IP-VRF SHOULD consider it
   reachable by every PE that has advertised an IP A-D per ES and IP A-D
   per EVI route for that ESI and IP-VRF.





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3.1.  Constructing the IP A-D per EVI Route

   The construction of the IP A-D per EVI route is the same as that of
   the Ethernet A-D per EVI route, as described in [RFC7432], with the
   following exceptions:

   -  The Route-Distinguisher is for the corresponding IP-VRF.

   -  The Ethernet Tag should be set to 0.

   -  The route SHOULD carry the RT of the corresponding IP-VRF.

   -  The route MUST carry the PE's MAC Extended Community if the
      encapsulation used between the PEs for inter-subnet forwarding is
      an Ethernet NVO tunnel [I-D.ietf-bess-evpn-prefix-advertisement].

   -  The route SHOULD carry the Layer 2 Extended Community [RFC8214].
      For all-active multihoming, all PEs attached to the specified ES
      will advertise P=1.  For backup path, the Primary PE will
      advertise P=1 and the Backup PE will advertise P=0, B=1.

      o  The Primary PE SHOULD be a PE with a routing adjacency to the
         attached CE.

      o  The Primary PE MAY be determined by policy or MAY be elected by
         a DF Election as in [RFC8584] as described in Section 2.

4.  Fast Convergence for Routed Traffic

   Host or Prefix reachability is learned via the BGP-EVPN control plane
   over the MPLS/NVO network.  EVPN IP routes for a given ES are
   advertised by one or more of the PEs attached to that ES.  When one
   of these PEs fails, a remote PE needs to quickly invalidate the EVPN
   IP routes received from it.

   To accomplish this, EVPN defined the fast convergence function
   specified in [RFC7432].  This document extends fast convergence to
   inter-subnet forwarding by having each PE advertise a set of one or
   more IP A-D per ES routes for each locally attached Ethernet segment
   (refer to Section 4.1 below for details on how these routes are
   constructed).  A PE may need to advertise more than one IP A-D per ES
   route for a given ES because the ES may be in a multiplicity of IP-
   VRFs and the Route-Targets for all of these IP-VRFs may not fit into
   a single route.  Advertising a set of IP A-D per ES routes for the ES
   allows each route to contain a subset of the complete set of Route
   Targets.  Each IP A-D per ES route is differentiated from the other
   routes in the set by a different Route Distinguisher (RD).




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   Upon failure in connectivity to the attached ES, the PE withdraws the
   corresponding set of IP A-D per ES routes.  This triggers all PEs
   that receive the withdrawal to update their next-hop adjacencies for
   all IP addresses associated with the Ethernet Segment in question,
   across IP-VRFs.  If no other PE has advertised an IP A-D per ES route
   for the same Ethernet Segment, then the PE that received the
   withdrawal simply invalidates the IP entries for that segment.
   Otherwise, the PE updates its next-hop adjacencies accordingly.

   These routes should be processed with higher priority than EVPN IP
   route withdrawals upon failure.  Similar priority processing is
   needed even on the intermediate Route Reflectors.

4.1.  Constructing IP A-D per Ethernet Segment Route

   This section describes the procedures used to construct the IP A-D
   per ES route, which is used for fast convergence (as discussed in
   Section 3).  The usage/construction of this route remains similar to
   that described in section 8.2.1. of [RFC7432] with a few notable
   exceptions as explained in following sections.

4.1.1.  IP A-D per ES Route Targets

   Each IP A-D per ES route MUST carry one or more Route Targets (RTs).
   The set of IP A-D per ES routes MUST carry the entire set of IP-VRF
   RTs for all the IP-VRFs defined on that ES.

4.2.  Avoiding convergence issues by synchronizing IP prefixes

   Consider a pair of multi-homing PEs, PE1 and PE2.  Let there be a
   host H1 attached to them.  Consider PE3 and a host H3 attached to it.

   If the host H1 is learned on both the PEs, the ECMP path list is
   formed on PE3 pointing to (PE1/PE2).  Traffic from H3 to H1 is not
   impacted even if one of the PEs fails as the path list gets corrected
   upon receiving the withdrawal of the fast convergence route(s) (IP
   A-D per ES routes).

   In a case where H1 is locally learned only on PE1 due to LAG hashing
   or a single routing protocol adjacency to PE1, at PE3, H1 has ECMP
   path list (PE1/PE2) using Aliasing as described in this document.
   Traffic from H3 can reach H1 via either PE1 or PE2.

