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Internet Engineering Task Force (IETF)                        P. MarquesRequest for Comments: 6368Category: Standards Track                                      R. RaszukISSN: 2070-1721                                                  NTT MCL                                                                K. Patel                                                           Cisco Systems                                                               K. Kumaki                                                             T. Yamagata                                                        KDDI Corporation                                                          September 2011Internal BGP as the Provider/Customer Edge Protocol forBGP/MPLS IP Virtual Private Networks (VPNs)Abstract   This document defines protocol extensions and procedures for BGP   Provider/Customer Edge router iteration in BGP/MPLS IP VPNs.  These   extensions and procedures have the objective of making the usage of   the BGP/MPLS IP VPN transparent to the customer network, as far as   routing information is concerned.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6368.Marques, et al.              Standards Track                    [Page 1]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................22. Requirements Language ...........................................33. IP VPN as a Route Server ........................................34. Path Attributes .................................................55. BGP Customer Route Attributes ...................................66. Next-Hop Handling ...............................................77. Exchanging Routes between Different VPN Customer Networks .......88. Deployment Considerations ......................................109. Security Considerations ........................................1210. IANA Considerations ...........................................1211. Acknowledgments ...............................................1212. References ....................................................1312.1. Normative References .....................................1312.2. Informative References ...................................131.  Introduction   In current deployments, when BGP is used as the Provider/Customer   Edge routing protocol, these peering sessions are typically   configured as an external peering between the VPN provider autonomous   system (AS) and the customer network autonomous system.  At each   External BGP boundary, BGP path attributes [RFC4271] are modified as   per standard BGP rules.  This includes prepending the AS_PATH   attribute with the autonomous-system number of the originating   Customer Edge (CE) router and the autonomous-system number(s) of the   Provider Edge (PE) router(s).Marques, et al.              Standards Track                    [Page 2]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   In order for such routes not to be rejected by AS_PATH loop   detection, a PE router advertising a route received from a remote PE   often remaps the customer network autonomous-system number to its   own.  Otherwise, the customer network can use different autonomous-   system numbers at different sites or configure their CE routers to   accept routes containing their own AS number.   While this technique works well in situations where there are no BGP   routing exchanges between the client network and other networks, it   does have drawbacks for customer networks that use BGP internally for   purposes other than interaction between CE and PE routers.   In order to make the usage of BGP/MPLS VPN services as transparent as   possible to any external interaction, it is desirable to define a   mechanism by which PE-CE routers can exchange BGP routes by means   other than External BGP.   One can consider a BGP/MPLS VPN as a provider-managed backbone   service interconnecting several customer-managed sites.  While this   model is not universal, it does constitute a good starting point.   Independently of the presence of VPN service, networks often use a   hierarchical design utilizing either BGP route reflection [RFC4456]   or confederations [RFC5065].  This document assumes that the IP VPN   service interacts with the customer network following a similar   model.2.  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 inRFC 2119 [RFC2119].3.  IP VPN as a Route Server   In a typical backbone/area hierarchical design, routers that attach   an area (or site) to the core use BGP route reflection (or   confederations) to distribute routes between the top-level core   Internal BGP (iBGP) mesh and the local area iBGP cluster.   To provide equivalent functionality in a network using a provider-   provisioned backbone, one can consider the VPN as the equivalent of   an Internal BGP Route Server that multiplexes information from _N_   VPN attachment points.Marques, et al.              Standards Track                    [Page 3]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   A route learned by any of the PEs in the IP VPN is available to all   other PEs that import the Route Target used to identify the customer   network.  This is conceptually equivalent to a centralized route   server.   In a PE router, PE-received routes are not advertised back to other   PEs.  It is this split-horizon technique that prevents routing loops   in an IP VPN environment.  This is also consistent with the behavior   of a top-level mesh of route reflectors (RRs).   In order to complete the Route Server model, it is necessary to be   able to transparently carry the Internal BGP path attributes of   customer network routes through the BGP/MPLS VPN core.  This is   achieved by using a new BGP path attribute, described below, that   allows the customer network attributes to be saved and restored at   the BGP/MPLS VPN boundaries.   When a route is advertised from PE to CE, if it is advertised as an   iBGP route, the CE will not advertise it further unless it is itself   configured as a route reflector (or has an External BGP session).   This is a consequence of the default BGP behavior of not advertising   iBGP routes back to iBGP peers.  This behavior is not modified.   On a BGP/MPLS VPN PE, a CE-received route MUST be advertised to other   VPN PEs that import the Route Targets that are associated with the   route.  This is independent of whether the CE route has been received   as an external or internal route.  However, a CE-received route is   not re-advertised back to other CEs unless route reflection is   explicitly configured.  This is the equivalent of disabling client-   to-client reflection in BGP route reflection implementations.   When reflection is configured on the PE router, with local CE routers   as clients, there is no need to internally mesh multiple CEs that may   exist in the site.   This Route Server model can also be used to support a confederation-   style abstraction to CE devices.  At this point, we choose not to   describe in detail the procedures for that mode of operation.   Confederations are considered to be less common than route reflection   in enterprise environments.Marques, et al.              Standards Track                    [Page 4]

RFC 6368             Internal BGP as PE/CE Protocol       September 20114.  Path Attributes             --> push path attributes --> vrf-export --> BGP/MPLS IP VPN   VRF route                                             PE-PE route                                                         advertisement             <--  pop path attributes <--  vrf-import <--   The diagram above shows the BGP path attribute stack processing in   relation to existing BGP/MPLS IP VPN [RFC4364] route processing   procedures.  BGP path attributes received from a customer network are   pushed into the stack, before adding the Export Route Targets to the   BGP path attributes.  Conversely, the stack is popped following the   Import Target processing step that identifies the VPN Routing and   Forwarding (VRF) table in which a PE-received route is accepted.   When the advertising PE performs a "push" operation at the   "vrf-export" processing stage, it SHOULD initialize the attributes of   the BGP IP VPN route advertisement as it would for a locally   originated route from the respective VRF context.   When a PE-received route is imported into a VRF, its IGP metric, as   far as BGP path selection is concerned, SHOULD be the metric to the   remote PE address, expressed in terms of the service provider metric   domain.   For the purposes of VRF route selection performed at the PE, between   routes received from local CEs and remote PEs, customer network IGP   metrics SHOULD always be considered higher (and thus least preferred)   than local site metrics.   When backdoor links are present, this would tend to direct the   traffic between two sites through the backdoor link for BGP routes   originated by a remote site.  However, BGP already has policy   mechanisms, such as the LOCAL_PREF attribute, to address this type of   situation.   When a given CE is connected to more than one PE, it will not   advertise the route that it receives from a PE to another PE unless   configured as a route reflector, due to the standard BGP route   advertisement rules.   When a CE reflects a PE-received route to another PE, the fact that   the original attributes of a route are preserved across the VPN   prevents the formation of routing loops due to mutual redistribution   between the two networks.Marques, et al.              Standards Track                    [Page 5]

RFC 6368             Internal BGP as PE/CE Protocol       September 20115.  BGP Customer Route Attributes   In order to transparently carry the BGP path attributes of customer   routes, this document defines a new BGP path attribute:      ATTR_SET (type code 128)      ATTR_SET is an optional transitive attribute that carries a set of      BGP path attributes.  An attribute set (ATTR_SET) can include any      BGP attribute that can occur in a BGP UPDATE message, except for      the MP_REACH and MP_UNREACH attributes.   The ATTR_SET attribute is encoded as follows:                      +------------------------------+                      | Attr Flags (O|T) Code = 128  |                      +------------------------------+                      | Attr. Length (1 or 2 octets) |                      +------------------------------+                      | Origin AS (4 octets)         |                      +------------------------------+                      | Path Attributes (variable)   |                      +------------------------------+   The Attribute Flags are encoded according toRFC 4271 [RFC4271].  The   Extended Length bit determines whether the Attribute Length is one or   two octets.   The attribute value consists of a 4-octet "Origin AS" value followed   by a variable-length field that conforms to the BGP UPDATE message   path attribute encoding rules.  The length of this attribute is 4   plus the total length of the encoded attributes.   The ATTR_SET attribute is used by a PE router to store the original   set of BGP attributes it receives from a CE.  When a PE router   advertises a PE-received route to a CE, it will use the path   attributes carried in the ATTR_SET attribute.   In other words, the BGP path attributes are "pushed" into this   attribute, which operates as a stack, when the route is received by   the VPN and "popped" when the route is advertised in the PE-to-CE   direction.   Using this mechanism isolates the customer network from the   attributes used in the customer network and vice versa.  Attributes   such as the route reflection cluster list attribute are segregated   such that customer network cluster identifiers won't be considered by   the customer network route reflectors and vice versa.Marques, et al.              Standards Track                    [Page 6]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   The Origin autonomous-system number is designed to prevent a route   originating in a given autonomous-system iBGP from being leaked into   a different autonomous system without proper AS_PATH manipulation.   It SHOULD contain the autonomous-system number of the customer   network that originates the given set of attributes.  The value is   encoded as a 32-bit unsigned integer in network byte order,   regardless of whether or not the originating PE supports 4-octet AS   numbers [RFC4893].   The AS_PATH and AGGREGATOR attributes contained within an ATTR_SET   attribute MUST be encoded using 4-octet AS numbers [RFC4893],   regardless of the capabilities advertised by the BGP speaker to which   the ATTR_SET attribute is transmitted.  BGP speakers that support the   extensions defined in this document MUST also supportRFC 4893   [RFC4893].  The reason for this requirement is to remove ambiguity   between 2-octet and 4-octet AS_PATH attribute encoding.   The NEXT_HOP attribute SHOULD NOT be included in an ATTR_SET.  When   present, it SHOULD be ignored by the receiving PE.  Future   applications of the ATTR_SET attribute MAY define meaningful   semantics for an included NEXT_HOP attribute.   The ATTR_SET attribute SHALL be considered malformed if any of the   following apply:   o  Its length is less than 4 octets.   o  The original path attributes carried in the variable-length      attribute data include the MP_REACH or MP_UNREACH attribute.   o  The included attributes are malformed themselves.   An UPDATE message with a malformed ATTR_SET attribute SHALL be   handled as follows.  If its Partial flag is set and its   Neighbor-Complete flag is clear, the UPDATE is treated as a route   withdraw as discussed in [OPT-TRANS-BGP].  Otherwise (i.e., Partial   flag is clear or Neighbor-Complete is set), the procedures of the   BGP-4 base specification [RFC4271] MUST be followed with respect to   an Optional Attribute Error.6.  Next-Hop Handling   When BGP/MPLS VPNs are not in use, the NEXT_HOP attribute in iBGP   routes carries the address of the border router advertising the route   into the domain.  The IGP distance to the NEXT_HOP of the route is an   important component of BGP route selection.Marques, et al.              Standards Track                    [Page 7]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   When a BGP/MPLS VPN service is used to provide interconnection   between different sites, since the customer network runs a different   IGP domain, metrics between the provider and customer networks are   not comparable.   However, the most important component of a metric is the inter-area   metric, which is known to the customer network.  The intra-area   metric is typically negligible.   The use of route reflection, for instance, requires metrics to be   configured so that inter-cluster/area metrics are always greater than   intra-cluster metrics.   The approach taken by this document is to rewrite the NEXT_HOP   attribute at the VRF import/export boundary.  PE routers take into   account the PE-PE IGP distance calculated by the customer network   IGP, when selecting between routes advertised from different PEs.   An advantage of the proposed method is that the customer network can   run independent IGPs at each site.7.  Exchanging Routes between Different VPN Customer Networks   In the traditional model, where External BGP sessions are used   between the BGP/MPLS VPN PE and CE, the PE router identifies itself   as belonging to the customer network autonomous system.   In order to use Internal BGP sessions, the PE router has to identify   itself as belonging to the customer AS.  More specifically, the VRF   that is used to interconnect to that customer site is assigned to the   customer AS rather than the VPN provider AS.   The Origin AS element in the ATTR_SET path attribute conveys the   AS number of the originating VRF.  This AS number is used in a   receiving PE in order to identify route exchanges between VRFs in   different ASes.Marques, et al.              Standards Track                    [Page 8]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   In scenarios such as what is commonly referred to as an "extranet"   VPN, routes MAY be advertised to both internal and external VPN   attachments belonging to different autonomous systems.                          +-----+                 +-----+                          | PE1 |-----------------| PE2 |                          +-----+                 +-----+                         /       \                   |                  +-----+         +-----+         +-----+                  | CE1 |         | CE2 |         | CE3 |                  +-----+         +-----+         +-----+                    AS 1            AS 2            AS 1   Consider the example given above, where (PE1, CE1) and (PE2, CE3)   sessions are iBGP.  In BGP/MPLS VPNs, a route received from CE1 above   may be distributed to the VRFs corresponding to the attachment points   for CEs 2 and 3.   The desired result in such a scenario is to present the internal peer   (CE3) with a BGP advertisement that contains the same BGP path   attributes received from CE1, and to present the external peer (CE2)   with a BGP advertisement that would correspond to a situation where   AS 1 and AS 2 have an External BGP session between them.   In order to achieve this goal, the following set of rules applies:      When importing a VPN route that contains the ATTR_SET attribute      into a destination VRF, a PE router MUST check that the "Origin      AS" number contained in the ATTR_SET attribute matches the      autonomous system associated with the VRF.      In case the autonomous-system numbers do match, the route is      imported into the VRF with the attributes contained in the      ATTR_SET attribute.  Otherwise, in the case of an autonomous-      system number mismatch, the set of attributes to be associated      with the route SHALL be constructed as follows:      1.  The path attributes are set to the attributes contained in the          ATTR_SET attribute.      2.  iBGP-specific attributes are discarded (LOCAL_PREF,          ORIGINATOR, CLUSTER_LIST, etc).      3.  The "Origin AS" number contained in the ATTR_SET attribute          is prepended to the AS_PATH following the rules that would          apply to an External BGP peering between the source and          destination ASes.Marques, et al.              Standards Track                    [Page 9]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011      4.  If the autonomous system associated with the VRF is the same          as the VPN provider autonomous system and the AS_PATH          attribute of the VPN route is not empty, it SHALL be prepended          to the AS_PATH attribute of the VRF route.      When advertising the VRF route to an External BGP peer, a PE      router SHALL apply steps 1 to 4 defined above and subsequently      prepend its own autonomous-system number to the AS_PATH attribute.      For example, if the route originated in a VRF that supports      Internal BGP peering and the ATTR_SET attribute and is advertised      to a CE that is configured in the traditional External BGP mode,      then the originator AS, the VPN AS_PATH segment, and the customer      network AS are prepended to the AS_PATH.      When importing a route without the ATTR_SET attribute to a VRF      that is configured in a different autonomous system, a PE router      MUST prepend the VPN provider AS number to the AS_PATH.   In all cases where a route containing the ATTR_SET attribute is   imported, attributes present on the VPN route other than the NEXT_HOP   attribute are ignored, both from the point of view of route selection   in the VRF Adj-RIB-In and route advertisement to a CE router.  In   other words, the information contained in the ATTR_SET attribute   overrides the VPN route attributes on "vrf-import".8.  Deployment Considerations   It is RECOMMENDED that different VRFs of the same VPN (i.e., in   different PE routers) that are configured with iBGP PE-CE peering   sessions use different Route Distinguisher (RD) values.  Otherwise   (in the case where the same RD is used), the BGP IP VPN   infrastructure may select a single BGP customer path for a given IP   Network Layer Reachability Information (NLRI) without access to the   detailed path information that is contained in the ATTR_SET   attribute.   As mentioned previously, the model for this service is a "Route   Server" where the IP VPN provides the customer network with all the   BGP paths known by the CEs.  This effectively implies the use of   unique RDs per VRF.   The stated goal of this extension is to isolate the customer network   from the BGP path attribute operations performed by the IP VPN and   conversely isolate the service provider network from any attributes   injected by the customer.  For instance, BGP communities can be used   to influence the behavior of the IP VPN infrastructure.  Using this   extension, the service provider network can transparently carry these   attributes without interfering with its operations.Marques, et al.              Standards Track                   [Page 10]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011   Another example of unwanted interaction between customer and IP VPN   BGP attributes is a scenario where the same service provider   autonomous-system number is used to provide Internet service as well   as the IP VPN service.  In this case, it is not uncommon to have a   VPN customer route contain the AS number of the service provider.   The IP VPN should work transparently in this case as in all others.   This protocol extension is designed to behave such that each PE VRF   operates as a router in the configured AS.  Previously, VRFs operated   in the provider network AS only.  The VPN backbone provides   interconnection between VRFs of the same AS, as well as   interconnection between different ASes (subject to the appropriate   policies).  When interconnecting VRFs in the same AS, the VPN   backbone operates as a top-level route reflection mesh.  When   interconnecting VRFs in different ASes, the provider network provides   an implicit peering relationship between the ASes that originate and   import a specific route.   This extension is also applicable to scenarios where the VPN backbone   spans multiple ASes.  When the VPN backbone Inter-AS operation   follows option b) or c) as defined inSection 10 of [RFC4364], the   provider networks are able to influence the route attributes and   route selection of the VPN routes while providing a transparent   service to the customer AS.  Either Internal BGP connectivity or   extranets can be provided to the customer AS.   When VPN provider networks interconnect via option a), there is no   possibility of providing a fully transparent service.  By definition,   option a) implies that each autonomous-system border router (ASBR)   has a VRF associated with the customer VPN that is configured to   operate in the respective provider AS.  These ASBR VRFs then   communicate via External BGP with their peer provider ASes.   In this case, it is still possible to have all the customer VRFs with   one provider network be configured in the same customer AS.  This   customer AS will then peer with the provider AS implicitly at the   ASBR, which will in turn peer explicitly with a second provider AS.   This is not, however, a scenario in which transparency to the   customer AS is possible.Marques, et al.              Standards Track                   [Page 11]

RFC 6368             Internal BGP as PE/CE Protocol       September 20119.  Security Considerations   It is worthwhile to consider the security implications of this   proposal from two independent perspectives: the IP VPN provider and   the IP VPN customer.   From an IP VPN provider perspective, this mechanism will assure   separation between the BGP path attributes advertised by the CE   router and the BGP attributes used within the provider network, thus   potentially improving security.   Although this behavior is largely implementation dependent, it is   currently possible for a CE device to inject BGP attributes (extended   communities, for example) that have semantics on the IP VPN provider   network, unless explicitly disabled by configuration in the PE.   With the rules specified for the ATTR_SET path attribute, any   attribute that has been received from a CE is pushed into the stack   before the route is advertised to other PEs.   As with any other field based on values received from an external   system, an implementation must consider the issues of input   validation and resource management.   From the perspective of the VPN customer network, it is our opinion   that there is no change to the security profile of PE-CE interaction.   While having an iBGP session allows the PE to specify additional   attributes not allowed on an External BGP session (e.g., LOCAL_PREF),   this does not significantly change the fact that the VPN customer   must trust its service provider to provide it with correct routing   information.10.  IANA Considerations   This document defines a new BGP path attribute that is part of a   registry space managed by IANA.  IANA has updated its BGP Path   Attributes registry with the value specified above (128) for the   ATTR_SET path attribute.11.  Acknowledgments   The authors would like to thank Stephane Litkowski and Bruno Decraene   for their comments.Marques, et al.              Standards Track                   [Page 12]

RFC 6368             Internal BGP as PE/CE Protocol       September 201112.  References12.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4271]   Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A               Border Gateway Protocol 4 (BGP-4)",RFC 4271,               January 2006.   [RFC4364]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private               Networks (VPNs)",RFC 4364, February 2006.   [RFC4456]   Bates, T., Chen, E., and R. Chandra, "BGP Route               Reflection: An Alternative to Full Mesh Internal BGP               (IBGP)",RFC 4456, April 2006.   [RFC4893]   Vohra, Q. and E. Chen, "BGP Support for Four-octet AS               Number Space",RFC 4893, May 2007.   [RFC5065]   Traina, P., McPherson, D., and J. Scudder, "Autonomous               System Confederations for BGP",RFC 5065, August 2007.12.2.  Informative References   [OPT-TRANS-BGP]               Scudder, J. and E. Chen, "Error Handling for Optional               Transitive BGP Attributes", Work in Progress,               September 2010.Marques, et al.              Standards Track                   [Page 13]

RFC 6368             Internal BGP as PE/CE Protocol       September 2011Authors' Addresses   Pedro Marques   EMail: pedro.r.marques@gmail.com   Robert Raszuk   NTT MCL   101 S. Ellsworth Avenue Suite 350   San Mateo, CA  94401   US   EMail: robert@raszuk.net   Keyur Patel   Cisco Systems   170 W. Tasman Dr.   San Jose, CA  95134   US   EMail: keyupate@cisco.com   Kenji Kumaki   KDDI Corporation   Garden Air Tower   Iidabashi   Chiyoda-ku, Tokyo  102-8460   Japan   EMail: ke-kumaki@kddi.com   Tomohiro Yamagata   KDDI Corporation   Garden Air Tower   Iidabashi   Chiyoda-ku, Tokyo  102-8460   Japan   EMail: to-yamagata@kddi.comMarques, et al.              Standards Track                   [Page 14]