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
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
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
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.
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
Voluntary BGP Session Teardown: The Operator initiates the teardown
of the potentially affected BGP session by issuing an
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
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
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
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
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
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
o The packet filter MUST affect all Address Family Identifiers.
Appendix A contains examples of correct packet filters for various
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/
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
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.
Appendix A. Example Packet Filters
This section includes examples of packet filters performing
Involuntary BGP Session Teardown at an IXP using peering LAN prefixes
192.0.2.0/24 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 192.0.2.0/24 eq bgp 192.0.2.0/24
20 deny tcp 192.0.2.0/24 192.0.2.0/24 eq bgp
30 permit ip any any
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
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
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