   PE2 should install local forwarding state for EVPN IP routes
   advertised by other PEs attached to the same ES (i.e., PE1) but not
   advertise them as local routes.  When the traffic from H3 reaches
   PE2, PE2 will be able forward the traffic to H1 without any
   convergence delay (caused by triggering ARP/ND to H1 or to the next-



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   hop to reach H1).  The synchronization of the EVPN IP routes across
   all PEs of the same Ethernet Segment is important to solve
   convergence issues.

4.3.  Handling Silent Host MAC/IP route for IP Aliasing

   Consider the example of Figure 1 for IP aliasing.  If PE1 fails, PE3
   will receive the withdrawal of the fast convergence route(s) and
   update the ECMP list for H1 to be just PE2.  When the EVPN IP route
   for H1 is also withdrawn, neither PE2 nor PE3 will have a route to
   H1, and traffic from H3 to H1 is blackholed until PE2 learns H1 and
   advertises an EVPN IP route for it.

   This blackholing can be much worse if the H1 behaves like a silent
   host.  IP address of H1 will not be re-learned on PE2 till H1 ARP/ND
   messages or some traffic triggers ARP/ND for H1.

   PE2 can detect the failure of PE1's reachability in different ways:

   a.  When PE1 fails, the next hop tracking to PE1 in the underlay
       routing protocols can help detect the failure.

   b.  Upon the failure of its link to CE1, PE1 will withdraw its IP A-D
       route(s) and PE2 can use this as a trigger to detect failure.

   Thus to avoid blackholing, when PE2 detects loss of reachability to
   PE1, it should trigger ARP/ND requests for all remote IP prefixes
   received from PE1 across all affected IP-VRFs.  This will force host
   H1 to reply to the solicited ARP/ND messages from PE2 and refresh
   both MAC and IP for the corresponding host in its tables.

   Even in core failure scenario on PE1, PE1 must withdraw all its local
   layer-2 connectivity, as Layer-2 traffic should not be received by
   PE1.  So when ARP/ND is triggered from PE2 the replies from host H1
   can only be received by PE2.  Thus H1 will be learned as local route
   and also advertised from PE2.

   It is recommended to have a staggered or delayed deletion of the EVPN
   IP routes from PE1, so that ARP/ND refresh can happen on PE2 before
   the deletion.

4.4.  MAC Aging

   In the same example as in Section 4.3, PE1 would do ARP/ND refresh
   for H1 before it ages out.  During this process, H1 can age out
   genuinely or due to the ARP/ND reply landing on PE2.  PE1 must
   withdraw the local entry from BGP when H1 entry ages out.  PE1




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   deletes the entry from the local forwarding only when there are no
   remote synced entries.

5.  Determining Reachability to Unicast IP Addresses

5.1.  Local Learning

   The procedures for local learning do not change from [RFC7432] or
   [I-D.ietf-bess-evpn-prefix-advertisement].

5.2.  Remote Learning

   The procedures for remote learning do not change from [RFC7432] or
   [I-D.ietf-bess-evpn-prefix-advertisement].

5.3.  Constructing the EVPN IP Routes

   The procedures for constructing MAC/IP Address or IP Prefix
   Advertisements do not change from [RFC7432] or
   [I-D.ietf-bess-evpn-prefix-advertisement].

5.3.1.  Route Resolution

   If the ESI field is set to reserved values of 0 or MAX-ESI, the EVPN
   IP route resolution MUST be based on the EVPN IP route alone.

   If the ESI field is set to a non-reserved ESI, the EVPN IP route
   resolution MUST happen only when both the EVPN IP route and the
   associated set of IP A-D per ES routes have been received.  To
   illustrate this with an example, consider a pair of multi-homed PEs,
   PE1 and PE2, connected to an all-active Ethernet Segment.  A given
   host with IP address H1 is learned by PE1 but not by PE2.  When the
   EVPN IP route from PE1 and a set of IP A-D per ES and IP A-D per EVI
   routes from PE1 and PE2 are received, then (1) PE3 can forward
   traffic destined to H1 to both PE1 and PE2.

   If after (1) PE1 withdraws the IP A-D per ES route, then PE3 will
   forward the traffic to PE2 only.

   If after (1) PE2 withdraws the IP A-D per ES route, then PE3 will
   forward the traffic to PE1 only.

   If after (1) PE1 withdraws the EVPN IP route, then PE3 will do
   delayed deletion of H1, as described in Section 4.3.

   If after (1) PE2 advertised the EVPN IP route, but PE1 withdraws it,
   PE3 will continue forwarding to both PE1 and PE2 as long as it has
   the IP A-D per ES and the IP A-D per EVI route from both.



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6.  Forwarding Unicast Packets

   Refer to Section 5 in [I-D.ietf-bess-evpn-inter-subnet-forwarding]
   and [I-D.ietf-bess-evpn-prefix-advertisement].

7.  Load Balancing of Unicast Packets

   The procedures for load balancing of Unicast Packets do not change
   from [RFC7432]

8.  IP Aliasing and Unequal ECMP for IP Prefix Routes

   [I-D.ietf-bess-evpn-unequal-lb] specifies the use of the EVPN Link
   bandwidth extended community to achieve weighted load balancing to an
   ES or Virtual ES for unicast traffic.  The procedures in
   [I-D.ietf-bess-evpn-unequal-lb] MAY be used along with the procedures
   described in this document for any of the three cases described in
   Section 1, with the following considerations:

   -  The ES weight is signaled by the multi-homed PEs in the IP A-D per
      ES routes.

   -  The remote ingress PE learning an EVPN IP Route to prefix/host P
      that is associated to a weighted load balancing ES, will follow
      the procedures in [I-D.ietf-bess-evpn-unequal-lb] to influence the
      load balancing for traffic to P.

   -  [I-D.ietf-bess-evpn-unequal-lb] also allows the use of the Link
      Bandwidth Extended Community along with RT5s.  If the ingress PE
      learns a prefix P via a non-reserved ESI RT5 route with a weight
      (for which IP A-D per ES routes also signal a weight) and a zero
      ESI RT5 that includes a weight, the ingress PE will consider all
      the PEs attached to the ES as a single PE when normalizing
      weights.

      As an example, consider PE1 and PE2 are attached to ES-1 and PE1
      advertises an RT-5 for prefix P with ESI-1 (and link bandwidth of
      1).  Consider PE3 advertises an RT-5 for P with ESI=0 and link
      bandwidth of 2.  If PE1 and PE2 advertise a link bandwidth of 1
      and 2, respectively, in the IP A-D per ES routes for ES-1, an
      ingress PE4 SHOULD assign a normalized weight of 1 to ES-1 and a
      normalized weight of 2 to PE3.  When PE4 sprays the flows to P, it
      will send twice as many flows to PE3.  For the flows sent to ES-1,
      the individual PE link bandwidths advertised in the IP A-D per ES
      routes will be considered.






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9.  Security Considerations

   The mechanisms in this document use EVPN control plane as defined in
   [RFC7432].  Security considerations described in [RFC7432] are
   equally applicable.  This document uses MPLS and IP-based tunnel
   technologies to support data plane transport.  Security
   considerations described in [RFC7432] and in [RFC8365] are equally
   applicable.

10.  IANA Considerations

   No IANA considerations.

11.  Contributors

12.  Acknowledgments

13.  References

13.1.  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>.

   [RFC8214]  Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
              Rabadan, "Virtual Private Wire Service Support in Ethernet
              VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
              <https://www.rfc-editor.org/info/rfc8214>.

   [RFC8365]  Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
              Uttaro, J., and W. Henderickx, "A Network Virtualization
              Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
              DOI 10.17487/RFC8365, March 2018,
              <https://www.rfc-editor.org/info/rfc8365>.

   [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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   [RFC8584]  Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
              J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet
              VPN Designated Forwarder Election Extensibility",
              RFC 8584, DOI 10.17487/RFC8584, April 2019,
              <https://www.rfc-editor.org/info/rfc8584>.

13.2.  Informative References

   [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 Ethernet VPN
              (EVPN)", draft-ietf-bess-evpn-inter-subnet-forwarding-15
              (work in progress), July 2021.

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

   [I-D.ietf-bess-evpn-virtual-eth-segment]
              Sajassi, A., Brissette, P., Schell, R., Drake, J. E., and
              J. Rabadan, "EVPN Virtual Ethernet Segment", draft-ietf-
              bess-evpn-virtual-eth-segment-07 (work in progress), July
              2021.

   [I-D.ietf-bess-evpn-unequal-lb]
              Malhotra, N., Sajassi, A., Rabadan, J., Drake, J.,
              Lingala, A., and S. Thoria, "Weighted Multi-Path
              Procedures for EVPN Multi-Homing", draft-ietf-bess-evpn-
              unequal-lb-14 (work in progress), May 2021.

Authors' Addresses

   A. Sajassi (editor)
   Cisco Systems

   Email: sajassi@cisco.com


   G. Badoni
   Cisco Systems

   Email: gbadoni@cisco.com







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   P. Warade
   Cisco Systems

   Email: pwarade@cisco.com


   S. Pasupula
   Cisco Systems

   Email: surpasup@cisco.com


   J. Drake (editor)
   Juniper

   Email: jdrake@juniper.net


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

   Email: jorge.rabadan@nokia.com


























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