faqs.org - Internet FAQ Archives


Or Display the document by number

Network Working Group                                           E. Rosen
Request for Comments: 2547                                    Y. Rekhter
Category: Informational                              Cisco Systems, Inc.
                                                              March 1999

                             BGP/MPLS VPNs

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice



   This document describes a method by which a Service Provider with an
   IP backbone may provide VPNs (Virtual Private Networks) for its
   customers.  MPLS (Multiprotocol Label Switching) is used for
   forwarding packets over the backbone, and BGP (Border Gateway
   Protocol) is used for distributing routes over the backbone.  The
   primary goal of this method is to support the outsourcing of IP
   backbone services for enterprise networks. It does so in a manner
   which is simple for the enterprise, while still scalable and flexible
   for the Service Provider, and while allowing the Service Provider to
   add value. These techniques can also be used to provide a VPN which
   itself provides IP service to customers.

Table of Contents

   1.3        VPNs with Overlapping Address Spaces  ...............   4
   1.4        VPNs with Different Routes to the Same System  ......   4
   1.5        Multiple Forwarding Tables in PEs  ..................   5
   3          Per-Site Forwarding Tables in the PEs  ..............   6
   4          VPN Route Distribution via BGP  .....................   8
   4.1        The VPN-IPv4 Address Family  ........................   9

   4.2.2      Route Distribution Among PEs by BGP  ................  12
   4.2.3      The VPN of Origin Attribute  ........................  13
   4.2.4      Building VPNs using Target and Origin Attributes  ...  14
   5          Forwarding Across the Backbone  .....................  15
   6          How PEs Learn Routes from CEs  ......................  16
   7          How CEs learn Routes from PEs  ......................  19
   8          What if the CE Supports MPLS?  ......................  19
   8.2        Representing an ISP VPN as a Stub VPN  ..............  20
   9.1        Point-to-Point Security Tunnels between CE Routers  .  21
   9.2        Multi-Party Security Associations  ..................  21

1. Introduction

1.1. Virtual Private Networks

   Consider a set of "sites" which are attached to a common network
   which we may call the "backbone". Let's apply some policy to create a
   number of subsets of that set, and let's impose the following rule:
   two sites may have IP interconnectivity over that backbone only if at
   least one of these subsets contains them both.

   The subsets we have created are "Virtual Private Networks" (VPNs).
   Two sites have IP connectivity over the common backbone only if there
   is some VPN which contains them both.  Two sites which have no VPN in
   common have no connectivity over that backbone.

   If all the sites in a VPN are owned by the same enterprise, the VPN
   is a corporate "intranet".  If the various sites in a VPN are owned
   by different enterprises, the VPN is an "extranet".  A site can be in
   more than one VPN; e.g., in an intranet and several extranets.  We
   regard both intranets and extranets as VPNs. In general, when we use
   the term VPN we will not be distinguishing between intranets and

   We wish to consider the case in which the backbone is owned and
   operated by one or more Service Providers (SPs).  The owners of the
   sites are the "customers" of the SPs.  The policies that determine

   whether a particular collection of sites is a VPN are the policies of
   the customers.  Some customers will want the implementation of these
   policies to be entirely the responsibility of the SP.  Other
   customers may want to implement these policies themselves, or to
   share with the SP the responsibility for implementing these policies.
   In this document, we are primarily discussing mechanisms that may be
   used to implement these policies.  The mechanisms we describe are
   general enough to allow these policies to be implemented either by
   the SP alone, or by a VPN customer together with the SP.  Most of the
   discussion is focused on the former case, however.

   The mechanisms discussed in this document allow the implementation of
   a wide range of policies. For example, within a given VPN, we can
   allow every site to have a direct route to every other site ("full
   mesh"), or we can restrict certain pairs of sites from having direct
   routes to each other ("partial mesh").

   In this document, we are particularly interested in the case where
   the common backbone offers an IP service.  We are primarily concerned
   service provider, or perhaps to a set of service providers, with
   which it maintains contractual relationships.  We are not focused on
   providing VPNs over the public Internet.

   In the rest of this introduction, we specify some properties which
   VPNs should have.  The remainder of this document outlines a VPN
   model which has all these properties.  The VPN Model of this document
   appears to be an instance of the framework described in [4].

1.2. Edge Devices

   We suppose that at each site, there are one or more Customer Edge
   (CE) devices, each of which is attached via some sort of data link
   (e.g., PPP, ATM, ethernet, Frame Relay, GRE tunnel, etc.)  to one or
   more Provider Edge (PE) routers.

   If a particular site has a single host, that host may be the CE
   device.  If a particular site has a single subnet, that the CE device
   may be a switch.  In general, the CE device can be expected to be a
   router, which we call the CE router.

   We will say that a PE router is attached to a particular VPN if it is
   attached to a CE device which is in that VPN.  Similarly, we will say
   that a PE router is attached to a particular site if it is attached
   to a CE device which is in that site.

   When the CE device is a router, it is a routing peer of the PE(s) to
   which it is attached, but is not a routing peer of CE routers at

   other sites.  Routers at different sites do not directly exchange
   routing information with each other; in fact, they do not even need
   to know of each other at all (except in the case where this is
   necessary for security purposes, see section 9).  As a consequence,
   very large VPNs (i.e., VPNs with a very large number of sites) are
   easily supported, while the routing strategy for each individual site
   is greatly simplified.

   It is important to maintain clear administrative boundaries between
   the SP and its customers (cf. [4]).  The PE and P routers should be
   administered solely by the SP, and the SP's customers should not have
   any management access to it.  The CE devices should be administered
   solely by the customer (unless the customer has contracted the
   management services out to the SP).

1.3. VPNs with Overlapping Address Spaces

   We assume that any two non-intersecting VPNs (i.e., VPNs with no
   sites in common) may have overlapping address spaces; the same
   address may be reused, for different systems, in different VPNs.  As
   long as a given endsystem has an address which is unique within the
   scope of the VPNs that it belongs to, the endsystem itself does not
   need to know anything about VPNs.

   In this model, the VPN owners do not have a backbone to administer,
   not even a "virtual backbone". Nor do the SPs have to administer a
   separate backbone or "virtual backbone" for each VPN.  Site-to-site
   routing in the backbone is optimal (within the constraints of the
   policies used to form the VPNs), and is not constrained in any way by
   an artificial "virtual topology" of tunnels.

1.4. VPNs with Different Routes to the Same System

   Although a site may be in multiple VPNs, it is not necessarily the
   case that the route to a given system at that site should be the same
   in all the VPNs.  Suppose, for example, we have an intranet
   consisting of sites A, B, and C, and an extranet consisting of A, B,
   C, and the "foreign" site D.  Suppose that at site A there is a
   server, and we want clients from B, C, or D to be able to use that
   server.  Suppose also that at site B there is a firewall.  We want
   all the traffic from site D to the server to pass through the
   firewall, so that traffic from the extranet can be access controlled.
   However, we don't want traffic from C to pass through the firewall on
   the way to the server, since this is intranet traffic.

   This means that it needs to be possible to set up two routes to the
   server.  One route, used by sites B and C, takes the traffic directly
   to site A.  The second route, used by site D, takes the traffic

   instead to the firewall at site B.  If the firewall allows the
   traffic to pass, it then appears to be traffic coming from site B,
   and follows the route to site A.

1.5. Multiple Forwarding Tables in PEs

   Each PE router needs to maintain a number of separate forwarding
   tables.  Every site to which the PE is attached must be mapped to one
   of those forwarding tables.  When a packet is received from a
   particular site, the forwarding table associated with that site is
   consulted in order to determine how to route the packet.  The
   forwarding table associated with a particular site S is populated
   only with routes that lead to other sites which have at least one VPN
   in common with S. This prevents communication between sites which
   have no VPN in common, and it allows two VPNs with no site in common
   to use address spaces that overlap with each other.

1.6. SP Backbone Routers

   The SP's backbone consists of the PE routers, as well as other
   routers (P routers) which do not attach to CE devices.

   If every router in an SP's backbone had to maintain routing
   information for all the VPNs supported by the SP, this model would
   have severe scalability problems; the number of sites that could be
   supported would be limited by the amount of routing information that
   could be held in a single router.  It is important to require
   therefore that the routing information about a particular VPN be
   present ONLY in those PE routers which attach to that VPN.  In
   particular, the P routers should not need to have ANY per-VPN routing
   information whatsoever.

   VPNs may span multiple service providers. We assume though that when
   the path between PE routers crosses a boundary between SP networks,
   it does so via a private peering arrangement, at which there exists
   mutual trust between the two providers. In particular, each provider
   must trust the other to pass it only correct routing information, and
   to pass it labeled (in the sense of MPLS [9]) packets only if those
   packets have been labeled by trusted sources. We also assume that it
   is possible for label switched paths to cross the boundary between
   service providers.

1.7. Security

   A VPN model should, even without the use of cryptographic security
   measures, provide a level of security equivalent to that obtainable
   when a level 2 backbone (e.g., Frame Relay) is used.  That is, in the
   absence of misconfiguration or deliberate interconnection of

   different VPNs, it should not be possible for systems in one VPN to
   gain access to systems in another VPN.

   It should also be possible to deploy standard security procedures.

2. Sites and CEs

   From the perspective of a particular backbone network, a set of IP
   systems constitutes a site if those systems have mutual IP
   interconnectivity, and communication between them occurs without use
   of the backbone. In general, a site will consist of a set of systems
   which are in geographic proximity.  However, this is not universally
   true; two geographic locations connected via a leased line, over
   which OSPF is running, will constitute a single site, because
   communication between the two locations does not involve the use of
   the backbone.

   A CE device is always regarded as being in a single site (though as
   we shall see, a site may consist of multiple "virtual sites"). A
   site, however, may belong to multiple VPNs.

   A PE router may attach to CE devices in any number of different
   sites, whether those CE devices are in the same or in different VPNs.
   A CE device may, for robustness, attach to multiple PE routers, of
   the same or of different service providers.  If the CE device is a
   router, the PE router and the CE router will appear as router
   adjacencies to each other.

   While the basic unit of interconnection is the site, the architecture
   described herein allows a finer degree of granularity in the control
   of interconnectivity. For example, certain systems at a site may be
   members of an intranet as well as members of one or more extranets,
   while other systems at the same site may be restricted to being
   members of the intranet only.

3. Per-Site Forwarding Tables in the PEs

   Each PE router maintains one or more "per-site forwarding tables".
   Every site to which the PE router is attached is associated with one
   of these tables.  A particular packet's IP destination address is
   looked up in a particular per-site forwarding table only if that
   packet has arrived directly from a site which is associated with that

   How are the per-site forwarding tables populated?

   As an example, let PE1, PE2, and PE3 be three PE routers, and let
   CE1, CE2, and CE3 be three CE routers. Suppose that PE1 learns, from
   CE1, the routes which are reachable at CE1's site.  If PE2 and PE3
   are attached respectively to CE2 and CE3, and there is some VPN V
   containing CE1, CE2, and CE3, then PE1 uses BGP to distribute to PE2
   and PE3 the routes which it has learned from CE1.  PE2 and PE3 use
   these routes to populate the forwarding tables which they associate
   respectively with the sites of CE2 and CE3.  Routes from sites which
   are not in VPN V do not appear in these forwarding tables, which
   means that packets from CE2 or CE3 cannot be sent to sites which are
   not in VPN V.

   If a site is in multiple VPNs, the forwarding table associated with
   that site can contain routes from the full set of VPNs of which the
   site is a member.

   A PE generally maintains only one forwarding table per site, even if
   it is multiply connected to that site.  Also, different sites can
   share the same forwarding table if they are meant to use exactly the
   same set of routes.

   Suppose a packet is received by a PE router from a particular
   directly attached site, but the packet's destination address does not
   match any entry in the forwarding table associated with that site.
   If the SP is not providing Internet access for that site, then the
   packet is discarded as undeliverable.  If the SP is providing
   Internet access for that site, then the PE's Internet forwarding
   table will be consulted.  This means that in general, only one
   forwarding table per PE need ever contain routes from the Internet,
   even if Internet access is provided.

   To maintain proper isolation of one VPN from another, it is important
   that no router in the backbone accept a labeled packet from any
   adjacent non-backbone device unless (a) the label at the top of the
   label stack was actually distributed by the backbone router to the
   non-backbone device, and (b) the backbone router can determine that
   use of that label will cause the packet to leave the backbone before
   any labels lower in the stack will be inspected, and before the IP
   header will be inspected.  These restrictions are necessary in order
   to prevent packets from entering a VPN where they do not belong.

   The per-site forwarding tables in a PE are ONLY used for packets
   which arrive from a site which is directly attached to the PE.  They
   are not used for routing packets which arrive from other routers that
   belong to the SP backbone.  As a result, there may be multiple
   different routes to the same system, where the route followed by a
   given packet is determined by the site from which the packet enters
   the backbone.  E.g., one may have one route to a given system for

   packets from the extranet (where the route leads to a firewall), and
   a different route to the same system for packets from the intranet
   (including packets that have already passed through the firewall).

3.1. Virtual Sites

   In some cases, a particular site may be divided by the customer into
   several virtual sites, perhaps by the use of VLANs.  Each virtual
   site may be a member of a different set of VPNs. The PE then needs to
   contain a separate forwarding table for each virtual site.  For
   example, if a CE supports VLANs, and wants each VLAN mapped to a
   separate VPN, the packets sent between CE and PE could be contained
   in the site's VLAN encapsulation, and this could be used by the PE,
   along with the interface over which the packet is received, to assign
   the packet to a particular virtual site.

   Alternatively, one could divide the interface into multiple "sub-
   interfaces" (particularly if the interface is Frame Relay or ATM),
   and assign the packet to a VPN based on the sub-interface over which
   it arrives.  Or one could simply use a different interface for each
   virtual site.  In any case, only one CE router is ever needed per
   site, even if there are multiple virtual sites.  Of course, a
   different CE router could be used for each virtual site, if that is

   Note that in all these cases, the mechanisms, as well as the policy,
   for controlling which traffic is in which VPN are in the hand of the

   If it is desired to have a particular host be in multiple virtual
   sites, then that host must determine, for each packet, which virtual
   site the packet is associated with.  It can do this, e.g., by sending
   packets from different virtual sites on different VLANs, our out
   different network interfaces.

   These schemes do NOT require the CE to support MPLS.  Section 8
   contains a brief discussion of how the CE might support multiple
   virtual sites if it does support MPLS.

4. VPN Route Distribution via BGP

   PE routers use BGP to distribute VPN routes to each other (more
   accurately, to cause VPN routes to be distributed to each other).

   A BGP speaker can only install and distribute one route to a given
   address prefix.  Yet we allow each VPN to have its own address space,
   which means that the same address can be used in any number of VPNs,
   where in each VPN the address denotes a different system.  It follows

   that we need to allow BGP to install and distribute multiple routes
   to a single IP address prefix.  Further, we must ensure that POLICY
   is used to determine which sites can be use which routes; given that
   several such routes are installed by BGP, only one such must appear
   in any particular per-site forwarding table.

   We meet these goals by the use of a new address family, as specified

4.1. The VPN-IPv4 Address Family

   The BGP Multiprotocol Extensions [3] allow BGP to carry routes from
   multiple "address families".  We introduce the notion of the "VPN-
   IPv4 address family".  A VPN-IPv4 address is a 12-byte quantity,
   beginning with an 8-byte "Route Distinguisher (RD)" and ending with a
   4-byte IPv4 address.  If two VPNs use the same IPv4 address prefix,
   the PEs translate these into unique VPN-IPv4 address prefixes.  This
   ensures that if the same address is used in two different VPNs, it is
   possible to install two completely different routes to that address,
   one for each VPN.

   The RD does not by itself impose any semantics; it contains no
   information about the origin of the route or about the set of VPNs to
   which the route is to be distributed.  The purpose of the RD is
   solely to allow one to create distinct routes to a common IPv4
   address prefix.  Other means are used to determine where to
   redistribute the route (see section 4.2).

   The RD can also be used to create multiple different routes to the
   very same system.  In section 3, we gave an example where the route
   to a particular server had to be different for intranet traffic than
   for extranet traffic.  This can be achieved by creating two different
   VPN-IPv4 routes that have the same IPv4 part, but different RDs.
   This allows BGP to install multiple different routes to the same
   system, and allows policy to be used (see section 4.2.3) to decide
   which packets use which route.

   The RDs are structured so that every service provider can administer
   service provider.  An RD consists of a two-byte type field, an
   the semantics of the administrator field.  The administrator field
   identifies an assigned number authority, and the assigned number
   field contains a number which has been assigned, by the identified
   authority, for a particular purpose.  For example, one could have an
   RD whose administrator field contains an Autonomous System number

   (ASN), and whose (4-byte) number field contains a number assigned by
   the SP to whom IANA has assigned that ASN.  RDs are given this
   structure in order to ensure that an SP which provides VPN backbone
   service can always create a unique RD when it needs to do so.
   However, the structuring provides no semantics. When BGP compares two
   such address prefixes, it ignores the structure entirely.

   If the Administrator subfield and the Assigned Number subfield of a
   VPN-IPv4 address are both set to all zeroes, the VPN-IPv4 address is
   considered to have exactly the same meaning as the corresponding
   globally unique IPv4 address. In particular, this VPN-IPv4 address
   and the corresponding globally unique IPv4 address will be considered
   comparable by BGP. In all other cases, a VPN-IPv4 address and its
   corresponding globally unique IPv4 address will be considered
   noncomparable by BGP.

   A given per-site forwarding table will only have one VPN-IPv4 route
   for any given IPv4 address prefix.  When a packet's destination
   address is matched against a VPN-IPv4 route, only the IPv4 part is
   actually matched.

   A PE needs to be configured to associate routes which lead to
   particular CE with a particular RD.  The PE may be configured to
   associate all routes leading to the same CE with the same RD, or it
   may be configured to associate different routes with different RDs,
   even if they lead to the same CE.

4.2. Controlling Route Distribution

   In this section, we discuss the way in which the distribution of the
   VPN-IPv4 routes is controlled.

4.2.1. The Target VPN Attribute

   Every per-site forwarding table is associated with one or more
   "Target VPN" attributes.

   When a VPN-IPv4 route is created by a PE router, it is associated
   with one or more "Target VPN" attributes.  These are carried in BGP
   as attributes of the route.

   Any route associated with Target VPN T must be distributed to every
   PE router that has a forwarding table associated with Target VPN T.
   When such a route is received by a PE router, it is eligible to be
   installed in each of the PE's per-site forwarding tables that is
   associated with Target VPN T. (Whether it actually gets installed
   depends on the outcome of the BGP decision process.)

   In essence, a Target VPN attribute identifies a set of sites.
   Associating a particular Target VPN attribute with a route allows
   that route to be placed in the per-site forwarding tables that are
   used for routing traffic which is received from the corresponding

   There is a set of Target VPNs that a PE router attaches to a route
   received from site S. And there is a set of Target VPNs that a PE
   router uses to determine whether a route received from another PE
   router could be placed in the forwarding table associated with site
   S. The two sets are distinct, and need not be the same.

   The function performed by the Target VPN attribute is similar to that
   performed by the BGP Communities Attribute.  However, the format of
   the latter is inadequate, since it allows only a two-byte numbering
   space.  It would be fairly straightforward to extend the BGP
   Communities Attribute to provide a larger numbering space.  It should
   also be possible to structure the format, similar to what we have
   described for RDs (see section 4.1), so that a type field defines the
   length of an administrator field, and the remainder of the attribute
   is a number from the specified administrator's numbering space.

   When a BGP speaker has received two routes to the same VPN-IPv4
   prefix, it chooses one, according to the BGP rules for route

   Note that a route can only have one RD, but it can have multiple
   Target VPNs.  In BGP, scalability is improved if one has a single
   route with multiple attributes, as opposed to multiple routes.  One
   could eliminate the Target VPN attribute by creating more routes
   (i.e., using more RDs), but the scaling properties would be less

   How does a PE determine which Target VPN attributes to associate with
   a given route?  There are a number of different possible ways.  The
   PE might be configured to associate all routes that lead to a
   particular site with a particular Target VPN.  Or the PE might be
   configured to associate certain routes leading to a particular site
   with one Target VPN, and certain with another.  Or the CE router,
   when it distributes these routes to the PE (see section 6), might
   specify one or more Target VPNs for each route.  The latter method
   shifts the control of the mechanisms used to implement the VPN
   policies from the SP to the customer.  If this method is used, it may
   still be desirable to have the PE eliminate any Target VPNs that,
   according to its own configuration, are not allowed, and/or to add in
   some Target VPNs that according to its own configuration are

   It might be more accurate, if less suggestive, to call this attribute
   the "Route Target" attribute instead of the "VPN Target" attribute.
   It really identifies only a set of sites which will be able to use
   the route, without prejudice to whether those sites constitute what
   might intuitively be called a VPN.

4.2.2. Route Distribution Among PEs by BGP

   If two sites of a VPN attach to PEs which are in the same Autonomous
   System, the PEs can distribute VPN-IPv4 routes to each other by means
   of an IBGP connection between them.  Alternatively, each can have an
   IBGP connection to a route reflector.

   If two sites of VPN are in different Autonomous Systems (e.g.,
   because they are connected to different SPs), then a PE router will
   need to use IBGP to redistribute VPN-IPv4 routes either to an
   Autonomous System Border Router (ASBR), or to a route reflector of
   which an ASBR is a client.  The ASBR will then need to use EBGP to
   redistribute those routes to an ASBR in another AS.  This allows one
   to connect different VPN sites to different Service Providers.
   However, VPN-IPv4 routes should only be accepted on EBGP connections
   at private peering points, as part of a trusted arrangement between
   SPs.  VPN-IPv4 routes should neither be distributed to nor accepted
   from the public Internet.

   If there are many VPNs having sites attached to different Autonomous
   Systems, there does not need to be a single ASBR between those two
   ASes which holds all the routes for all the VPNs; there can be
   multiple ASBRs, each of which holds only the routes for a particular
   subset of the VPNs.

   When a PE router distributes a VPN-IPv4 route via BGP, it uses its
   own address as the "BGP next hop".  It also assigns and distributes
   an MPLS label.  (Essentially, PE routers distribute not VPN-IPv4
   routes, but Labeled VPN-IPv4 routes. Cf. [8]) When the PE processes a
   received packet that has this label at the top of the stack, the PE
   will pop the stack, and send the packet directly to the site from to
   which the route leads.  This will usually mean that it just sends the
   packet to the CE router from which it learned the route.  The label
   may also determine the data link encapsulation.

   In most cases, the label assigned by a PE will cause the packet to be
   sent directly to a CE, and the PE which receives the labeled packet
   will not look up the packet's destination address in any forwarding
   table.  However, it is also possible for the PE to assign a label
   which implicitly identifies a particular forwarding table.  In this
   case, the PE receiving a packet that label would look up the packet's
   destination address in one of its forwarding tables.  While this can

   be very useful in certain circumstances, we do not consider it
   further in this paper.

   Note that the MPLS label that is distributed in this way is only
   usable if there is a label switched path between the router that
   installs a route and the BGP next hop of that route.  We do not make
   any assumption about the procedure used to set up that label switched
   path.  It may be set up on a pre-established basis, or it may be set
   up when a route which would need it is installed.  It may be a "best
   effort" route, or it may be a traffic engineered route.  Between a
   particular PE router and its BGP next hop for a particular route
   there may be one LSP, or there may be several, perhaps with different
   QoS characteristics.  All that matters for the VPN architecture is
   that some label switched path between the router and its BGP next hop

   All the usual techniques for using route reflectors [2] to improve
   scalability, e.g., route reflector hierarchies, are available.  If
   route reflectors are used, there is no need to have any one route
   reflector know all the VPN-IPv4 routes for all the VPNs supported by
   the backbone.  One can have separate route reflectors, which do not
   communicate with each other, each of which supports a subset of the
   total set of VPNs.

   If a given PE router is not attached to any of the Target VPNs of a
   particular route, it should not receive that route; the other PE or
   route reflector which is distributing routes to it should apply
   outbound filtering to avoid sending it unnecessary routes.  Of
   course, if a PE router receives a route via BGP, and that PE is not
   attached to any of the route's target VPNs, the PE should apply
   inbound filtering to the route, neither installing nor redistributing

   A router which is not attached to any VPN, i.e., a P router, never
   installs any VPN-IPv4 routes at all.

   These distribution rules ensure that there is no one box which needs
   to know all the VPN-IPv4 routes that are supported over the backbone.
   As a result, the total number of such routes that can be supported
   over the backbone is not bound by the capacity of any single device,
   and therefore can increase virtually without bound.

4.2.3. The VPN of Origin Attribute

   A VPN-IPv4 route may be optionally associated with a VPN of Origin
   attribute.  This attribute uniquely identifies a set of sites, and
   identifies the corresponding route as having come from one of the
   sites in that set.  Typical uses of this attribute might be to

   identify the enterprise which owns the site where the route leads, or
   to identify the site's intranet.  However, other uses are also
   possible.  This attribute could be encoded as an extended BGP
   communities attribute.

   In situations in which it is necessary to identify the source of a
   route, it is this attribute, not the RD, which must be used.  This
   attribute may be used when "constructing" VPNs, as described below.

   the "Route Origin" attribute instead of the "VPN of Origin"
   attribute.  It really identifies the route only has having come from
   one of a particular set of sites, without prejudice as to whether
   that particular set of sites really constitutes a VPN.

4.2.4. Building VPNs using Target and Origin Attributes

   By setting up the Target VPN and VPN of Origin attributes properly,
   one can construct different kinds of VPNs.

   Suppose it is desired to create a Closed User Group (CUG) which
   contains a particular set of sites. This can be done by creating a
   particular Target VPN attribute value to represent the CUG. This
   value then needs to be associated with the per-site forwarding tables
   for each site in the CUG, and it needs to be associated with every
   route learned from a site in the CUG.  Any route which has this
   Target VPN attribute will need to be redistributed so that it reaches
   every PE router attached to one of the sites in the CUG.

   Alternatively, suppose one desired, for whatever reason, to create a
   "hub and spoke" kind of VPN.  This could be done by the use of two
   Target Attribute values, one meaning "Hub" and one meaning "Spoke".
   Then routes from the spokes could be distributed to the hub, without
   causing routes from the hub to be distributed to the spokes.

   Suppose one has a number of sites which are in an intranet and an
   extranet, as well as a number of sites which are in the intranet
   only.  Then there may be both intranet and extranet routes which have
   a Target VPN identifying the entire set of sites.  The sites which
   are to have intranet routes only can filter out all routes with the
   "wrong" VPN of Origin.

   These two attributes allow great flexibility in allowing one to
   control the distribution of routing information among various sets of
   sites, which in turn provides great flexibility in constructing VPNs.

5. Forwarding Across the Backbone

   If the intermediate routes in the backbone do not have any
   information about the routes to the VPNs, how are packets forwarded
   from one VPN site to another?

   This is done by means of MPLS with a two-level label stack.

   PE routers (and ASBRs which redistribute VPN-IPv4 addresses) need to
   insert /32 address prefixes for themselves into the IGP routing
   tables of the backbone.  This enables MPLS, at each node in the
   backbone network, to assign a label corresponding to the route to
   each PE router.  (Certain procedures for setting up label switched
   paths in the backbone may not require the presence of the /32 address

   When a PE receives a packet from a CE device, it chooses a particular
   per-site forwarding table in which to look up the packet's
   destination address.  Assume that a match is found.

   If the packet is destined for a CE device attached to this same PE,
   the packet is sent directly to that CE device.

   If the packet is not destined for a CE device attached to this same
   PE, the packet's "BGP Next Hop" is found, as well as the label which
   that BGP next hop assigned for the packet's destination address. This
   label is pushed onto the packet's label stack, and becomes the bottom
   label.  Then the PE looks up the IGP route to the BGP Next Hop, and
   thus determines the IGP next hop, as well as the label assigned to
   the address of the BGP next hop by the IGP next hop.  This label gets
   pushed on as the packet's top label, and the packet is then forwarded
   to the IGP next hop.  (If the BGP next hop is the same as the IGP
   next hop, the second label may not need to be pushed on, however.)

   At this point, MPLS will carry the packet across the backbone and
   into the appropriate CE device.  That is, all forwarding decisions by
   P routers and PE routers are now made by means of MPLS, and the
   packet's IP header is not looked at again until the packet reaches
   the CE device.  The final PE router will pop the last label from the
   MPLS label stack before sending the packet to the CE device, thus the
   CE device will just see an ordinary IP packet.  (Though see section 8
   for some discussion of the case where the CE desires to received
   labeled packets.)

   When a packet enters the backbone from a particular site via a
   particular PE router, the packet's route is determined by the
   contents of the forwarding table which that PE router associated with
   that site.  The forwarding tables of the PE router where the packet

   leaves the backbone are not relevant.  As a result, one may have
   multiple routes to the same system, where the particular route chosen
   for a particular packet is based on the site from which the packet
   enters the backbone.

   Note that it is the two-level labeling that makes it possible to keep
   all the VPN routes out of the P routers, and this in turn is crucial
   to ensuring the scalability of the model.  The backbone does not even
   need to have routes to the CEs, only to the PEs.

6. How PEs Learn Routes from CEs

   The PE routers which attach to a particular VPN need to know, for
   each of that VPN's sites, which addresses in that VPN are at each

   In the case where the CE device is a host or a switch, this set of
   addresses will generally be configured into the PE router attaching
   to that device.  In the case where the CE device is a router, there
   are a number of possible ways that a PE router can obtain this set of

   The PE translates these addresses into VPN-IPv4 addresses, using a
   configured RD.  The PE then treats these VPN-IPv4 routes as input to
   BGP.  In no case will routes from a site ever be leaked into the
   backbone's IGP.

   Exactly which PE/CE route distribution techniques are possible
   depends on whether a particular CE is in a "transit VPN" or not.  A
   "transit VPN" is one which contains a router that receives routes
   from a "third party" (i.e., from a router which is not in the VPN,
   but is not a PE router), and that redistributes those routes to a PE
   router.  A VPN which is not a transit VPN is a "stub VPN".  The vast
   majority of VPNs, including just about all corporate enterprise
   networks, would be expected to be "stubs" in this sense.

   The possible PE/CE distribution techniques are:

      1. Static routing (i.e., configuration) may be used. (This is
         likely to be useful only in stub VPNs.)

      2. PE and CE routers may be RIP peers, and the CE may use RIP to
         tell the PE router the set of address prefixes which are
         reachable at the CE router's site.  When RIP is configured in
         the CE, care must be taken to ensure that address prefixes from
         other sites (i.e., address prefixes learned by the CE router
         from the PE router) are never advertised to the PE.  More
         precisely: if a PE router, say PE1, receives a VPN-IPv4 route

         R1, and as a result distributes an IPv4 route R2 to a CE, then
         R2 must not be distributed back from that CE's site to a PE
         router, say PE2, (where PE1 and PE2 may be the same router or
         different routers), unless PE2 maps R2 to a VPN-IPv4 route
         which is different than (i.e., contains a different RD than)

      3. The PE and CE routers may be OSPF peers.  In this case, the
         site should be a single OSPF area, the CE should be an ABR in
         that area, and the PE should be an ABR which is not in that
         area.  Also, the PE should report no router links other than
         those to the CEs which are at the same site. (This technique
         should be used only in stub VPNs.)

      4. The PE and CE routers may be BGP peers, and the CE router may
         use BGP (in particular, EBGP to tell the PE router the set of
         address prefixes which are at the CE router's site. (This
         technique can be used in stub VPNs or transit VPNs.)

         From a purely technical perspective, this is by far the best

              a) Unlike the IGP alternatives, this does not require the
                 PE to run multiple routing algorithm instances in order
                 to talk to multiple CEs

              b) BGP is explicitly designed for just this function:
                 passing routing information between systems run by
                 different administrations

              c) If the site contains "BGP backdoors", i.e., routers
                 with BGP connections to routers other than PE routers,
                 this procedure will work correctly in all
                 circumstances.  The other procedures may or may not
                 work, depending on the precise circumstances.

              d) Use of BGP makes it easy for the CE to pass attributes
                 of the routes to the PE.  For example, the CE may
                 suggest a particular Target for each route, from among
                 the Target attributes that the PE is authorized to
                 attach to the route.

          On the other hand, using BGP is likely to be something new for
          the CE administrators, except in the case where the customer
          itself is already an Internet Service Provider (ISP).

          If a site is not in a transit VPN, note that it need not have
          a unique Autonomous System Number (ASN).  Every CE whose site
          which is not in a transit VPN can use the same ASN.  This can
          be chosen from the private ASN space, and it will be stripped
          out by the PE.  Routing loops are prevented by use of the Site
          of Origin Attribute (see below).

          If a set of sites constitute a transit VPN, it is convenient
          to represent them as a BGP Confederation, so that the internal
          structure of the VPN is hidden from any router which is not
          within the VPN.  In this case, each site in the VPN would need
          two BGP connections to the backbone, one which is internal to
          the confederation and one which is external to it.  The usual
          intra-confederation procedures would have to be slightly
          modified in order to take account for the fact that the
          backbone and the sites may have different policies.  The
          backbone is a member of the confederation on one of the
          connections, but is not a member on the other.  These
          techniques may be useful if the customer for the VPN service
          is an ISP.  This technique allows a customer that is an ISP to
          obtain VPN backbone service from one of its ISP peers.

