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Internal BGP as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6368.
Authors Kenji Kumaki , Keyur Patel , Robert Raszuk , Pedro Marques , Tomohiro Yamagata
Last updated 2020-01-21 (Latest revision 2011-06-23)
Replaces draft-marques-l3vpn-ibgp
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Ben Niven-Jenkins
IESG IESG state RFC 6368 (Proposed Standard)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Stewart Bryant
IESG note Ben Niven-Jenkins ( is the document shepherd.
Send notices to (None)
Network Working Group                                         P. Marques
Intended status: Standards Track                               R. Raszuk
Expires: December 26, 2011                                      K. Patel
                                                           Cisco Systems
                                                               K. Kumaki
                                                             T. Yamagata
                                                        KDDI Corporation
                                                           June 24, 2011

Internal BGP as Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
                        Private Networks (VPNs)


   This document defines protocol extensions and procedures for BGP
   Provider/Customer edge router iteration in BGP/MPLS IP VPN networks.
   These have the objective of making the usage of the BGP/MPLS IP VPN
   transparent to the customer network, as far as routing information is

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 26, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   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 Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  IP VPN network as a Route Server . . . . . . . . . . . . . . .  5
   4.  Path attributes  . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  BGP customer route attributes  . . . . . . . . . . . . . . . .  8
   6.  Next-hop handling  . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Exchanging routes between different VPN customer networks  . . 11
   8.  Deployment considerations  . . . . . . . . . . . . . . . . . . 13
   9.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   10. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 16
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 18
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  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).

   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 an
   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

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2.  Requirements Language

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

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3.  IP VPN network 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 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 which multiplexes information from _N_
   VPN attachment points.

   A route learned by any of the PEs in the IP VPN network, 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 RRs.

   In order to complete the Route Server model, 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 which 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 readvertised back to other CEs unless Route Reflection (RR) is
   explicitly configured.  This is the equivalent of disabling client to
   client reflection in BGP RR 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

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   style abstraction to CE devices.  We choose not to describe in detail
   the procedures for that mode of operation, at this point.
   Confederations are considered to be less common than route reflection
   in enterprise environments.

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4.  Path attributes

              --> push path attributes --> vrf-export --> BGP/MPLS IP VPN
    VRF route                                             PE-PE route
              <--  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 after 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 if 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

   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 (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 to address this type of situations such as the LOCAL_PREF

   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.

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5.  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 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 to RFC4271 [RFC4271].  The
   Extended Length bit determines whether the Attribute Length is one or
   two octets.

   The attribute value consists on a 4 octet "Origin AS" value followed
   by a variable length field which conforms to the BGP UPDATE message
   path attribute encoding rules.  The attribute length is 4 plus the
   total length of the encoded attributes.

   This 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 stack
   like attribute 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
   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.

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   The Origin autonomous-system number is designed to prevent a route
   originating in a given autonomous system iBGP to be 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 Four-octet AS Numbers

   The AS_PATH and AGGREGATOR attributes contained within an ATTR_SET
   attribute MUST be encoded using Four-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 support RFC4893
   [RFC4893].  The reason for this requirement is to remove ambiguity
   between two-octet and four-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 meaninful semantics
   for an included NEXT_HOP attribute.

   The ATTR_SET attribute SHALL be considered malformed if any of the
   following applies:

   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 [I-D.ietf-idr-optional-transitive].  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.

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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.

   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.

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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

   In scenarios such as what is commonly referred to 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 the external peer (CE 2) a BGP
   advertisement that would correspond to a situation where AS 1 and 2
   have an external BGP session between them.

   It order to achieve this goal the following set of rules apply:

      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.

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      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.  Internal BGP specific attributes are discarded (LOCAL_PREF,

      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

      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 Exterior 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 Exterior BGP mode
      then both 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 ATTR_SET attribute
   overrides the VPN route attributes on "vrf-import".

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8.  Deployment considerations

   It is RECOMMENDED that different VRFs of the same VPN (i.e. in
   different PE routers) which 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 interference with its operations.

   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 both 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 network should work transparently in this case as in all

   This protocol extension is designed to behave such that each PE VRF
   operates as a router in the configured AS.  Previously VRFs operate
   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 in Section 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.  Both internal BGP connectivity or

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   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 eBGP with their peer Provider ASes.

   In this case it is still possible to have all the customer VRFs with
   one Provider network to be configured in the same customer AS.  This
   customer AS will then peer with the Provider AS implicitly at the
   ABSR.  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.

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9.  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 customer
   CE router and the BGP attributes used within the provider network,
   thus potentially improving security.

   Although this behavior is largely implementation dependent, currently
   it is 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 out 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 eBGP session (e.g. local-pref), this
   does not significantly change the fact that the VPN customer must
   trust its service provider to provide it correct routing information.

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10.  IANA considerations

   This document defines a new BGP path attribute which is part of a
   registry space managed by IANA.  We request that IANA update its BGP
   Path Attributes registry with the value specified above (128) for the
   ATTR_SET path attribute.

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11.  Acknowledgments

   The authors would like to thank Stephane Litkowski and Bruno Decraene
   for their comments.

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12.  References

12.1.  Normative References

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

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "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.

   [RFC4893]  Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", RFC 4893, May 2007.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

12.2.  Informative References

              Scudder, J. and E. Chen, "Error Handling for Optional
              Transitive BGP Attributes",
              draft-ietf-idr-optional-transitive-03 (work in progress),
              September 2010.

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Authors' Addresses

   Pedro Marques


   Robert Raszuk
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134


   Keyur Patel
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA  95134


   Kenji Kumaki
   KDDI Corporation
   Garden Air Tower
   Chiyoda-ku, Tokyo  102-8460


   Tomohiro Yamagata
   KDDI Corporation
   Garden Air Tower
   Chiyoda-ku, Tokyo  102-8460


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