This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 5280
Internet Engineering Task Force (IETF)                       W. Hargrave
Request for Comments: 8327                                         LONAP
BCP: 214                                                     M. Griswold
Category: Best Current Practice                                      20C
ISSN: 2070-1721                                              J. Snijders
                                                             N. Hilliard
                                                              March 2018

         Mitigating the Negative Impact of Maintenance through
                          BGP Session Culling


   This document outlines an approach to mitigate the negative impact on
   networks resulting from maintenance activities.  It includes guidance
   for both IP networks and Internet Exchange Points (IXPs).  The
   approach is to ensure BGP-4 sessions that will be affected by
   maintenance are forcefully torn down before the actual maintenance
   activities commence.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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).  Further information on
   BCPs is available in 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) 2018 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  BGP Session Culling . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Voluntary BGP Session Teardown Recommendations  . . . . .   4
       3.1.1.  Maintenance Considerations  . . . . . . . . . . . . .   4
     3.2.  Involuntary BGP Session Teardown Recommendations  . . . .   4
       3.2.1.  Packet-Filter Considerations  . . . . . . . . . . . .   5
       3.2.2.  Hardware Considerations . . . . . . . . . . . . . . .   5
     3.3.  Procedural Considerations . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Example Packet Filters . . . . . . . . . . . . . . .   8
     A.1.  Example Configuration for Cisco IOS, IOS XR, and Arista
           EOS . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     A.2.  Example Configuration for Nokia SR OS . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   BGP Session Culling is the practice of ensuring BGP sessions are 
forcefully torn down before maintenance activities on a lower-layer
network commence -- activities that otherwise would affect the flow
of data between the BGP speakers.
EID 5280 (Verified) is as follows:

Section: 1

Original Text:

BGP Session Culling is the practice of ensuring BGP sessions are
forcefully torn down before maintenance activities on a lower-layer
network commence -- activities that otherwise would affect the flow
of data between the BGP speakers.  BGP Session Culling is the
practice of ensuring BGP sessions are forcefully torn down before
commencing maintenance activities (that otherwise would affect the
flow of data between the BGP speakers) on a lower-layer network.

Corrected Text:

BGP Session Culling is the practice of ensuring BGP sessions are
forcefully torn down before maintenance activities on a lower-layer
network commence -- activities that otherwise would affect the flow
of data between the BGP speakers.
The original version of a corrected sentence was left in the document in the editing phase
BGP Session Culling minimizes the amount of disruption that lower- layer network maintenance activities cause, by making BGP speakers preemptively converge onto alternative paths while the lower-layer network's forwarding plane remains fully operational. The grace period required for a successful application of BGP Session Culling is the sum of the time needed to detect the loss of the BGP session plus the time required for the BGP speaker to converge onto alternative paths. The first value is often governed by the BGP Hold Timer (see Section 6.5 of [RFC4271]), which is commonly between 90 and 180 seconds. The second value is implementation specific, but it could be as much as 15 minutes when a router with a slow control plane is receiving a full set of Internet routes. Throughout this document, the "Caretaker" is defined to be in control of the lower-layer network, while "Operators" directly administrate the BGP speakers. Operators and Caretakers implementing BGP Session Culling are encouraged to avoid using a fixed grace period, and instead to monitor forwarding-plane activity while the culling is taking place and to consider it complete once traffic levels have dropped to a minimum (Section 3.3). 2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. BGP Session Culling From the viewpoint of the Operator, there are two types of BGP Session Culling: Voluntary BGP Session Teardown: The Operator initiates the teardown of the potentially affected BGP session by issuing an Administrative Shutdown. Involuntary BGP Session Teardown: The Caretaker of the lower-layer network disrupts (higher-layer) BGP control-plane traffic, causing the BGP Hold Timers of the affected BGP session to expire, subsequently triggering rerouting of end-user traffic. 3.1. Voluntary BGP Session Teardown Recommendations Before an Operator commences activities that can cause disruption to the flow of data through the lower-layer network, an Operator can reduce loss of traffic by issuing an administrative shutdown to all BGP sessions running across the lower-layer network and wait a few minutes for data-plane traffic to subside. While architectures exist to facilitate quick network reconvergence (such as BGP Prefix Independent Convergence (PIC) [BGP_PIC]), an Operator cannot assume the remote side has such capabilities. As such, a grace period between the Administrative Shutdown and the impacting maintenance activities is warranted. After the maintenance activities have concluded, the Operator is expected to restore the BGP sessions to their original Administrative state. 3.1.1. Maintenance Considerations Initiators of the Administrative Shutdown MAY consider using Graceful Shutdown [RFC8326] to facilitate smooth drainage of traffic prior to session tear down, and the Shutdown Communication [RFC8203] to inform the remote side on the nature and duration of the maintenance activities. 3.2. Involuntary BGP Session Teardown Recommendations In the case where multilateral interconnection between BGP speakers is facilitated through a switched Layer 2 fabric, such as commonly seen at Internet Exchange Points (IXPs), different operational considerations can apply. Operational experience shows that many Operators are unable to carry out the Voluntary BGP Session Teardown recommendations, because of the operational cost and risk of coordinating the two configuration changes required. This has an adverse affect on Internet performance. In the absence of notifications from the lower layer (e.g., Ethernet link down) consistent with the planned maintenance activities in a switched Layer 2 fabric, the Caretaker of the fabric could choose to cull BGP sessions on behalf of the Operators connected to the fabric. Such culling of control-plane traffic will preempt the loss of end- user traffic by causing the expiration of BGP Hold Timers ahead of the moment where the expiration would occur without intervention from the fabric's Caretaker. In this scenario, BGP Session Culling is accomplished as described in the next subsection, through the application of a combined Layer 3 and Layer 4 (Layer 3/4) packet filter deployed in the Caretaker's switched fabric. 3.2.1. Packet-Filter Considerations The peering LAN prefixes used by the IXP form the control plane, and the following considerations apply to the packet-filter design: o The packet filter MUST only affect BGP traffic specific to the Layer 2 fabric, i.e., traffic forming part of the control plane of the system described, rather than multihop BGP traffic that merely transits. o The packet filter MUST only affect BGP, i.e., TCP port 179. o The packet filter SHOULD make provision for the bidirectional nature of BGP, i.e., sessions may be established in either direction. o The packet filter MUST affect all Address Family Identifiers. Appendix A contains examples of correct packet filters for various platforms. 3.2.2. Hardware Considerations Not all hardware is capable of deploying combined Layer 3/4 filters on Layer 2 ports; even on platforms that claim support for such a feature, limitations may exist or hardware resource allocation failures may occur during filter deployment, which may cause unexpected results. These problems may include: o Platform inability to apply Layer 3/4 filters on ports that already have Layer 2 filters applied. o Layer 3/4 filters supported for IPv4 but not for IPv6. o Layer 3/4 filters supported on physical ports, but not on IEEE 802.1AX Link Aggregate ports [IEEE802.1AX]. o Failure of the Caretaker to apply filters to all IEEE 802.1AX Link Aggregate ports [IEEE802.1AX]. o Limitations in Access Control List (ACL) hardware mechanisms causing filters not to be applied. o Fragmentation of ACL lookup memory causing transient ACL application problems that are resolved after ACL removal/ reapplication. o Temporary service loss during hardware programming. o Reduction in hardware ACL capacity if the platform enables lossless ACL application. It is advisable for the Caretaker to be aware of the limitations of their hardware and to thoroughly test all complicated configurations in advance to ensure that problems don't occur during production deployments. 3.3. Procedural Considerations The Caretaker of the lower-layer network can monitor data-plane traffic (e.g., interface counters) and carry out the maintenance without impact to traffic once session culling is complete. It is recommended that the packet filters be deployed for the duration of the maintenance only and be removed immediately after the maintenance is completed. To prevent unnecessary troubleshooting, it is RECOMMENDED that Caretakers notify the affected Operators before the maintenance takes place and make it explicit that the Involuntary BGP Session Culling methodology will be applied. 4. Security Considerations There are no security considerations. 5. IANA Considerations This document has no actions for IANA. 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <>. [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, <>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <>. 6.2. Informative References [BGP_PIC] Bashandy, A., Ed., Filsfils, C., and P. Mohapatra, "BGP Prefix Independent Convergence", Work in Progress, draft-ietf-rtgwg-bgp-pic-06, November 2017. [IEEE802.1AX] IEEE, "IEEE Standard for Local and metropolitan area networks -- Link Aggregation", IEEE Std 802.1AX-2014, DOI 10.1109/IEEESTD.2014.7055197, December 2014, < opac?punumber=6997981>. [RFC8203] Snijders, J., Heitz, J., and J. Scudder, "BGP Administrative Shutdown Communication", RFC 8203, DOI 10.17487/RFC8203, July 2017, <>. [RFC8326] Francois, P., Ed., Decraene, B., Ed., Pelsser, C., Patel, K., and C. Filsfils, "Graceful BGP Session Shutdown", RFC 8326, DOI 10.17487/8326, March 2018, <>. Appendix A. Example Packet Filters This section includes examples of packet filters performing Involuntary BGP Session Teardown at an IXP using peering LAN prefixes and 2001:db8:2::/64 as its control plane. A repository of configuration examples for a number of assorted platforms can be found at <>. A.1. Example Configuration for Cisco IOS, IOS XR, and Arista EOS ipv6 access-list acl-ipv6-permit-all-except-bgp 10 deny tcp 2001:db8:2::/64 eq bgp 2001:db8:2::/64 20 deny tcp 2001:db8:2::/64 2001:db8:2::/64 eq bgp 30 permit ipv6 any any ! ip access-list acl-ipv4-permit-all-except-bgp 10 deny tcp eq bgp 20 deny tcp eq bgp 30 permit ip any any ! interface Ethernet33 description IXP Participant Affected by Maintenance ip access-group acl-ipv4-permit-all-except-bgp in ipv6 access-group acl-ipv6-permit-all-except-bgp in ! A.2. Example Configuration for Nokia SR OS ip-filter 10 create filter-name "ACL IPv4 Permit All Except BGP" default-action forward entry 10 create match protocol tcp dst-ip src-ip port eq 179 exit action drop exit exit exit ipv6-filter 10 create filter-name "ACL IPv6 Permit All Except BGP" default-action forward entry 10 create match next-header tcp dst-ip 2001:db8:2::/64 src-ip 2001:db8:2::/64 port eq 179 exit action drop exit exit exit interface "port-1/1/1" description "IXP Participant Affected by Maintenance" ingress filter ip 10 filter ipv6 10 exit exit Acknowledgments The authors would like to thank the following people for their contributions to this document: Saku Ytti, Greg Hankins, James Bensley, Wolfgang Tremmel, Daniel Roesen, Bruno Decraene, Tore Anderson, John Heasley, Warren Kumari, Stig Venaas, and Brian Carpenter. Authors' Addresses Will Hargrave LONAP Ltd 5 Fleet Place London EC4M 7RD United Kingdom Email: Matt Griswold 20C 1658 Milwaukee Ave # 100-4506 Chicago, IL 60647 United States of America Email: Job Snijders NTT Communications Theodorus Majofskistraat 100 Amsterdam 1065 SZ The Netherlands Email: Nick Hilliard INEX 4027 Kingswood Road Dublin 24 Ireland Email: