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Versions: 00 01

Network Working Group                                      Eric C. Rosen
Internet Draft                                              Peter Psenak
Expiration Date: January 2002                        Cisco Systems, Inc.

                                                    Padma Pillay-Esnault
                                                  Juniper Networks, Inc.

                                                               July 2001


              OSPF as the PE/CE Protocol in BGP/MPLS VPNs


                   draft-ietf-ppvpn-ospf-2547-00.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   [VPN] describes a method of providing a VPN service.  That method
   allows a variety of different protocols to be used as the routing
   protocol between the Customer Edge (CE) router and the Provider Edge
   (PE) router.  However, it does not fully specify the procedures which
   must be implemented within the Provider's network when OSPF is used
   as the PE/CE routing protocol.  This document provides that
   specification.







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Table of Contents

    1        Specification of Requirements  ........................   2
    2        Introduction  .........................................   2
    3        Requirements  .........................................   3
    4        BGP/OSPF Interaction Procedures for PE routers  .......   5
    4.1      Overview  .............................................   5
    4.2      Details  ..............................................   7
    4.2.1    General  ..............................................   7
    4.2.2    Handling LSAs from the CE  ............................   9
    4.2.3    Sham Links  ...........................................  11
    4.2.3.1  Intra-Area Routes  ....................................  11
    4.2.3.2  Creating Sham Links  ..................................  12
    4.2.3.3  Treatment of Sham Links  ..............................  13
    4.2.4    VPN-IP routes received via BGP  .......................  13
    5        ID Summary for Sub-IP Area  ...........................  15
    5.1      Summary  ..............................................  15
    5.2      Related Documents  ....................................  15
    5.3      Targeted at PPVPN WG  .................................  15
    6        Acknowledgments  ......................................  15
    7        Authors' Address  .....................................  16
    8        Bibliography  .........................................  16





1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.


2. Introduction

   [VPN] describes a method by which a Service Provider (SP) can use its
   IP backbone to provide a VPN service to customers.  In that method, a
   customer's edge devices (CE devices) are connected to the provider's
   edge routers (PE routers).  If the CE device is a router, then the PE
   router may become a routing peer of the CE router (in some routing
   protocol), and may as a result learn the routes which lead to the
   CE's site and which need to be distributed to other PE routers that
   attach to the same VPN.

   The PE routers which attach to a common VPN use BGP to distribute the



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   VPN's routes to each other.  A CE router can then learn the routes to
   other sites in the VPN by peering with its attached PE router in a
   routing protocol.  CE routers at different sites do not, however,
   peer with each other.

   It can be expected that many VPNs will use OSPF as their internal
   routing protocol.  This does not necessarily mean that the PE routers
   need to use OSPF to peer with the CE routers.  Each site in a VPN can
   use OSPF as its intra-site routing protocol, while using, e.g., BGP
   or RIP to distribute routes to a PE router.  However, it is certainly
   convenient, when OSPF is being used intra-site, to use it on the PE-
   CE link as well, and [VPN] explicitly allows this.

   Like anything else, the use of OSPF on the PE-CE link has advantages
   and disadvantages.  The disadvantage to using OSPF on the PE-CE link
   is that it gets the PE router involved in a VPN site's IGP, however
   peripherally.  The advantages though are:

     - The administrators of the CE router need not have any expertise
       in any routing protocol other than OSPF.

     - The CE routers do not need to have support for any routing
       protocols other than OSPF.

     - If a customer is transitioning his network from a traditional
       OSPF backbone to the VPN service described in [VPN], the use of
       OSPF on the PE-CE link eases the transitional issues.

   It seems likely that some SPs and their customers will resolve these
   trade-offs in favor of the use of OSPF on the PE-CE link.  Thus we
   need to specify the procedures which must be implemented by a PE
   router in order to make this possible.  (No special procedures are
   needed in the CE router though; CE routers just run whatever OSPF
   implementations they may have.)


3. Requirements

   Consider a set of VPN sites which are thought of as a common "OSPF
   domain".  These will almost certainly be a set of sites which
   together constitute an "intranet", and each of which runs OSPF as its
   intra-site routing protocol.

   Per [VPN], the VPN routes are distributed among the PE routers by
   BGP.  If the PE uses OSPF to distribute routes to the CE router, the
   standard procedures governing BGP/OSPF interactions [OSPF] would
   cause routes from one site to be delivered to another as AS-external
   routes (in type 5 LSAs).  This is undesirable; it would be much



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   better to deliver such routes in type 3 LSAs (as inter-area routes),
   so that they can be distinguished from any "real" AS-external routes
   that may be circulating in the VPN. (That is, so that they can be
   distinguished by OSPF from routes which really do not come from
   within the VPN.)  Hence it is necessary for the PE routers to
   implement a modified version of the BGP/OSPF interaction procedures.

   In fact, we would like to have a very general set of procedures which
   allows a customer to easily replace a legacy private OSPF backbone
   with the VPN service.  We would like this procedure to meet the
   following set of requirements:

     - The procedures should not make assumptions about the OSPF
       topology.  In particular, it should not be assumed that customer
       sites are OSPF stub sites or NSSA sites.  Nor should it be
       assumed that a customer site contains only one OSPF area, or that
       it has no area 0 routers.

     - If VPN sites A and B are in the same OSPF domain, then routes
       from one should be presented to the other as OSPF intra-network
       routes.  In general, this can be done by presenting such routes
       in type 3 LSAs.

       Note that this allows two VPN sites to be connected via an "OSPF
       backdoor link".  That is, one can have an OSPF link between the
       two sites which is used only when the VPN backbone is
       unavailable.  (This would not be possible with the ordinary
       BGP/OSPF interaction procedures.  The ordinary procedures would
       present routes via the VPN backbone as AS-external routes, and
       these could never be preferred to intra-network routes.)  This
       may be very useful during a period of transition from a legacy
       OSPF backbone to a VPN backbone.

     - It should be possible to make use of an "OSPF backdoor link"
       between two sites, even if the two sites are in the same OSPF
       area, and neither of the routers attached to the inter-site
       backdoor link is an area 0 router.  This can also be very useful
       during a transition period, and eliminates any need to
       reconfigure the sites' routers to be ABRs.

       Assuming that it is desired to have the route via the VPN
       backbone be preferred to the backdoor route, the VPN backbone
       itself must be presented to the CE routers at each site as a link
       between the two PE routers to which the CE routers are
       respectively attached.






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     - CE routers, connected to PE routers of the VPN service, may
       themselves function as OSPF backbone (area 0) routers.  An OSPF
       backbone may even consist of several "segments" which are
       interconnected themselves only via the VPN service. In such a
       scenario, full intercommunication between sites connected to
       different segments of the OSPF backbone should still be possible.

     - The transition from the legacy private OSPF backbone to the VPN
       service must be simple and straightforward.  The transition is
       likely to be phased, such that customer sites are migrated one by
       one from the legacy private OSPF backbone to the VPN service.
       During the transition, any given site might be connected to the
       VPN service, to the legacy OSPF backbone, or to both. Complete
       connectivity among all such sites must be maintained.

       Since the VPN service is to replace the legacy backbone, it must
       be possible, by suitable adjustment of the OSPF metrics, to make
       OSPF prefer routes which traverse the SP's VPN backbone to
       alternative routes which do not.

     - The OSPF metric assigned to a given route should be carried
       transparently over the VPN backbone.

   Routes from sites which are not in the same OSPF domain will appear
   as AS-external routes.

   We presuppose familiarity with the contents of [OSPF], including the
   OSPF LSA types, and will refer without further exegesis to type 1, 2,
   3, etc., LSAs.  Familiarity with [VPN] is also presupposed.


4. BGP/OSPF Interaction Procedures for PE routers

4.1. Overview

   [VPN] defines the notion of a Per-Site Routing and Forwarding Table,
   or VRF. A PE router must be capable of running multiple instances of
   OSPF, where each instance is associated with a particular VRF.
   (Whether these instances are realized as separate processes, or
   merely as separate contexts of a common process, is an implementation
   matter.)

   BGP is used to distribute routes among the set of PE routers that
   attach to a single OSPF domain.  Per [VPN], these routes are
   distributed as VPN-IP routes.  Import/export to/from particular VRFs
   is governed via Route Targets.  To meet the above requirements, a PE
   which imports a particular route into a particular VRF needs to know
   whether that route comes from the same OSPF domain and the same OSPF



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   area as the CEs to which it is attached.  Our procedure is to encode
   this information as BGP Extended Communities attributes [EXT], and
   have BGP distribute it along with the VPN-IP route.  The OSPF metric
   of a route is also carried as a BGP attribute of the route.

