Network Working Group                                       P. Mohapatra
Internet-Draft                                          Sproute Networks
Intended status: Standards Track                             R. Fernando
Expires: September 6, 2018                                 Cisco Systems
                                                           March 5, 2018

                 BGP Link Bandwidth Extended Community


   This document describes an application of BGP extended communities
   that allows a router to perform unequal cost load balancing.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   material may not have granted the IETF Trust the right to allow
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Link Bandwidth Extended Community . . . . . . . . . . . . . .   2
   3.  Deployment Considerations . . . . . . . . . . . . . . . . . .   3
   4.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   When a BGP speaker receives multiple paths from its internal peers,
   it could select more than one path to send traffic to.  In doing so,
   it might be useful to provide the speaker with information that would
   help it distribute the traffic based on the bandwidth of the external
   (DMZ) link.  This document suggests that the external link bandwidth
   be carried in the network using a new extended community [RFC4360]  -
   the link bandwidth extended community.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Link Bandwidth Extended Community

   When a BGP speaker receives a route from an external neighbor and
   advertises this route (via IBGP) to internal neighbors, as part of
   this advertisement the router may carry the cost to reach the
   external neighbor.  The cost can be either configured per neighbor or
   derived from the bandwidth of the link that connects the router to a
   directly connected external neighbor.  This value is carried in the

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   Link Bandwidth Extended Community.  No more than one link bandwidth
   extended community SHALL be attached to a route.  Additionally, if a
   route is received with link bandwidth extended community and the BGP
   speaker sets itself as next-hop while announcing that route to other
   peers, the link bandwidth extended community should be removed.

   The extended community is optional non-transitive.  The value of the
   high-order octet of the extended Type Field is 0x40.  The value of
   the low-order octet of the extended type field for this community is
   0x04.  The value of the Global Administrator subfield in the Value
   Field SHOULD represent the Autonomous System of the router that
   attaches the Link Bandwidth Community.  If four octet AS numbering
   scheme is used [RFC6793], AS_TRANS should be used in the Global
   Administrator subfield.  The bandwidth of the link is expressed as 4
   octets in IEEE floating point format, units being bytes (not bits!)
   per second.  It is carried in the Local Administrator subfield of the
   Value Field.

3.  Deployment Considerations

   The usage of this community is restricted to the cases where BGP
   multipath can be safely deployed.  If the path between the load
   sharing router and the exit point is not tunneled, then the IGP
   distance between the load balancing router and the exit points should
   be the same.

   If the path between the load sharing router and the exit point is
   tunneled, then the choice to use this community is a purely local
   matter to the load sharing router.

   In the context of BGP/MPLS VPNs [RFC4364], link bandwidth community
   could be used to support inbound load balancing for multihomed sites,
   as follows.  Consider a site that is connected to PE1 and PE2.  Both
   PE1 and PE2 would advertise VPN-IP routes associated with the
   destinations within the site.  One way to enable other PEs to receive
   all these routes is to require the RD of the routes advertised by PE1
   to be different from the RD of the routes advertised by PE2.  The
   VPN-IP routes advertised by PE1 should carry the link bandwidth
   community; likewise for the VPN-IP routes advertised by PE2.  The
   bandwidth value carried in the community could be locally determined
   by PE1 and PE2.  Alternatively CEs of the site, when advertising IP
   routes to PE1 and PE2, could add the link bandwith community to these
   advertisements, in which case PE1 and PE2, when originating VPN-IP
   routes, would use the bandwidth value from the IP routes they
   received from the CEs to construct the link bandwidth community
   carried by these VPN-IP routes.

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   An ingress PE, when sending traffic to destinations within the site,
   can use the bandwidth value carried in the community of the routes
   advertised by PE1 and PE2 to perform load sharing, where some of the
   traffic would go via PE1, while other traffic would go via PE2.

   If there are multiple paths to reach a destination and if only some
   of them have link bandwidth community, the load sharing router should
   not perform unequal cost load balancing based on link bandwidths.

4.  Acknowledgments

   The authors would like to thank Yakov Rekhter, Srihari Sangli and Dan
   Tappan for proposing unequal cost load balancing as one possible
   application of the extended community attribute.

   The authors would like to thank Bruno Decraene, Robert Raszuk, Joel
   Halpern, Aleksi Suhonen, Randy Bush, and John Scudder for their
   comments and contributions.

5.  IANA Considerations

   This document defines a specific application of the two-octet AS
   specific extended community.  IANA is requested to assign a sub- type
   value of 0x04 for the link bandwidth extended community.

       Name                                           Value
       ----                                           -----
       non-transitive Link Bandwidth Ext. Community  0x4004

6.  Security Considerations

   There are no additional security risks introduced by this design.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <>.

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   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,

Authors' Addresses

   Pradosh Mohapatra
   Sproute Networks


   Rex Fernando
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134


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