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For this RFC, original HTML is available from the RFC-Editor: RFC8971


Internet Engineering Task Force (IETF)                S. Pallagatti, Ed.
Request for Comments: 8971                                        VMware
Category: Informational                                   G. Mirsky, Ed.
ISSN: 2070-1721                                                ZTE Corp.
                                                             S. Paragiri
                                                  Individual Contributor
                                                             V. Govindan
                                                            M. Mudigonda
                                                           December 2020

 Bidirectional Forwarding Detection (BFD) for Virtual eXtensible Local
                          Area Network (VXLAN)


   This document describes the use of the Bidirectional Forwarding
   Detection (BFD) protocol in point-to-point Virtual eXtensible Local
   Area Network (VXLAN) tunnels used to form an overlay network.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
     2.1.  Abbreviations
     2.2.  Requirements Language
   3.  Deployment
   4.  Use of the Management VNI
   5.  BFD Packet Transmission over VXLAN Tunnel
   6.  Reception of BFD Packet from VXLAN Tunnel
   7.  Echo BFD
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Authors' Addresses

1.  Introduction

   "Virtual eXtensible Local Area Network (VXLAN)" [RFC7348] provides an
   encapsulation scheme that allows the building of an overlay network
   by decoupling the address space of the attached virtual hosts from
   that of the network.

   One use of VXLAN is in data centers interconnecting virtual machines
   (VMs) of a tenant.  VXLAN addresses the requirements of the Layer 2
   and Layer 3 data-center network infrastructure in the presence of VMs
   in a multi-tenant environment by providing a Layer 2 overlay scheme
   on a Layer 3 network [RFC7348].  Another use is as an encapsulation
   for Ethernet VPN [RFC8365].

   This document is written assuming the use of VXLAN for virtualized
   hosts and refers to VMs and VXLAN Tunnel End Points (VTEPs) in
   hypervisors.  However, the concepts are equally applicable to non-
   virtualized hosts attached to VTEPs in switches.

   In the absence of a router in the overlay, a VM can communicate with
   another VM only if they are on the same VXLAN segment.  VMs are
   unaware of VXLAN tunnels, because a VXLAN tunnel is terminated on a
   VTEP.  VTEPs are responsible for encapsulating and decapsulating
   frames exchanged among VMs.

   The ability to monitor path continuity -- i.e., perform proactive
   continuity check (CC) for point-to-point (p2p) VXLAN tunnels -- is
   important.  The asynchronous mode of BFD, as defined in [RFC5880], is
   used to monitor a p2p VXLAN tunnel.

   In the case where a Multicast Service Node (MSN) (as described in
   Section 3.3 of [RFC8293]) participates in VXLAN, the mechanisms
   described in this document apply and can, therefore, be used to test
   the continuity of the path between the source Network Virtualization
   Endpoint (NVE) and the MSN.

   This document describes the use of the Bidirectional Forwarding
   Detection (BFD) protocol to enable monitoring continuity of the path
   between VXLAN VTEPs that are performing as VNEs, and/or between the
   source NVE and a replicator MSN using a Management VXLAN Network
   Identifier (VNI) (Section 4).  All other uses of the specification to
   test toward other VXLAN endpoints are out of scope.

2.  Conventions Used in This Document

2.1.  Abbreviations

   BFD:     Bidirectional Forwarding Detection

   CC:      Continuity Check

   FCS:     Frame Check Sequence

   MSN:     Multicast Service Node

   NVE:     Network Virtualization Endpoint

   p2p:     Point-to-point

   VFI:     Virtual Forwarding Instance

   VM:      Virtual Machine

   VNI:     VXLAN Network Identifier (or VXLAN Segment ID)

   VTEP:    VXLAN Tunnel End Point

   VXLAN:   Virtual eXtensible Local Area Network

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Deployment

   Figure 1 illustrates a scenario with two servers: each hosting two
   VMs.  The servers host VTEPs that terminate two VXLAN tunnels with
   VNI number 100 and 200, respectively.  Separate BFD sessions can be
   established between the VTEPs (IP1 and IP2) for monitoring each of
   the VXLAN tunnels (VNI 100 and 200).  Using a BFD session to monitor
   a set of VXLAN VNIs between the same pair of VTEPs might help to
   detect and localize problems caused by misconfiguration.  An
   implementation that supports this specification MUST be able to
   control the number of BFD sessions that can be created between the
   same pair of VTEPs.  This method is applicable whether the VTEP is a
   virtual or physical device.

      |        Server 1          |
      | +----+----+  +----+----+ |
      | |VM1-1    |  |VM1-2    | |
      | |VNI 100  |  |VNI 200  | |
      | |         |  |         | |
      | +---------+  +---------+ |
      |        VTEP (IP1)        |
                            |   +-------------+
                            |   |   Layer 3   |
                            +---|   Network   |
                                         |         VTEP (IP2)       |
                                         | +----+----+  +----+----+ |
                                         | |VM2-1    |  |VM2-2    | |
                                         | |VNI 100  |  |VNI 200  | |
                                         | |         |  |         | |
                                         | +---------+  +---------+ |
                                         |      Server 2            |

                      Figure 1: Reference VXLAN Domain

   At the same time, a service-layer BFD session may be used between the
   tenants of VTEPs IP1 and IP2 to provide end-to-end fault management;
   this use case is outside the scope of this document.  In such a case,
   for VTEPs, the BFD Control packets of that session are
   indistinguishable from data packets.

   For BFD Control packets encapsulated in VXLAN (Figure 2), the inner
   destination IP address SHOULD be set to one of the loopback addresses
   from 127/8 range for IPv4 or to one of IPv4-mapped IPv6 loopback
   addresses from ::ffff: range for IPv6.

4.  Use of the Management VNI

   In most cases, a single BFD session is sufficient for the given VTEP
   to monitor the reachability of a remote VTEP, regardless of the
   number of VNIs.  BFD control messages MUST be sent using the
   Management VNI, which acts as the control and management channel
   between VTEPs.  An implementation MAY support operating BFD on
   another (non-Management) VNI, although the implications of this are
   outside the scope of this document.  The selection of the VNI number
   of the Management VNI MUST be controlled through a management plane.
   An implementation MAY use VNI number 1 as the default value for the
   Management VNI.  All VXLAN packets received on the Management VNI
   MUST be processed locally and MUST NOT be forwarded to a tenant.

5.  BFD Packet Transmission over VXLAN Tunnel

   BFD packets MUST be encapsulated and sent to a remote VTEP as
   explained in this section.  Implementations SHOULD ensure that the
   BFD packets follow the same forwarding path as VXLAN data packets
   within the sender system.

   BFD packets are encapsulated in VXLAN as described below.  The VXLAN
   packet format is defined in Section 5 of [RFC7348].  The value in the
   VNI field of the VXLAN header MUST be set to the value selected as
   the Management VNI.  The outer IP/UDP and VXLAN headers MUST be
   encoded by the sender, as defined in [RFC7348].

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                                                               |
    ~                      Outer Ethernet Header                    ~
    |                                                               |
    |                                                               |
    ~                        Outer IPvX Header                      ~
    |                                                               |
    |                                                               |
    ~                        Outer UDP Header                       ~
    |                                                               |
    |                                                               |
    ~                           VXLAN Header                        ~
    |                                                               |
    |                                                               |
    ~                    Inner Ethernet Header                      ~
    |                                                               |
    |                                                               |
    ~                        Inner IPvX Header                      ~
    |                                                               |
    |                                                               |
    ~                         Inner UDP Header                      ~
    |                                                               |
    |                                                               |
    ~                       BFD Control Packet                     ~
    |                                                               |
    |                   Outer Ethernet FCS                          |

            Figure 2: VXLAN Encapsulation of BFD Control Packet

   The BFD packet MUST be carried inside the inner Ethernet frame of the
   VXLAN packet.  The choice of destination Media Access Control (MAC)
   and destination IP addresses for the inner Ethernet frame MUST ensure
   that the BFD Control packet is not forwarded to a tenant but is
   processed locally at the remote VTEP.  The inner Ethernet frame
   carrying the BFD Control packet has the following format:

   Ethernet Header:
      Destination MAC:  A Management VNI, which does not have any
         tenants, will have no dedicated MAC address for decapsulated
         traffic.  The value 00-52-02 SHOULD be used in this field.

      Source MAC:  MAC address associated with the originating VTEP.

      Ethertype:  This is set to 0x0800 if the inner IP header is IPv4
         and set to 0x86DD if the inner IP header is IPv6.

   IP header:
      Destination IP:  This IP address MUST NOT be of one of tenant's IP
         addresses.  The IP address SHOULD be selected from the range
         127/8 for IPv4 and from the range ::ffff: for
         IPv6.  Alternatively, the destination IP address MAY be set to
         VTEP's IP address.

      Source IP:  IP address of the originating VTEP.

      TTL or Hop Limit:  MUST be set to 255, in accordance with

   The destination UDP port is set to 3784 and the fields of the BFD
   Control packet are encoded as specified in [RFC5881].

6.  Reception of BFD Packet from VXLAN Tunnel

   Once a packet is received, the VTEP MUST validate the packet.  If the
   packet is received on the Management VNI and is identified as a BFD
   Control packet addressed to the VTEP, then the packet can be
   processed further.  Processing of BFD Control packets received on a
   non-Management VNI is outside the scope of this specification.

   The received packet's inner IP payload is then validated according to
   Sections 4 and 5 in [RFC5881].

7.  Echo BFD

   Support for echo BFD is outside the scope of this document.

8.  IANA Considerations

   IANA has assigned a single MAC address of the value 00-52-02 from the
   "Unassigned (small allocations)" block of the "IANA Unicast 48-bit
   MAC Addresses" registry as follows: the "Usage" field is "BFD for
   VXLAN".  The "Reference" is this document.

9.  Security Considerations

   Security issues discussed in [RFC5880], [RFC5881], and [RFC7348]
   apply to this document.

   This document recommends using an address from the internal host
   loopback addresses 127/8 range for IPv4, or an IP4-mapped IPv6
   loopback address from the ::ffff: range for IPv6, as the
   destination IP address in the inner IP header.  Using such an address
   prevents the forwarding of the encapsulated BFD control message by a
   transient node, in case the VXLAN tunnel is broken, in accordance
   with [RFC1812].

      |  A router SHOULD NOT forward, except over a loopback interface,
      |  any packet that has a destination address on network 127.  A
      |  router MAY have a switch that allows the network manager to
      |  disable these checks.  If such a switch is provided, it MUST
      |  default to performing the checks.

   The use of IPv4-mapped IPv6 addresses has the same property as using
   the IPv4 network 127/8.  Moreover, the IPv4-mapped IPv6 addresses'
   prefix is not advertised in any routing protocol.

   If the implementation supports establishing multiple BFD sessions
   between the same pair of VTEPs, there SHOULD be a mechanism to
   control the maximum number of such sessions that can be active at the
   same time.

10.  References

10.1.  Normative References

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,

   [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>.

10.2.  Informative References

   [RFC8293]  Ghanwani, A., Dunbar, L., McBride, M., Bannai, V., and R.
              Krishnan, "A Framework for Multicast in Network
              Virtualization over Layer 3", RFC 8293,
              DOI 10.17487/RFC8293, January 2018,

   [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,


   The authors would like to thank Jeff Haas of Juniper Networks for his
   reviews and feedback on this material.

   The authors would also like to thank Nobo Akiya, Marc Binderberger,
   Shahram Davari, Donald E. Eastlake 3rd, Anoop Ghanwani, Dinesh Dutt,
   Joel Halpern, and Carlos Pignataro for the extensive reviews and the
   most detailed and constructive comments.


   Reshad Rahman

   Email: rrahman@cisco.com

Authors' Addresses

   Santosh Pallagatti (editor)

   Email: santosh.pallagatti@gmail.com

   Greg Mirsky (editor)
   ZTE Corp.

   Email: gregimirsky@gmail.com

   Sudarsan Paragiri
   Individual Contributor

   Email: sudarsan.225@gmail.com

   Vengada Prasad Govindan

   Email: venggovi@cisco.com

   Mallik Mudigonda

   Email: mmudigon@cisco.com

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