   If two PEs attach to different VPN sites that are in the same OSPF
   area (as indicated by the OSPF area number), the PE/CE links to those
   site MAY be treated as links within that area. In addition, each PE
   MAY flood, into that area, a type 1 LSA advertising a link to the
   other PE.  If this procedure is followed, two VPN sites in the same
   OSPF area will see the VPN backbone as a link within that area, a
   link between the two PE routers.  We refer to this link as a "sham
   link".  This allows routes from one site to the other to be treated
   as intra-area routes.  The procedures governing the use of sham links
   are specified in Section 4.2.3.

   Every PE attached to a particular OSPF network MUST be an OSPF area 0
   router. This allows it to distribute inter-area routes to the CE via
   Type 3 LSAs.  The CE router might or might not be an area 0 router,
   and the PE/CE link might or might not be an area 0 link.

   If the OSPF network contains area 0 routers (other than the PE
   routers), at least one PE router must have an area 0 link to a non-PE
   area 0 router in that OSPF network. (The non-PE area 0 router
   functions as a CE router.) This ensures that inter-area routes and
   AS-external routes can be leaked between the PE routers and the non-
   PE OSPF backbone.

   When a type 3 LSA is sent over an area 0 link from a PE router to a
   CE router, a special bit in the LSA Options field is set.  This is
   used to ensure that if any CE router sends this type 3 LSA to a PE
   router, the PE router will not further redistribute it.

   Two sites which are not in the same OSPF area will see the VPN
   backbone as being an integral part of the OSPF backbone. However, if
   there are area 0 routers which are NOT PE routers, then the VPN
   backbone actually functions as a sort of higher level backbone,
   providing a third level of hierarchy above area 0.  This allows a
   legacy OSPF backbone to become disconnected during a transition
   period, as long as the various segments all attach to the VPN
   backbone.

   When a PE router needs to distribute to a CE router a route which
   comes from a site outside the latter's OSPF domain, the PE router
   presents itself as an ASBR, and distributes the route in a type 5
   LSA.  OSPF route tagging is used to ensure that a type 5 LSA
   generated by a PE router will be ignored by any other PE router that
   may receive it.  A special OSPF route tag, which we will call the VPN



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   Route Tag, will be used for this purpose.


4.2. Details

4.2.1. General

   If a PE and a CE are communicating via OSPF, the PE MUST create, and
   MUST flood to the CE, a type 1 LSA advertising its link to the CE.
   The PE MUST have an OSPF router id which is valid (i.e., unique)
   within the OSPF domain. The PE MUST also be configured to know which
   OSPF area the link is in. A PE-CE link may be in any area, including
   area 0; this is a matter of the OSPF configuration.

   Whether or not the PE-CE link is an area 0 link, the PE itself MUST
   be an OSPF area 0 router.  That is, the link state topology from a
   site will not be passed along by the PE.

   The PE MUST support at least one OSPF instance for each OSPF domain
   to which it attaches.  Each instance of OSPF MUST be associated with
   a single VRF.  If n CEs associated with that VRF are running OSPF on
   their respective PE/CE links, then those n CEs are OSPF adjacencies
   of the PE in the corresponding instance of OSPF.  Generally, though
   not necessarily, if the PE attaches to several CEs in the same OSPF
   domain, it will associate the interfaces to those PEs with a single
   VRF.

   If the OSPF domain has any area 0 routers (other than the PE
   routers), then at least one of those MUST be a CE router, and MUST
   have an area 0 link to at least one PE router. This adjacency MAY be
   via an OSPF virtual link.

   Each OSPF domain MUST be associated with a Domain Identifier.  This
   MUST be configurable, and the default value (if none is configured)
   MUST be zero.  More precisely, each VRF associated with an OSPF
   instance will either be configured with a Domain Identifier, or else
   will use zero as its Domain Identifier.  All VRFs which correspond to
   the same OSPF domain MUST be configured with the same value of this
   attribute, or else all must use the default value of zero.

   If a non-zero Domain Identifier value is configured, it must be a
   globally unique value which can be encoded in a BGP Extended
   Communities attribute.  Routes from a particular OSPF domain will,
   when distributed in BGP as VPN-IPv4 routes, carry this attribute.
   (However, if the attribute value is zero, then the attribute SHOULD
   not be carried.) The OSPF Domain will be encoded as the VPN of Origin
   Extended Community [VPN, EXT] attribute.  (It is shown in [VPN] how
   to assign globally unique values to this attribute.)



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   If a Domain Identifier is not configured for a particular OSPF
   domain, then care must be taken to avoid having routes from that OSPF
   domain and routes from another OSPF domain imported into the same
   VRF.

   If a particular VRF in a PE is associated with an instance of OSPF,
   then it will be configured with a special OSPF route tag value, which
   we call the VPN Route Tag.  This route tag MUST be included in the
   Type 5 LSAs which the PE originates (as the result of receiving a
   BGP-distributed VPN-IP route, see section 4.2.4) and sends to any of
   the attached CEs. Its value is arbitrary, but must be distinct from
   any OSPF Route Tag being used within the OSPF domain.  Its value MUST
   therefore be configurable.  The default value SHOULD be an
   automatically computed tag based on the Autonomous System number of
   the VPN backbone:


   Tag = <Automatic = 1, Complete = 1, PathLength = 01>

          0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |1|1|0|1|     ArbitraryTag      |       AutonomousSystem        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 _AS number of the VPN Backbone_



   If a PE router needs to use OSPF to distribute to a CE router a route
   which comes from a site outside the CE router's OSPF domain, the PE
   router SHOULD present itself to the CE router as an Autonomous System
   Border Router (ASBR), and SHOULD report such routes as AS-external
   routes.  That is, these PE routers originate Type 5 LSAs reporting
   the extra-domain routes as AS-external routes. Each such Type 5 LSA
   MUST contain an OSPF route tag whose value is that of the VPN Route
   Tag.  This tag identifies the route as having come from a PE router.
   The VPN Route Tag MUST be used to ensure that a Type 5 LSA originated
   by a PE router is not redistributed through the OSPF area to another
   PE router.

   When a PE/CE link is an area 0 link, the high-order bit of the LSA
   Options field (previously unused) is used to distinguish type 3 LSAs
   which report routes across the VPN backbone from other VPN sites. We
   refer to this as the DN bit. The DN bit is set in a type 3 LSA which
   is sent from a PE router to a CE router across an area 0 link. When a
   PE router receives, from a CE router, a type 3 LSA with the DN bit
   set, the route is ignored. Without this mechanism, the type 3 LSA



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   could be sent by a PE to a CE, flooded through a number of OSPF
   routers, and then sent to another PE; this could cause a loop.  This
   mechanism prevents such loops.


4.2.2. Handling LSAs from the CE

   This section specifies the way in which a PE router handles the OSPF
   LSAs it receives from a CE router.

   If a route is received in a Type 5 LSA, and if it has an OSPF route
   tag value equal to the VPN Route Tag, then that route is ignored.

   If a route is received in a Type 3 LSA, and if it has the DN bit set
   in its Options Field, then the route is ignored.

   Otherwise, there is normal OSPF processing, for the relevant instance
   of OSPF in the PE.

   Next, the PE must examine the corresponding VRF. For every address
   prefix which appears there, the PE must create a VPN-IPv4 route in
   BGP. Each such route will have some of the following Extended
   Community attributes:

     - The VPN of Origin Extended Community attribute.  This MUST be
       present, UNLESS the VRF has the (default) Domain Identifier value
       of zero.  In this case, the attribute MAY be omitted.

     - OSPF Route Type Extended Community Attribute. This attribute MUST
       be present, and is encoded as follows:

         * Type: 0x8000

         * Area Number: 4 bytes, encoding a 32-bit area number. For AS-
           external routes, the value is 0. A non-zero value identifies
           the route as being internal to the OSPF domain, and as being
           within the identified area. Area numbers are relative to a
           particular OSPF domain.

         * OSPF Route Type: 1 byte, encoded as follows:

             ** 1 or 2 for intra-area routes (depending on whether the
                route came from a type 1 or a type 2 LSA -- however this
                difference is not significant to the procedures
                specified herein)






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             ** 3 for summary routes

             ** 5 for external routes (area number must be 0)

             ** 7 for NSSA routes.

             ** 129 for Sham Link Endpoint Addresses.  See section
                4.2.3.2 for the specification of when this value must be
                used.

         * Options: 1 byte. Currently this is only used if the route
           type is 5 or 7. Setting the least significant bit in the
           field indicates that the route carries a type 2 metric.

     - OSPF Router Id Extended Community.  This OPTIONAL attribute
       specifies the OSPF Router Id of the system that is identified in
       the BGP Next Hop attribute.  More precisely, it specifies the
       Router Id of the particular VRF from which this route was
       exported.

         * Type: 0x8001

         * Router Address: 4 Bytes

         * Unused: 2 Bytes.

     - MED. By default, this SHOULD be set to the value of the OSPF
       distance associated with the route, plus 1.

   The intention of all this is the following. OSPF Routes from one site
   are converted to BGP, distributed across the VPN backbone, and
   possibly converted back to OSPF routes before being distributed into
   another site. With these attributes, BGP carries enough information
   about the route to enable the route to be converted back into OSPF
   "transparently", just as if BGP had not been involved.

   Routes which a PE receives in type 4 LSAs MUST be ignored.

   The attributes specified above are in addition to any other
   attributes which routes must carry in accord with the [VPN].

   The Site of Origin attribute, which is usually required by [VPN], is
   OPTIONAL for routes which a PE learns from a CE via OSPF.

   Use of the Site of Origin attribute would, in the case of a multiply
   homed site (i.e., a site attached to several PE routers), prevent an
   intra-site route from being reinjected into a site from the VPN
   backbone. Such a reinjection would not harm the routing, because the



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   route via the VPN backbone would be advertised in a type 3 LSA, and
   hence would appear to be an inter-area route; the real intra-area
   route would be preferred. But unnecessary overhead would be
   introduced. On the other hand, if the Site of Origin attribute is not
   used, a partitioned site will find itself automatically repaired,
   since traffic from one partition to the other will automatically
   travel via the VPN backbone. Therefore the use of a Site of Origin
   attribute is optional, so that a trade-off can be made between the
   cost of the increased overhead and the value of automatic partition
   repair.


4.2.3. Sham Links

4.2.3.1. Intra-Area Routes

   Suppose there are two sites in the same OSPF area.  Each site is
   attached to a different PE router, and there is also an intra-area
   OSPF link connecting the two sites.

   It is possible to treat these two sites as a single VPN site which
   just happens to be multihomed to the backbone.  This is in fact the
   simplest thing to do, and is perfectly adequate, providing that the
   preferred route between the two sites is via the intra-area OSPF link
   (a "backdoor link"), rather than via the VPN backbone.  There will be
   routes between sites that go through the PE routers, but these routes
   will appear to be inter-area routes, and OSPF will consider them to
   be less preferable than the intra-area routes through the backdoor
   link.

   If it is desired to have OSPF prefer the routes through the backbone
   over the routes through the backdoor link, then the routes through
   the backbone must be appear to be intra-area routes.  To make a route
   through the backbone appear to be an inter-area route, it is
   necessary to make it appear as if there is an intra-area link
   connecting the two PE routers.  This is what we refer to as a "sham
   link".  (If the two sites attach to the same PE router, this of
   course is not necessary.)

   A sham link can be thought of as a relation between two VRFs.  If two
   VRFs are to be connected by a sham link, each VRF must be associated
   with a "Sham Link Endpoint Address", a /32 address which is treated
   as an address of the PE router containing that VRF.  The Sham Link
   Endpoint Address associated with a VRF MUST be configurable, and it
   MAY default to the VRF's router id.  The Sham Link Endpoint Address
   is an address in the VPN's address space, not the SP's address space.

   A VRF needs only a single Sham Link Endpoint Address, no matter how



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   many sham links it has.  It MUST be distributed by BGP as a VPN-IPv4
   address, and it MAY be distributed by OSPF.


4.2.3.2. Creating Sham Links

   Sham links may be manually configured, or they may be auto-
   configured.

   Each VRF that is associated with a PE-CE link on which OSPF is
   running MUST be configurable as to whether auto-configuration of sham
   links to/from that VRF is allowed.  The default MUST be "manual
   configuration".

   If a VRF is configured for manual configuration of the sham links, it
   will never initiate auto-configuration of a sham link, nor will it
   ever create a sham link as the result of a remotely initiated auto-
   configuration procedure.  If a VRF is configured for auto-
   configuration of sham links, manual configuration of particular sham
   links is still allowed.  In any event, no more than one sham link
   with the same pair of sham link endpoint addresses will ever be
   created.