          (However, if a VPN customer is itself an ISP, and its CE
          routers support MPLS, a much simpler technique can be used,
          wherein the ISP is regarded as a stub VPN.  See section 8.)

   When we do not need to distinguish among the different ways in which
   a PE can be informed of the address prefixes which exist at a given
   site, we will simply say that the PE has "learned" the routes from
   that site.

   Before a PE can redistribute a VPN-IPv4 route learned from a site, it
   must assign certain attributes to the route. There are three such

      - Site of Origin

        This attribute uniquely identifies the site from which the PE
        router learned the route.  All routes learned from a particular
        site must be assigned the same Site of Origin attribute, even if
        a site is multiply connected to a single PE, or is connected to
        multiple PEs.  Distinct Site of Origin attributes must be used
        for distinct sites.  This attribute could be encoded as an
        extended BGP communities attribute (section 4.2.1).

      - VPN of Origin

        See section 4.2.1.

      - Target VPN

        See section 4.2.1.

7. How CEs learn Routes from PEs

   In this section, we assume that the CE device is a router.

   In general, a PE may distribute to a CE any route which the PE has
   placed in the forwarding table which it uses to route packets from
   that CE.  There is one exception: if a route's Site of Origin
   attribute identifies a particular site, that route must never be
   redistributed to any CE at that site.

   In most cases, however, it will be sufficient for the PE to simply
   distribute the default route to the CE.  (In some cases, it may even
   be sufficient for the CE to be configured with a default route
   pointing to the PE.)  This will generally work at any site which does
   not itself need to distribute the default route to other sites.
   (E.g., if one site in a corporate VPN has the corporation's access to
   the Internet, that site might need to have default distributed to the
   other site, but one could not distribute default to that site

   Whatever procedure is used to distribute routes from CE to PE will
   also be used to distribute routes from PE to CE.

8. What if the CE Supports MPLS?

   In the case where the CE supports MPLS, AND is willing to import the
   complete set of routes from its VPNs, the PE can distribute to it a
   label for each such route.  When the PE receives a packet from the CE
   with such a label, it (a) replaces that label with the corresponding
   label that it learned via BGP, and (b) pushes on a label
   corresponding to the BGP next hop for the corresponding route.

8.1. Virtual Sites

   If the CE/PE route distribution is done via BGP, the CE can use MPLS
   to support multiple virtual sites.  The CE may itself contain a
   separate forwarding table for each virtual site, which it populates
   as indicated by the VPN of Origin and Target VPN attributes of the
   routes it receives from the PE.  If the CE receives the full set of
   routes from the PE, the PE will not need to do any address lookup at
   all on packets received from the CE.  Alternatively, the PE may in
   some cases be able to distribute to the CE a single (labeled) default
   route for each VPN.  Then when the PE receives a labeled packet from

   the CE, it would know which forwarding table to look in; the label
   placed on the packet by the CE would identify only the virtual site
   from which the packet is coming.

8.2. Representing an ISP VPN as a Stub VPN

   If a particular VPN is actually an ISP, but its CE routers support
   MPLS, then the VPN can actually be treated as a stub VPN.  The CE and
   PE routers need only exchange routes which are internal to the VPN.
   The PE router would distribute to the CE router a label for each of
   these routes.  Routers at different sites in the VPN can then become
   BGP peers.  When the CE router looks up a packet's destination
   address, the routing lookup always resolves to an internal address,
   usually the address of the packet's BGP next hop.  The CE labels the
   packet appropriately and sends the packet to the PE.

9. Security

   Under the following conditions:

      a) labeled packets are not accepted by backbone routers from
         untrusted or unreliable sources, unless it is known that such
         packets will leave the backbone before the IP header or any
         labels lower in the stack will be inspected, and

      b) labeled VPN-IPv4 routes are not accepted from untrusted or
         unreliable sources,

   the security provided by this architecture is virtually identical to
   that provided to VPNs by Frame Relay or ATM backbones.

   It is worth noting that the use of MPLS makes it much simpler to
   provide this level of security than would be possible if one
   attempted to use some form of IP-within-IP tunneling in place of
   MPLS.  It is a simple matter to refuse to accept a labeled packet
   unless the first of the above conditions applies to it.  It is rather
   more difficult to configure the a router to refuse to accept an IP
   packet if that packet is an IP-within-IP tunnelled packet which is
   going to a "wrong" place.

   The use of MPLS also allows a VPN to span multiple SPs without
   depending in any way on the inter-domain distribution of IPv4 routing

   It is also possible for a VPN user to provide himself with enhanced
   security by making use of Tunnel Mode IPSEC [5].  This is discussed
   in the remainder of this section.

9.1. Point-to-Point Security Tunnels between CE Routers

   A security-conscious VPN user might want to ensure that some or all
   of the packets which traverse the backbone are authenticated and/or
   encrypted. The standard way to obtain this functionality today would
   be to create a "security tunnel" between every pair of CE routers in
   a VPN, using IPSEC Tunnel Mode.

   However, the procedures described so far do not enable the CE router
   transmitting a packet to determine the identify of the next CE router
   that the packet will traverse.  Yet that information is required in
   order to use Tunnel Mode IPSEC.  So we must extend those procedures
   to make this information available.

   A way to do this is suggested in [6].  Every VPN-IPv4 route can have
   an attribute which identifies the next CE router that will be
   traversed if that route is followed.  If this information is provided
   to all the CE routers in the VPN, standard IPSEC Tunnel Mode can be

   If the CE and PE are BGP peers, it is natural to present this
   information as a BGP attribute.

   Each CE that is to use IPSEC should also be configured with a set of
   address prefixes, such that it is prohibited from sending insecure
   traffic to any of those addresses.  This prevents the CE from sending
   insecure traffic if, for some reason, it fails to obtain the
   necessary information.

   When MPLS is used to carry packets between the two endpoints of an
   IPSEC tunnel, the IPSEC outer header does not really perform any
   function.  It might be beneficial to develop a form of IPSEC tunnel
   mode which allows the outer header to be omitted when MPLS is used.

9.2. Multi-Party Security Associations

   Instead of setting up a security tunnel between each pair of CE
   routers, it may be advantageous to set up a single, multiparty
   security association. In such a security association, all the CE
   routers which are in a particular VPN would share the same security
   parameters (.e.g., same secret, same algorithm, etc.). Then the
   ingress CE wouldn't have to know which CE is the next one to receive
   the data, it would only have to know which VPN the data is going to.
   A CE which is in multiple VPNs could use different security
   parameters for each one, thus protecting, e.g., intranet packets from
   being exposed to the extranet.

   With such a scheme, standard Tunnel Mode IPSEC could not be used,
   because there is no way to fill in the IP destination address field
   of the "outer header".  However, when MPLS is used for forwarding,
   there is no real need for this outer header anyway; the PE router can
   use MPLS to get a packet to a tunnel endpoint without even knowing
   the IP address of that endpoint; it only needs to see the IP
   destination address of the "inner header".

   A significant advantage of a scheme like this is that it makes
   routing changes (in particular, a change of egress CE for a
   particular address prefix) transparent to the security mechanism.
   This could be particularly important in the case of multi-provider
   VPNs, where the need to distribute information about such routing
   changes simply to support the security mechanisms could result in
   scalability issues.

   Another advantage is that it eliminates the need for the outer IP
   header, since the MPLS encapsulation performs its role.

10. Quality of Service

   Although not the focus of this paper, Quality of Service is a key
   component of any VPN service.  In MPLS/BGP VPNs, existing L3 QoS
   capabilities can be applied to labeled packets through the use of the
   "experimental" bits in the shim header [10], or, where ATM is used as
   the backbone, through the use of ATM QoS capabilities.  The traffic
   engineering work discussed in [1] is also directly applicable to
   MPLS/BGP VPNs.  Traffic engineering could even be used to establish
   LSPs with particular QoS characteristics between particular pairs of
   sites, if that is desirable.  Where an MPLS/BGP VPN spans multiple
   SPs, the architecture described in [7] may be useful.  An SP may
   apply either intserv or diffserv capabilities to a particular VPN, as

11. Scalability

   We have discussed scalability issues throughout this paper.  In this
   section, we briefly summarize the main characteristics of our model
   with respect to scalability.

   The Service Provider backbone network consists of (a) PE routers, (b)
   BGP Route Reflectors, (c) P routers (which are neither PE routers nor
   Route Reflectors), and, in the case of multi-provider VPNs, (d)

   P routers do not maintain any VPN routes.  In order to properly
   forward VPN traffic, the P routers need only maintain routes to the
   PE routers and the ASBRs. The use of two levels of labeling is what
   makes it possible to keep the VPN routes out of the P routers.

   A PE router to maintains VPN routes, but only for those VPNs to which
   it is directly attached.

   Route reflectors and ASBRs can be partitioned among VPNs so that each
   partition carries routes for only a subset of the VPNs provided by
   the Service Provider. Thus no single Route Reflector or ASBR is
   required to maintain routes for all the VPNs.

   As a result, no single component within the Service Provider network
   has to maintain all the routes for all the VPNs.  So the total
   capacity of the network to support increasing numbers of VPNs is not
   limited by the capacity of any individual component.

12. Intellectual Property Considerations

   Cisco Systems may seek patent or other intellectual property
   protection for some of all of the technologies disclosed in this
   document. If any standards arising from this document are or become
   protected by one or more patents assigned to Cisco Systems, Cisco
   intends to disclose those patents and license them on reasonable and
   non-discriminatory terms.

13. Security Considerations

   Security issues are discussed throughout this memo.

14. Acknowledgments

   Significant contributions to this work have been made by Ravi
   Chandra, Dan Tappan and Bob Thomas.

15. Authors' Addresses

   Eric C. Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824

   EMail: erosen@cisco.com

   Yakov Rekhter
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134

   EMail: yakov@cisco.com

16. References

   [1] Awduche, Berger,  Gan, Li, Swallow, and Srinavasan,  "Extensions
       to RSVP for LSP Tunnels", Work in Progress.

   [2] Bates, T. and R. Chandrasekaran, "BGP Route Reflection: An

       Extensions for BGP4", RFC 2283, February 1998.

   [4] Gleeson, Heinanen, and Armitage, "A Framework for IP Based
       Virtual Private Networks", Work in Progress.

   [5] Kent and Atkinson, "Security Architecture for the Internet
       Protocol", RFC 2401, November 1998.

   [6] Li, "CPE based VPNs using MPLS", October 1998, Work in Progress.

   [7] Li, T. and Y. Rekhter, "A Provider Architecture for
       Differentiated Services and Traffic Engineering (PASTE)", RFC
       2430, October 1998.

   [8] Rekhter and Rosen, "Carrying Label Information in BGP4", Work in

   [9] Rosen, Viswanathan, and Callon, "Multiprotocol Label Switching
       Architecture", Work in Progress.

  [10] Rosen, Rekhter, Tappan, Farinacci, Fedorkow, Li, and Conta, "MPLS
       Label Stack Encoding", Work in Progress.

17.  Full Copyright Statement


   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


User Contributions:
