Internet DRAFT - draft-ietf-grow-bgp-gshut
draft-ietf-grow-bgp-gshut
Network Working Group P. Francois, Ed.
Internet-Draft Individual Contributor
Intended status: Standards Track B. Decraene, Ed.
Expires: June 17, 2018 Orange
C. Pelsser
Strasbourg University
K. Patel
Arrcus, Inc.
C. Filsfils
Cisco Systems
December 14, 2017
Graceful BGP session shutdown
draft-ietf-grow-bgp-gshut-13
Abstract
This draft standardizes a new well-known BGP community
GRACEFUL_SHUTDOWN to signal the graceful shutdown of paths. This
draft also describes operational procedures which use this community
to reduce the amount of traffic lost when BGP peering sessions are
about to be shut down deliberately, e.g. for planned maintenance.
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
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This Internet-Draft will expire on June 17, 2018.
Copyright Notice
Copyright (c) 2017 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Packet loss upon manual EBGP session shutdown . . . . . . . . 3
4. EBGP graceful shutdown procedure . . . . . . . . . . . . . . 4
4.1. Pre-configuration . . . . . . . . . . . . . . . . . . . . 4
4.2. Operations at maintenance time . . . . . . . . . . . . . 4
4.3. BGP implementation support for graceful shutdown . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Alternative techniques with limited applicability . 7
A.1. Multi Exit Discriminator tweaking . . . . . . . . . . . . 7
A.2. IGP distance Poisoning . . . . . . . . . . . . . . . . . 7
Appendix B. Configuration Examples . . . . . . . . . . . . . . . 7
B.1. Cisco IOS XR . . . . . . . . . . . . . . . . . . . . . . 7
B.2. BIRD . . . . . . . . . . . . . . . . . . . . . . . . . . 8
B.3. OpenBGPD . . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix C. Beyond EBGP graceful shutdown . . . . . . . . . . . 9
C.1. IBGP graceful shutdown . . . . . . . . . . . . . . . . . 9
C.2. EBGP session establishment . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Routing changes in BGP can be caused by planned maintenance
operations. This document defines a well-known community [RFC1997],
called GRACEFUL_SHUTDOWN, for the purpose of reducing the management
overhead of gracefully shutting down BGP sessions. The well-known
community allows implementers to provide an automated graceful
shutdown mechanism that does not require any router reconfiguration
at maintenance time.
This document discusses operational procedures to be applied in order
to reduce or eliminate loss of packets during a maintenance
operation. Loss comes from transient lack of reachability during BGP
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convergence which follows the shutdown of an EBGP peering session
between two Autonomous System Border Routers (ASBR).
This document presents procedures for the cases where the forwarding
plane is impacted by the maintenance, hence when the use of Graceful
Restart does not apply.
The procedures described in this document can be applied to reduce or
avoid packet loss for outbound and inbound traffic flows initially
forwarded along the peering link to be shut down. These procedures
trigger, in both Autonomous Sytems (AS), rerouting to alternate paths
if they exist within the AS, while allowing the use of the old path
until alternate ones are learned. This ensures that routers always
have a valid route available during the convergence process.
The goal of the document is to meet the requirements described in
[RFC6198] at best, without changing the BGP protocol.
Other maintenance cases, such as the shutdown of an IBGP session or
the establishement of an EBGP session, are out of scope of this
document. For information, they are briefly discussed in Appendix C.
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
[BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Terminology
graceful shutdown initiator: a router on which the session shutdown
is performed for the maintenance.
graceful shutdown receiver: a router that has a BGP session, to be
shutdown, with the graceful shutdown initiator.
3. Packet loss upon manual EBGP session shutdown
Packets can be lost during the BGP convergence following a manual
shutdown of an EBGP session for two reasons.
First, some routers can have no path toward an affected prefix, and
drop traffic destined to this prefix. This is because alternate
paths can be hidden by nodes of an AS. This happens when [RFC7911]
is not used and the paths are not selected as best by the ASBR that
receive them on an EBGP session, or by Route Reflectors that do not
propagate them further in the IBGP topology because they do not
select them as best.
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Second, the FIB can be inconsistent between routers within the AS,
and packets toward affected prefixes can loop and be dropped unless
encapsulation is used within the AS.
This document only addresses the first reason.
4. EBGP graceful shutdown procedure
This section describes configurations and actions to be performed for
the graceful shutdown of EBGP peering links.
The goal of this procedure is to retain the paths to be shutdown
between the peers, but with a lower LOCAL_PREF value, allowing the
paths to remain in use while alternate paths are selected and
propagated, rather than simply withdrawing the paths. The LOCAL_PREF
value SHOULD be lower than any of the alternative paths. The
RECOMMENDED value is 0.
Note that some alternative techniques with limited applicability are
discussed for information in Appendix A.
4.1. Pre-configuration
On each ASBR supporting the graceful shutdown receiver procedure, an
inbound BGP route policy is applied on all EBGP sessions of the ASBR,
that:
o matches the GRACEFUL_SHUTDOWN community.
o sets the LOCAL_PREF attribute of the paths tagged with the
GRACEFUL_SHUTDOWN community to a low value.
For information purpose, example of configurations are provided in
Appendix B.
4.2. Operations at maintenance time
On the graceful shutdown initiator, at maintenance time, the
operator:
o applies an outbound BGP route policy on the EBGP session to be
shutdown. This policy tags the paths propagated over the session
with the GRACEFUL_SHUTDOWN community. This will trigger the BGP
implementation to re-advertise all active routes previously
advertised, and tag them with the GRACEFUL_SHUTDOWN community.
o applies an inbound BGP route policy on the EBGP session to be
shutdown. This policy tags the paths received over the session
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with the GRACEFUL_SHUTDOWN community and sets LOCAL_PREF to a low
value.
o wait for route readvertisement over the EBGP session, and BGP
routing convergence on both ASBRs.
o shutdown the EBGP session, optionally using [RFC8203] to
communicate the reason of the shutdown.
In the case of a shutdown of the whole router, in addition to the
graceful shutdown of all EBGP sessions, there is a need to gracefully
shutdown the routes originated by this router (e.g, BGP aggregates
redistributed from other protocols, including static routes). This
can be performed by tagging these routes with the GRACEFUL_SHUTDOWN
community and setting LOCAL_PREF to a low value.
4.3. BGP implementation support for graceful shutdown
BGP Implementers SHOULD provide configuration knobs that utilize the
GRACEFUL_SHUTDOWN community to drain BGP neighbors in preparation of
impending neighbor shutdown. Implementation details are outside the
scope of this document.
5. IANA Considerations
The IANA has assigned the community value 0xFFFF0000 to the planned-
shut community in the "BGP Well-known Communities" registry. IANA is
requested to change the name planned-shut to GRACEFUL_SHUTDOWN and
set this document as the reference.
6. Security Considerations
By providing the graceful shutdown service to a neighboring AS, an
ISP provides means to this neighbor and possibly its downstream ASes
to lower the LOCAL_PREF value assigned to the paths received from
this neighbor.
The neighbor could abuse the technique and do inbound traffic
engineering by declaring some prefixes as undergoing a maintenance so
as to switch traffic to another peering link.
If this behavior is not tolerated by the ISP, it SHOULD monitor the
use of the graceful shutdown community.
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7. Acknowledgments
The authors wish to thank Olivier Bonaventure, Pradosh Mohapatra, Job
Snijders, John Heasley, and Christopher Morrow for their useful
comments.
8. References
8.1. Normative References
[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<https://www.rfc-editor.org/info/rfc1997>.
[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>.
[RFC6198] Decraene, B., Francois, P., Pelsser, C., Ahmad, Z.,
Elizondo Armengol, A., and T. Takeda, "Requirements for
the Graceful Shutdown of BGP Sessions", RFC 6198,
DOI 10.17487/RFC6198, April 2011,
<https://www.rfc-editor.org/info/rfc6198>.
[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>.
8.2. Informative References
[I-D.ietf-idr-best-external]
Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
Gredler, "Advertisement of the best external route in
BGP", draft-ietf-idr-best-external-05 (work in progress),
January 2012.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC8203] Snijders, J., Heitz, J., and J. Scudder, "BGP
Administrative Shutdown Communication", RFC 8203,
DOI 10.17487/RFC8203, July 2017,
<https://www.rfc-editor.org/info/rfc8203>.
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Appendix A. Alternative techniques with limited applicability
A few alternative techniques have been considered to provide graceful
shutdown capabilities but have been rejected due to their limited
applicability. This section describes them for possible reference.
A.1. Multi Exit Discriminator tweaking
The MED attribute of the paths to be avoided can be increased so as
to force the routers in the neighboring AS to select other paths.
The solution only works if the alternate paths are as good as the
initial ones with respect to the LOCAL_PREF value and the AS Path
Length value. In the other cases, increasing the MED value will not
have an impact on the decision process of the routers in the
neighboring AS.
A.2. IGP distance Poisoning
The distance to the BGP NEXT_HOP corresponding to the maintained
session can be increased in the IGP so that the old paths will be
less preferred during the application of the IGP distance tie-break
rule. However, this solution only works for the paths whose
alternates are as good as the old paths with respect to their
LOCAL_PREF value, their AS Path length, and their MED value.
Also, this poisoning cannot be applied when BGP NEXT_HOP self is used
as there is no BGP NEXT_HOP specific to the maintained session to
poison in the IGP.
Appendix B. Configuration Examples
This appendix is non-normative.
Example routing policy configurations to honor the GRACEFUL_SHUTDOWN
well-known BGP community.
B.1. Cisco IOS XR
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community-set comm-graceful-shutdown
65535:0
end-set
!
route-policy AS64497-ebgp-inbound
! normally this policy would contain much more
if community matches-any comm-graceful-shutdown then
set local-preference 0
endif
end-policy
!
router bgp 64496
neighbor 2001:db8:1:2::1
remote-as 64497
address-family ipv6 unicast
send-community-ebgp
route-policy AS64497-ebgp-inbound in
!
!
!
B.2. BIRD
function honor_graceful_shutdown() {
if (65535, 0) ~ bgp_community then {
bgp_local_pref = 0;
}
}
filter AS64497_ebgp_inbound
{
# normally this policy would contain much more
honor_graceful_shutdown();
}
protocol bgp peer_64497_1 {
neighbor 2001:db8:1:2::1 as 64497;
local as 64496;
import keep filtered;
import filter AS64497_ebgp_inbound;
}
B.3. OpenBGPD
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AS 64496
router-id 192.0.2.1
neighbor 2001:db8:1:2::1 {
remote-as 64497
}
# normally this policy would contain much more
match from any community GRACEFUL_SHUTDOWN set { localpref 0 }
Appendix C. Beyond EBGP graceful shutdown
C.1. IBGP graceful shutdown
For the shutdown of an IBGP session, provided the IBGP topology is
viable after the maintenance of the session, i.e, if all BGP speakers
of the AS have an IBGP signaling path for all prefixes advertised on
this graceful shutdown IBGP session, then the shutdown of an IBGP
session does not lead to transient unreachability. As a consequence,
no specific graceful shutdown action is required.
C.2. EBGP session establishment
We identify two potential causes for transient packet losses upon the
establishment of an EBGP session. The first one is local to the
startup initiator, the second one is due to the BGP convergence
following the injection of new best paths within the IBGP topology.
C.2.1. Unreachability local to the ASBR
An ASBR that selects as best a path received over a newly established
EBGP session may transiently drop traffic. This can typically happen
when the NEXT_HOP attribute differs from the IP address of the EBGP
peer, and the receiving ASBR has not yet resolved the MAC address
associated with the IP address of that "third party" NEXT_HOP.
A BGP speaker implementation MAY avoid such losses by ensuring that
"third party" NEXT_HOPs are resolved before installing paths using
these in the RIB.
Alternatively, the operator (script) MAY ping third party NEXT_HOPs
that are expected to be used before establishing the session. By
proceeding like this, the MAC addresses associated with these third
party NEXT_HOPs are resolved by the startup initiator.
C.2.2. IBGP convergence
During the establishment of an EBGP session, in some corner cases a
router may have no path toward an affected prefix, leading to loss of
connectivity.
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A typical example for such transient unreachability for a given
prefix is the following:
Let's consider three Route Reflectors (RR): RR1, RR2, RR3. There is
a full mesh of IBGP sessions between them.
1. RR1 is initially advertising the current best path to the
members of its IBGP RR full-mesh. It propagated that path within
its RR full-mesh. RR2 knows only that path toward the prefix.
2. RR3 receives a new best path originated by the startup
initiator, being one of its RR clients. RR3 selects it as best,
and propagates an UPDATE within its RR full-mesh, i.e., to RR1 and
RR2.
3. RR1 receives that path, reruns its decision process, and picks
this new path as best. As a result, RR1 withdraws its previously
announced best-path on the IBGP sessions of its RR full-mesh.
4. If, for any reason, RR3 processes the withdraw generated in
step 3, before processing the update generated in step 2, RR3
transiently suffers from unreachability for the affected prefix.
The use of [RFC7911] or [I-D.ietf-idr-best-external] among the RR of
the IBGP full-mesh can solve these corner cases by ensuring that
within an AS, the advertisement of a new route is not translated into
the withdraw of a former route.
Indeed, "best-external" ensures that an ASBR does not withdraw a
previously advertised (EBGP) path when it receives an additional,
preferred path over an IBGP session. Also, "best-intra-cluster"
ensures that a RR does not withdraw a previously advertised (IBGP)
path to its non clients (e.g. other RRs in a mesh of RR) when it
receives a new, preferred path over an IBGP session.
Authors' Addresses
Pierre Francois (editor)
Individual Contributor
Email: pfrpfr@gmail.com
Bruno Decraene (editor)
Orange
Email: bruno.decraene@orange.com
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Cristel Pelsser
Strasbourg University
Email: pelsser@unistra.fr
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Clarence Filsfils
Cisco Systems
Email: cfilsfil@cisco.com
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