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Network Working Group                                           E. RosenRequest for Comments: 4576                                     P. PsenakCategory: Standards Track                              P. Pillay-Esnault                                                     Cisco Systems, Inc.                                                               June 2006Using a Link State Advertisement (LSA) Options Bit toPrevent 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 2006Table of Contents1. Introduction ....................................................22. Specification of Requirements ...................................33. Information Loss and Loops ......................................34. Using the LSA Options to Prevent Loops ..........................45. Security Considerations .........................................56. Acknowledgements ................................................57. Normative References ............................................61.  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 privateRFC 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 andRosen, 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.   This methodology provides a "peer model".  CE routers peer with PE   routers, but CE routers at different sites do not peer with each   other.   If a VPN uses OSPFv2 as its internal routing protocol, it is not   necessarily the case that the CE routers of that VPN use OSPFv2 to   peer with the PE routers.  Each site in a VPN can use OSPFv2 as its   intra-site routing protocol while using BGP or RIP (for example) to   distribute routes to a PE router.  However, it is certainly   convenient when OSPFv2 is being used intra-site to use it on the PE-   CE link as well, and [VPN] explicitly allows this.  In this case, a   PE will run a separate instance of OSPFv2 for each VPN that is   attached to the PE; the PE will in general have multiple VPN-specific   OSPFv2 routing tables.   When OSPFv2 is used on a PE-CE link that belongs to a particular VPN,   the PE router must redistribute to that VPN's OSPFv2 instance certain   routes that have been installed in the BGP routing table.  Similarly,   a PE router must redistribute to BGP routes that have been installed   in the VPN-specific OSPF routing tables.  Procedures for this are   specified in [VPN-OSPF].   The routes that are redistributed from BGP to OSPFv2 are advertised   in LSAs that are originated by the PE.  The PE acts as an OSPF border   router, advertising some of these routes in AS-external LSAs, and   some in summary LSAs, as specified in [VPN-OSPF].   Similarly, when a PE router receives an LSA from a CE router, it runs   the OSPF routing computation.  Any route that gets installed in the   OSPF routing table must be translated into a VPN-IPv4 route and then   redistributed into BGP.  BGP will then distribute these routes to the   other PE routers.2.  Specification of Requirements   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 inRFC 2119.3.  Information Loss and Loops   A PE, say PE1, may learn a route to a particular VPN-IPv4 address   prefix via BGP.  This may cause it to generate a summary LSA or an   AS-external LSA in which it reports that address prefix.  This LSA   may then be distributed to a particular CE, say CE1.  The LSA mayRosen, et al.               Standards Track                     [Page 3]

RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 2006   then be distributed throughout a particular OSPF area, reaching   another CE, say CE2.  CE2 may then distribute the LSA to another PE,   say PE2.   As stated in the previous section, PE2 must run the OSPF routing   computation to determine whether a particular address prefix,   reported in an LSA from CE2, is reachable from CE2 via a path that   does not include any PE router.  Unfortunately, there is no standard   way to do this.  The OSPFv2 LSAs do not necessarily carry the   information needed to enable PE2 to determine that the path to   address prefix X in a particular LSA from CE2 is actually a path that   includes, say PE1.  If PE2 then leaks X into BGP as a VPN-IPv4 route,   then PE2 is violating one of the constraints for loop-freedom in BGP;   viz., that routes learned from a particular BGP domain are not   redistributed back into that BGP domain.  This could cause a routing   loop to be created.   It is therefore necessary to have a means by which an LSA may carry   the information that a particular address prefix has been learned   from a PE router.  Any PE router that receives an LSA with this   information would omit the information in this LSA from its OSPF   routing computation, and thus it would not leak the information back   into BGP.   When a PE generates an AS-external LSA, it could use a distinct tag   value to indicate that the LSA is carrying information about an   address prefix for whom the path includes a PE router.  However, this   method is not available in the case where the PE generates a Summary   LSA.  Per [VPN-OSPF], each PE router must function as an OSPF area 0   router.  If the PE-CE link is an area 0 link, then it is possible for   the PE to receive, over that link, a summary LSA that originated at   another PE router.  Thus, we need some way of marking a summary LSA   to indicate that it is carrying information about a path via a PE   router.4.  Using the LSA Options to Prevent Loops   The high-order bit of the LSA Options field (a previously unused bit)   is used to solve the problem described in the previous section.  We   refer to this bit as the DN bit.  When a type 3, 5, or 7 LSA is sent   from a PE to a CE, the DN bit MUST be set.  The DN bit MUST be clear   in all other LSA types.Rosen, et al.               Standards Track                     [Page 4]

RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 2006                  +-------------------------------------+                  | DN | * | DC | EA | N/P | MC | E | * |                  +-------------------------------------+                         Options Field with DN Bit                          (RFC 2328, Section A.2)   When the PE receives, from a CE router, a type 3, 5, or 7 LSA with   the DN bit set, the information from that LSA MUST NOT be used during   the OSPF route calculation.  As a result, the LSA is not translated   into a BGP route.  The DN bit MUST be ignored in all other LSA types.   This prevents routes learned via BGP from being redistributed to BGP.   (This restriction is analogous to the usual OSPF restriction that   inter-area routes that are learned from area 0 are not passed back to   area 0.)   Note that the DN bit has no other effect on LSA handling.  In   particular, an LSA with the DN bit set will be put in the topological   database, aged, flooded, etc., just as if DN were not set.5.  Security Considerations   An attacker may cause the DN bit to be set, in an LSA traveling from   CE to PE, when the DN bit should really be clear.  This may cause the   address prefixes mentioned in that LSA to be unreachable from other   sites of the VPN.  Similarly, an attacker may cause the DN bit to be   clear, in an LSA traveling in either direction, when the DN bit   should really be set.  This may cause routing loops for traffic that   is destined to the address prefixes mentioned in that LSA.   These possibilities may be eliminated by using cryptographic   authentication as specified in Section D of [OSPFv2].6.  Acknowledgements   The idea of using the high-order options bit for this purpose is due   to Derek Yeung.  Thanks to Yakov Rekhter for his contribution to this   work.  We also wish to thank Acee Lindem for his helpful comments.Rosen, et al.               Standards Track                     [Page 5]

RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 20067.  Normative References   [OSPFv2]   Postel, J., "Suggested Telnet Protocol Changes",RFC 328,              April 1972.   [VPN]      Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private              Networks (VPNs)",RFC 4364, February 2006.   [VPN-OSPF] Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the              Provider/Customer Edge Protocol for BGP/MPLS IP Virtual              Private Networks (VPNs)",RFC 4577, June 2006.Authors' Addresses   Eric C. Rosen   Cisco Systems, Inc.   1414 Massachusetts Avenue   Boxborough, MA 01719   EMail: erosen@cisco.com   Peter Psenak   Cisco Systems   BA Business Center, 9th Floor   Plynarenska 1   Bratislava 82109   Slovakia   EMail: ppsenak@cisco.com   Padma Pillay-Esnault   Cisco Systems   3750 Cisco Way   San Jose, CA 95134   EMail: ppe@cisco.comRosen, et al.               Standards Track                     [Page 6]

RFC 4576          Prevent Looping in BGP/MPLS IP VPNs          June 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Rosen, et al.               Standards Track                     [Page 7]