Internet DRAFT - draft-zhang-idr-transitive-gr
draft-zhang-idr-transitive-gr
Network Working Group H. Zhang
Internet-Draft HangZhou H3C Co. Limited
Updates: RFC 4724 (if approved) A. Retana
Intended status: Standards Track Cisco Systems, Inc.
Expires: September 26, 2013 March 25, 2013
Transitive BGP Graceful Restart
draft-zhang-idr-transitive-gr-02
Abstract
This document defines an extension to BGP Graceful Restart that
reduces the negative impact of multiple inter-connected routers
restarting. The proposed mechanism does not require any changes to
the BGP protocol.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 18, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 5
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1. Introduction
The BGP Graceful Restart [RFC4724] process defines a mechanism that a
restarting router can use with its non-restarting peers. The
existence of other restarting routers results in the use of the base
route exchange mechanism [RFC4271] with them, even if the forwarding
state has indeed been preserved for (and by) those peers during the
restart. As a result, traffic forwarding between restarting routers
is disrupted.
This document defines an extension to BGP Graceful Restart that
reduces the negative impact of multiple inter-connected restarting
routers. The proposed mechanism does not require any changes to the
BGP protocol.
The current process [RFC4724] states that routes from restarting
peers are to be removed from the local forwarding state when the non-
restarting peers converge (the End-of-RIB marker is received from all
of them). Assuming a simple topology:
NR1 - R2 - R3 - NR2
where NRx are non-restarting routers, Rx are restarting routers
and the lines between them represent BGP sessions.
There are two types of routes affected (from R2's point of view) by
the current process:
1. Routes that are only reachable through R3. These routes will be
removed from the forwarding table when the non-restarting routers
converge, and installed back in when the convergence with R3 is
done.
2. Routes that are reachable through both R3 and NR1. These routes
will first change to NR1 when the non-restarting routers
converge, and later back to R3 (assuming that is in fact still
the preferred path).
Both types can clearly cause disruption in traffic forwarding, micro-
loops, traffic loss, etc.
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 [RFC2119].
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3. Proposed Solution
The extension proposed to BGP Graceful Restart to accommodate for
multiple restarting routers, when the forwarding state has been
preserved between them, is simply to delay sending the End-of-RIB
marker to non-restarting routers.
Specifically, to allow a restarting router the ability to reduce the
impact due to other restarting routers, the following paragraph is
added as the fifth one in section 4.1 (Procedures for the Restarting
Speaker) [RFC4724]:
Before updating the corresponding forwarding states, the
Restarting Speaker MAY start a path calculation after all non-
retarting peers's End-Of-RIB marker have been received, and
advertise the Adj-RIB-Out to its restarting peers (ones with the
"Restart State" bit set in the received capability), including the
End-of-RIB marker, and wait for the corresponding End-of-RIB
marker from them.
In order to maintain the transitive property when more than two BGP
speakers peering with each other restart, the following paragraph is
added as the sixth one in section 4.1 (Procedures for the Restarting
Speaker) [RFC4724]:
If the Restarting Speaker has multiple restarting peers, sending
the End-of-RIB marker SHOULD be delayed until all the markers from
those restarting peers have been received. The BGP speaker on a
given connection SHOULD send its End-of-RIB marker if the pair
hasn't sent or received UPDATES for a locally configured time
period (which SHOULD be significantly less than the
Selection_Deferral_Timer).
During the recovery period of multiple restarting routers, a BGP
speaker may advertise routing information that is not being used at
the time. Because the forwarding state of the speakers remains
unchanged (from that at the restart), it is clear that this
transitive property of sharing routing information between restarting
routers doesn't cause any issues in the actual forwarding of traffic.
Furthermore, it has the advantage if avoiding further disruptions in
the forwarding of traffic through the restarting routers.
4. Security Considerations
This document proposes an extension to an existing mechanism. The
same security considerations explained there apply to this extension.
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The propagation of routing information that is not in use may cause
forwarding loops and an inconsistent state in a network. However,
the risk in this document is mitigated by the fact that the
information is validated by all peers once the convergence process
completes.
5. IANA Considerations
This document has no IANA actions.
6. Acknowledgements
The authors would like to thank Enke Chen, John Scudder, Robert
Raszuk and Abhay Roy for their feedback.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
January 2007.
7.2. Informative References
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
Authors' Addresses
Haifeng Zhang
HangZhou H3C Co. Limited
310 Liuhe Road, Zhijiang Science Park
Hangzhou
P.R. China
Email: zhanghf@h3c.com
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Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Email: aretana@cisco.com
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