draft-ietf-idr-best-external-02.txt   draft-ietf-idr-best-external-03.txt 
Network Working Group P. Marques Network Working Group P. Marques
Internet-Draft R. Fernando Internet-Draft
Intended status: Standards Track E. Chen Intended status: Standards Track R. Fernando
Expires: February 8, 2011 P. Mohapatra Expires: September 10, 2011 E. Chen
P. Mohapatra
Cisco Systems Cisco Systems
August 7, 2010 March 9, 2011
Advertisement of the best external route in BGP Advertisement of the best external route in BGP
draft-ietf-idr-best-external-02.txt draft-ietf-idr-best-external-03.txt
Abstract Abstract
The base BGP specifications prevent a BGP speaker from advertising The base BGP specifications prevent a BGP speaker from advertising
any route that is not the best route for a BGP destination. This any route that is not the best route for a BGP destination. This
document specifies a modification of this rule. Routes are divided document specifies a modification of this rule. Routes are divided
into two categories, "external" and "internal". A specification is into two categories, "external" and "internal". A specification is
provided for choosing a "best external route" (for a particular value provided for choosing a "best external route" (for a particular value
of the Network Layer Reachability Information). A BGP speaker is of the Network Layer Reachability Information). A BGP speaker is
then allowed to advertise its "best external route" to its internal then allowed to advertise its "best external route" to its internal
BGP peers, even if that is not the best route for the destination. BGP peers, even if that is not the best route for the destination.
The document explains why advertising the best external route can The document explains why advertising the best external route can
improve convergence time without causing routing loops. Additional improve convergence time without causing routing loops. Additional
benefits include reduction of inter-domain churn and avoidance of benefits include reduction of inter-domain churn and avoidance of
permanent route oscillation. The document also generalizes the permanent route oscillation. The document also generalizes the
notions of "internal" and "external" so that they can be applied to notions of "internal" and "external" so that they can be applied to
Route Reflector Clusters and Autonomous System Confederations. Route Reflector Clusters and Autonomous System Confederations.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF). Note that other groups may also distribute
other groups may also distribute working documents as Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at This Internet-Draft will expire on September 10, 2011.
http://www.ietf.org/ietf/1id-abstracts.txt.
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This Internet-Draft will expire on February 8, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the BSD License. described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this 10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
skipping to change at page 3, line 16 skipping to change at page 3, line 16
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Algorithm for selection of best external route . . . . . . . . 5 2. Algorithm for selection of best external route . . . . . . . . 5
3. Advertisement Rules . . . . . . . . . . . . . . . . . . . . . 6 3. Advertisement Rules . . . . . . . . . . . . . . . . . . . . . 6
4. Consistency between routing and forwarding . . . . . . . . . . 6 4. Consistency between routing and forwarding . . . . . . . . . . 6
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Fast Connectivity Restoration . . . . . . . . . . . . . . 8 5.1. Fast Connectivity Restoration . . . . . . . . . . . . . . 8
5.2. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . 9 5.2. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . 9
5.3. Reducing Persistent IBGP oscillation . . . . . . . . . . . 9 5.3. Reducing Persistent IBGP oscillation . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 6. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 9 9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
The base BGP specifications prevent a BGP speaker from advertising The base BGP specifications prevent a BGP speaker from advertising
any route that is not the best route for a BGP destination. This any route that is not the best route for a BGP destination. This
document specifies a modification of this rule. Routes are divided document specifies a modification of this rule. Routes are divided
into two categories, "external" and "internal". A specification is into two categories, "external" and "internal". A specification is
provided for choosing a "best external route" (for a particular value provided for choosing a "best external route" (for a particular value
of the Network Layer Reachability Information). A BGP speaker is of the Network Layer Reachability Information). A BGP speaker is
skipping to change at page 9, line 31 skipping to change at page 9, line 31
a circular dependency between paths occurs such that the selection of a circular dependency between paths occurs such that the selection of
a new best path by a router, in response to a received IBGP a new best path by a router, in response to a received IBGP
advertisement, causes the withdrawal of information that another advertisement, causes the withdrawal of information that another
router depends on in order to generate the original event. router depends on in order to generate the original event.
In vanilla BGP, when only the best overall route is advertised, as in In vanilla BGP, when only the best overall route is advertised, as in
most implementations, oscillation can occur whenever there are 2 or most implementations, oscillation can occur whenever there are 2 or
clusters/sub-ASes such that at least one cluster has more than one clusters/sub-ASes such that at least one cluster has more than one
path that can potentially contribute to the dependency. path that can potentially contribute to the dependency.
6. Acknowledgments 6. Deployment Considerations
The mechanism specified in the draft allows a BGP speaker to
advertise a route that is not the best route used for forwarding.
This is a departure from the current behavior. However, consistency
in the path selection process across the AS is still guaranteed since
the ingress routers will not choose the best-external route as the
best route for a destination in steady state (for the same reason
that the BGP speaker announcing the best-external route chose an IBGP
route as best instead of the externally learnt route). Though it is
possible to alter this assurance by defining route policies on IBGP
sessions, use of such policies in IBGP is not recommended, especially
with best-external announcement turned on in the network. It is also
worth noting that such inconsistency in routing and forwarding is
mitigated in a tunneled network.
7. Acknowledgments
This document greatly benefits from the comments of Yakov Rekhter, This document greatly benefits from the comments of Yakov Rekhter,
John Scudder, Eric Rosen, and Jenny Yuan. John Scudder, Eric Rosen, Jenny Yuan, and Jay Borkenhagen.
7. IANA Considerations 8. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
8. Security Considerations 9. Security Considerations
There are no additional security risks introduced by this design. There are no additional security risks introduced by this design.
9. Normative References 10. Normative References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 [RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995. (BGP-4)", RFC 1771, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route "Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, August 2002. Oscillation Condition", RFC 3345, August 2002.
skipping to change at page 10, line 25 skipping to change at page 10, line 38
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006. (IBGP)", RFC 4456, April 2006.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007. System Confederations for BGP", RFC 5065, August 2007.
Authors' Addresses Authors' Addresses
Pedro Marques Pedro Marques
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Phone: Email: pedro.r.marques@gmail.com
Email: roque@cisco.com
Rex Fernando Rex Fernando
Cisco Systems Cisco Systems
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone:
Email: rex@cisco.com Email: rex@cisco.com
Enke Chen Enke Chen
Cisco Systems Cisco Systems
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone:
Email: enkechen@cisco.com Email: enkechen@cisco.com
Pradosh Mohapatra Pradosh Mohapatra
Cisco Systems Cisco Systems
170 W. Tasman Drive 170 W. Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone:
Email: pmohapat@cisco.com Email: pmohapat@cisco.com
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