Global Routing Operations W. Hargrave
Internet-Draft LONAP
Intended status: Best Current Practice M. Griswold
Expires: October 8, 2017 20C
J. Snijders
N. Hilliard
April 6, 2017

Mitigating Negative Impact of Maintenance through BGP Session Culling


This document outlines an approach to mitigate 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 affected by the maintenance are forcefully torn down before the actual maintenance activities commence.

Status of This Memo

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Table of Contents

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, which otherwise would affect the flow of data between the BGP speakers.

BGP Session Culling ensures that lower layer network maintenance activities cause the minimum possible amount of disruption, by causing BGP speakers to preemptively gracefully 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 governed by the BGP Hold Timer (section 6.5 of [RFC4271]), commonly between 90 and 180 seconds, The second value is implementation specific, but 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 the operator 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, but instead monitor forwarding plane activity while the culling is taking place and consider it complete once traffic levels have dropped to a minimum [Procedural_Considerations].

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. BGP Session Culling

From the viewpoint of the IP network operator, there are two types of BGP Session Culling:

Voluntary BGP Session Teardown:
The operator initiates the tear down of the potentially affected BGP session by issuing an Administrative Shutdown.
Involuntary BGP Session Teardown:
The caretaker of the lower layer network disrupts BGP control-plane traffic in the upper layer, 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 which can cause disruption to the flow of data through the lower layer network, an operator can reduce loss of traffic by issuing an Administratively 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 PIC [I-D.ietf-rtgwg-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 could consider to use [Graceful Shutdown] to facilitate smooth drainage of traffic prior to session tear down, and the Shutdown Communication [I-D.ietf-idr-shutdown] 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 many network operators are unable to carry out the Voluntary BGP Session Teardown recommendations, because of the operational cost and risk of co-ordinating 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 densely meshed multi-node layer-2 fabric, the caretaker of the fabric could opt to cull BGP sessions on behalf of the stakeholders connected to the fabric.

Such culling of control-plane traffic will pre-empt 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 through the application of a combined layer-3 and layer-4 packet filter deployed in the switched fabric itself.

3.2.1. Packet Filter Considerations

The following considerations apply to the packet filter design: Appendix A contains examples of correct packet filters for various platforms.

3.2.2. Hardware Considerations

Not all hardware is capable of deploying layer 3 / layer 4 filters on layer 2 ports, and even on platforms which support the feature, documented limitations may exist or hardware resource allocation failures may occur during filter deployment which may cause unexpected results. These problems may include:

It is advisable for the operator 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 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 are only deployed for the duration of the maintenance and immediately removed after the maintenance. To prevent unnecessarily 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. 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 Rösen, Bruno Decraene, and Tore Anderson.

5. Security Considerations

There are no security considerations.

6. IANA Considerations

This document has no actions for IANA.

7. References

7.1. 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., Li, T. and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006.

7.2. Informative References

[I-D.ietf-idr-shutdown] Snijders, J., Heitz, J. and J. Scudder, "BGP Administrative Shutdown Communication", Internet-Draft draft-ietf-idr-shutdown-07, March 2017.
[I-D.ietf-rtgwg-bgp-pic] Bashandy, A., Filsfils, C. and P. Mohapatra, "BGP Prefix Independent Convergence", Internet-Draft draft-ietf-rtgwg-bgp-pic-01, June 2016.

Appendix A. Example packet filters

Example packet filters for "Involuntary BGP Session Teardown" at an IXP with LAN prefixes and 2001:db8:2::/64.

A repository of configuration examples for a number of assorted platforms can be found at

A.1. Cisco IOS, IOS XR & Arista EOS Firewall Example Configuration

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. Nokia SR OS Filter Example Configuration

ip-filter 10 create
    filter-name "ACL IPv4 Permit All Except BGP"
    default-action forward
    entry 10 create
        match protocol tcp
            port eq 179

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

interface "port-1/1/1"
    description "IXP Participant Affected by Maintenance"
        filter ip 10
        filter ipv6 10

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: