Using a Link State Advertisement (LSA) Options Bit to Prevent Looping in BGP/MPLS IP Virtual Private Networks (VPNs)
RFC 4576

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Network Working Group                                           E. Rosen
Request for Comments: 4576                                     P. Psenak
Category: Standards Track                              P. Pillay-Esnault
                                                     Cisco Systems, Inc.
                                                               June 2006

         Using a Link State Advertisement (LSA) Options Bit to
     Prevent Looping in BGP/MPLS IP Virtual Private Networks (VPNs)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies a procedure that deals with a particular
   issue that may arise when a Service Provider (SP) provides "BGP/MPLS
   IP VPN" service to a customer and the customer uses OSPFv2 to
   advertise its routes to the SP.  In this situation, a Customer Edge
   (CE) Router and a Provider Edge (PE) Router are OSPF peers, and
   customer routes are sent via OSPFv2 from the CE to the PE.  The
   customer routes are converted into BGP routes, and BGP carries them
   across the backbone to other PE routers.  The routes are then
   converted back to OSPF routes sent via OSPF to other CE routers.  As
   a result of this conversion, some of the information needed to
   prevent loops may be lost.  A procedure is needed to ensure that once
   a route is sent from a PE to a CE, the route will be ignored by any
   PE that receives it back from a CE.  This document specifies the
   necessary procedure, using one of the options bits in the LSA (Link
   State Advertisements) to indicate that an LSA has already been
   forwarded by a PE and should be ignored by any other PEs that see it.

Rosen, et al.               Standards Track                     [Page 1]
RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 2006

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................3
   3. Information Loss and Loops ......................................3
   4. Using the LSA Options to Prevent Loops ..........................4
   5. Security Considerations .........................................5
   6. Acknowledgements ................................................5
   7. Normative References ............................................6

1.  Introduction

   [VPN] describes a method by which a Service Provider (SP) can use its
   IP backbone to provide an "IP VPN" service to customers.  In that
   sort of service, a customer's edge devices (CE devices) are connected
   to the provider's edge routers (PE routers).  Each CE device is in a
   single Virtual Private Network (VPN).  Each PE device may attach to
   multiple CEs of the same or of different VPNs.  A VPN thus consists
   of a set of "network segments" connected by the SP's backbone.

   A CE exchanges routes with a PE, using a routing protocol to which
   the customer and the SP jointly agree.  The PE runs that routing
   protocol's decision process (i.e., it performs the routing
   computation) to determine the set of IP address prefixes for which
   the following two conditions hold:

      -  Each address prefix in the set can be reached via that CE.

      -  The path from that CE to each such address prefix does NOT
         include the SP backbone (i.e., it does not include any PE
         routers).

   The PE routers that attach to a particular VPN redistribute routes to
   these address prefixes into BGP, so that they can use BGP to
   distribute the VPN's routes to each other.  BGP carries these routes
   in the "VPN-IPv4 address family", so that they are distinct from
   ordinary Internet routes.  The VPN-IPv4 address family also extends
   the IP addresses on the left so that address prefixes from two
   different VPNs are always distinct to BGP, even if both VPNs use the
   same piece of the private RFC 1918 address space.  Thus, routes from
   different VPNs can be carried by a single BGP instance and can be
   stored in a common BGP table without fear of conflict.

   If a PE router receives a particular VPN-IPv4 route via BGP, and if
   that PE is attached to a CE in the VPN to which the route belongs,
   then BGP's decision process may install that route in the BGP route
   table.  If so, the PE translates the route back into an IP route and

Rosen, et al.               Standards Track                     [Page 2]
RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 2006

   redistributes it to the routing protocol that is running on the link
   to that CE.