Global Routing Operations J. Snijders
Internet-Draft NTT
Intended status: Informational M. Schmidt
Expires: June 10, 2017 i3D.net
December 7, 2016

Usage of Large BGP Communities
draft-ietf-grow-large-communities-usage-01

Abstract

Examples and inspiration for operators on how to use Large BGP Communities.

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].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 10, 2017.

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Table of Contents

1. Introduction

Large BGP Communities [I-D.ietf-idr-large-community] provide a mechanism to signal opaque information between Autonomous Systems. This document presents a set of examples on how Large BGP Communities could be implemented by an operator to achieve various goals. This document draws from experience in Operational Communities such as NANOG and NLNOG.

The opaque nature of Large BGP Communities allows for rapid deployment of new features or changes to the product. Operators are encouraged to publicly publish an up to date version of their routing policy in which they document what each Large BGP Community means.

2. The Generic Design Pattern

Large BGP Communities are composed of a 4-octet Global Administrator field followed by two 4-octet Local Data fields. The design pattern described in this document uses a "ASN:Function:Parameter"-approach to fill the three fields.

In deployments of both BGP Communities [RFC1997] and Large BGP Communities, two categories of Communities are recognised:

For each context ideas are provided regarding the contents of each of the three fields in Large BGP Communities.

        AS 65551
            |
            ^
            |
        AS 64497
          /  \
         ^    \
        /      ^
   AS 64498     \
       |        | 
       `<->- AS 64499

Throughout the document a topology of four Autonomous Systems is used to illustrate the usage of Communities in the following configuration:

2.1. Informational Communities

Informational Communites serve as markers regarding the origin of the route announcement, the relation with the EBGP neighbor or for instance the intended propagation audience. Informational Communities also assist in network operations such as debugging.

The Global Administrator field is set to the ASN which is marking the routes with the Informational Communities. As an example: on a route which AS 64497 announces to AS 64498, AS 64497 might add the Large BGP Community 64497:100:31 to signal to AS 64498 that the route was learned in the Netherlands.

In general the intended audience of Informational Communities are downstream networks, but any adjacent Autonomous System could benefit from receiving these communities.

2.2. Action Communities

Action Communities are attached to routes to request non-default behaviour in an adjacent Autonomous System. For instance, Action Communities are used to change the route's propagation characteristics, the route's LOCAL_PREF value or the amount of AS_PATH prepends that should be added when exporting or importing a route.

The Global Administrator field is set to the ASN which is expected to perform a non-default action upon receiving the route. For instance, if AS 64499 would want to request AS 64497 to lower the LOCAL_PREFERENCE below the default, AS 64499 could tag the route with 64497:20:50.

In general the intended audience of Action Communities is an upstream provider.

3. Examples of Informational Communities

3.1. Location

AS 64497 can inform its downstream networks about the geographical entity where AS 64497 learned a route by marking the route with Large BGP Communities following one or a combination of the following schemes.

3.1.1. An ISO 3166-1 numeric function

AS 64497 could assign a value of 1 to the first Local Data field to designate the function of the parameter in the second Local Data field as ISO-3166-1 numeric country identifiers.

Information: ISO 3166-1
Large BGP Community Meaning
64497:1:528 Route learned in Netherlands
64497:1:392 Route learned in Japan
64497:1:840 Route learned in United States of America

Example documentation for AS 64497 using Informational Communties describing the origin of routes using ISO 3166-1 numeric identifiers.

3.1.2. An UNSD region function

AS 64497 could assign a value of 2 to the first Local Data field to designate the function of the parameter in the second Local Data field as an identifier for the macro geographical (continental) regions, geographical sub-regions, or selected economic and other groupings following a set of published identifiers by the United Nations Statistics Division.

Information: Regions
Large BGP Community Meaning
64497:2:2 Route learned in Africa
64497:2:9 Route learned in Oceania
64497:2:145 Route learned in Western Asia
64497:2:150 Route learned in Europe

Example documentation for AS 64497 using Informational Communties describing the origin of routes using numeric identifiers provided by the UN Statistics Division.

3.2. Relation

AS 64497 could assign a value of 3 to the first Local Data field to designate that the second Local Data field contains an identifier showing the relation with the EBGP neighbor from whom the route was received.

Information: Relation
Large BGP Community Meaning
64497:3:1 Route learned from a customer
64497:3:2 Route learned from a peering partner
64497:3:3 Route learned from an upstream provider

Example documentation for AS 64497 using Informational Communties describing the relation with the ASN from which the route was received.

3.3. Combining Informational Communities

Multiple Informational Communities can be tagged on a route, for example: a route learned in the Netherlands from a customer can contain both 64497:1:528 and 64497:2:150 and 64497:3:1.

4. Examples of Action Communities

4.1. Selective NO_EXPORT

As part of the commercial agreement between AS 64497 and AS 64498, AS 64497 might offer AS 64498 certain BGP Traffic Engineering features such as selectively not export routes learned from 64498 to certain EBGP neighbors of AS 64497.

4.1.1. Peer ASN Based Selective NO_EXPORT

AS 64497 might assign function identifier 4 to allow preventing propagation of routes to the ASN listed in the second Local Data field.

Action: Peer ASN NO_EXPORT
Large BGP Community Meaning
64497:4:2914 Do not export route to AS 2914
64497:4:7018 Do not export route to AS 7018
64497:4:65551 Do not export route to AS 65551

Example documentation for AS 64497 offering Action Communties to limit propagation of routes based on the Peer ASN described in the third field.

4.1.2. Location Based Selective NO_EXPORT

AS 64497 might assign function identifier 5 to allow its customers to request selectively not exporting routes on EBGP sessions within a certain geographical area. This example follows the ISO 3166-1 numeric encoding.

Action: NO_EXPORT in Region
Large BGP Community Meaning
64497:5:528 Do not export to EBGP neighbors in the Netherlands
64497:5:392 Do not export to EBGP neighbors in Japan
64497:5:840 Do not export to EBGP neighbors in United States of America

Example documentation for AS 64497 offering Action Communties to trigger NO_EXPORT on routes only when propagating the route to a certain geographical region.

4.2. Selective AS_PATH Prepending

As part of the commercial agreement between AS 64497 and AS 64498, AS 64497 might offer AS 64498 certain BGP Traffic Engineering features such as selectively prepending the AS_PATH with 64497's ASN to certain EBGP neighbors of AS 64497.

4.2.1. Peer ASN Based Selective AS_PATH Prepending

AS 64497 might assign function identifier 6 to allow prepending the AS_PATH on propagation of routes to the ASN listed in the second Local Data field.

Action: Prepend to Peer ASN
Large BGP Community Meaning
64497:6:2914 Prepend 64497 once on export to AS 2914
64497:6:7018 Prepend 64497 once on export to AS 7018
64497:6:65551 Prepend 64497 once on export to AS 65551

Example documentation for AS 64497 offering Action Communties to trigger prepending of the AS_PATH only when propagating the route to a certain Peer ASN.

4.2.2. Location Based Selective AS_PATH Prepending

AS 64497 might assign function identifier 7 to allow prepending of the AS_PATH on propagation of routes to on any EBGP neighbor's interconnection in the geographical entity listed in the second Local Data field. This example follows the ISO 3166-1 numeric regions codes in the Local Data 2 field.

Action: Prepend in Region
Large BGP Community Meaning
64497:7:528 Prepend once to EBGP neighbors in the Netherlands
64497:7:392 Prepend once to EBGP neighbors in Japan
64497:7:840 Prepend once to EBGP neighbors in United States of America

Example documentation for AS 64497 offering Action Communties to trigger prepending of the AS_PATH only when propagating the route to a certain geographical region.

4.3. Location based manipulation of LOCAL_PREF

In some cases, it can be desirable for an autonomous system to allow adjacent Autonomous Systems to directly influence the degree of preference associated with a route, usually expressed within the LOCAL_PREF attribute.

Furthermore, in the case of large networks spanning significant geography, it is often also useful to be able to extend this capability and scope its effect to a geographic region. This is a more powerful mechanism than AS_PATH prepending, but since degree of preference determines BGP route selection and thus onward advertisement, it can also be self-limiting in its scope.

Since the LOCAL_PREF attribute which influences degree of preference is locally significant within each autonomous system, it is not usually practical or useful to compare LOCAL_PREF attribute values between autonomous systems. Instead it can be useful to classify the major types of route likely to exist within an autonomous system's routing hierarchy and provide an ability to set one's route to that preference:

  • A qualified customer route. Usually the highest preference.
  • A peer, or network-share, route. A co-operating network provider engaged in a partnership for customer coverage ("peering").
  • A last resort, or backup route.

It is entirely possible that some providers may have more classes of route preference but it is possible to codify both the route preference class and the regional scope within the Local Data fields of the Large Community attribute.

For example, AS64497 might establish the following function identifiers to set route preference class, which could allow pairing with a location or peer-based operand to determine scope.

Action: Preference Function Identifiers
Function Preference Class
10 Qualified customer route. Highest preference.
15 Peering partner. Median preference.
19 Route of last resort. Lowest preference.

Once established, these route preference setting functions can be linked with a scoping operand such as per-peer or per-location based identifiers in order to provide AS64497's customers with a comprehensive and rich toolset to influence route preference.

Action: Regional Preference Communities
Large BGP Community Meaning
64497:15:528 Set as peer route in Netherlands
64497:19:840 Set as backup route in United States of America

Since the degree of preference influences BGP best path selection (which in turn influences onward route propagation) Operators should take special care with a traffic engineering tool such as location based local preference influence (BGP Wedgies [RFC4264]).

5. Security Considerations

Network operators should note the recommendations in Section 11 of BGP Operations and Security [RFC7454].

6. IANA Considerations

None.

7. Acknowledgements

The authors would like to gratefully acknowledge the insightful comments, contributions, critique and support from John Heasley, Adam Chappell and Jonathan Stewart.

8. Normative References

[I-D.ietf-idr-large-community] Heitz, J., Snijders, J., Patel, K., Bagdonas, I. and N. Hilliard, "BGP Large Communities", Internet-Draft draft-ietf-idr-large-community-11, December 2016.
[RFC1997] Chandra, R., Traina, P. and T. Li, "BGP Communities Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4264] Griffin, T. and G. Huston, "BGP Wedgies", RFC 4264, DOI 10.17487/RFC4264, November 2005.
[RFC7454] Durand, J., Pepelnjak, I. and G. Doering, "BGP Operations and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454, February 2015.

Authors' Addresses

Job Snijders NTT Communications Theodorus Majofskistraat 100 Amsterdam, 1065 SZ NL EMail: job@ntt.net
Martijn Schmidt i3D.net Rivium 1e Straat 1 Capelle aan den IJssel, 2909 LE NL EMail: martijnschmidt@i3d.net