   To manually configure a sham link between two VRFs, each VRF has to
   be configured to create a sham link to the other, where the "other"
   is identified by its sham link endpoint address.  Procedures for
   single-ended manual configuration of the sham link are for further
   study.

   IF a VRF is configured for auto-configuration of sham links, it MUST
   distribute, via BGP, a VPN-IP route corresponding to the Sham Link
   Endpoint Address.  This route MUST have the OSPF Route Type Extended
   Community attribute, with the OSPF Route Type field set to "Sham Link
   Endpoint Address".

   When a PE receives such a route, with a RT value that allows the
   route to be imported into a particular VRF, then if

     - that route has an OSPF Domain Id which matches that of the VRF
       (an OSPF Domain Id with value 0 is considered a match is the VRF
       has no OSPF Domain Id), AND

     - that route has an OSPF area number which matches that of the VRF,

   then a sham link may be created from the local VRF to the VRF
   identified by the sham link endpoint address.

   If all VRFs are configured for auto-configuration of sham links, a



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   full mesh of sham links will be created among all the sites which are
   in the same OSPF area.  This may not be what is desired.  To obtain
   more control over the set of sham links which are created, some or
   all of the VRFs can be configured to disable auto-configuration of
   the sham links.

   Note that sham links may be created for any area, including area 0.


4.2.3.3. Treatment of Sham Links

   Sham links SHOULD be treated by OSPF as OSPF Demand Circuits.  This
   means that LSAs will be flooded over them, but periodic refresh
   traffic is avoided.  Note that, as long as the backdoor link is up,
   flooding the LSAs over the sham link serves no purpose.  However, if
   the backdoor link goes down, OSPF does not have mechanisms enabling
   the routers in one site to rapidly flush the LSAs from the other
   site.  Therefore it is still necessary to maintain synchronization
   among the LSA databases at the two sites, hence the flooding over the
   sham link.

   The sham link is an unnumbered point-to-point intra-area link, and is
   advertised by means of a type 1 LSA.

   The OSPF metric associated with a sham link must be configurable (and
   there must be a configurable default). Whether traffic between the
   sites flows via a backdoor link or via the VPN backbone (i.e., via
   the sham link) depends on the settings of the OSPF link metrics.  The
   metrics can be set so that the backdoor link is not used unless
   connectivity via the VPN backbone fails, for example.

   If a PE determines that a particular route traverses the sham link,
   then the PE SHOULD NOT redistribute that route into BGP as a VPN-IPv4
   route.



4.2.4. VPN-IP routes received via BGP

   This section describes how the PE router handles VPN-IP routes
   received via BGP.

   If a received BGP VPN-IP route is not installed in the VRF, nothing
   is reported to the CE. A received route will not be installed into
   the VRF if some other route is preferable. (Note that a route which
   is not installed in the VRF may still cause the PE to create an OSPF
   link to another PE as specified in the previous section.)




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   Note that according to the usual OSPF route preference rules, intra-
   area routes, as computed by the OSPF, will be installed in the VRF in
   preference to any other routes received over BGP. This means that the
   CE will simply not hear about inter-area or external routes to
   address prefixes for which there is an intra-area route.

   In the following, we specify what is reported, in OSPF LSAs, by the
   PE to the CE, assuming that the PE is not configured to do any
   further summarization or filtering of the routing information before
   reporting it to the CE.

   If ONE of the following conditions holds for a given route that is
   received via BGP:

     - the VRF has a non-zero Domain Identifier, but the route does not
       have a non-zero VPN of Origin attribute, or

     - the route has a VPN of Origin attribute whose value is not the
       same as the Domain Identifier associated with the VRF, or

   then the route MUST be distributed to the CE in a type 5 LSA with a
   type 2 metric. By default, the MED, if present, is converted to a
   type 2 metric. If the MED is not present, a default type 2 metric
   value is used.

   Otherwise, if the route has an OSPF route type of external route, it
   MUST be advertised to the CE in a type 5 LSA. The VPN Route Tag (see
   section 4.2.1) MUST be placed in the LSA.  By default, the MED, if
   present, is converted to a type 1 or type 2 metric, as determined by
   the external route property of the VPN-IPv4 route.  If no MED is
   present, a default type 2 metric value is used.

   Otherwise, if the route has an OSPF route type of "summary route",
   the route should be treated as if it had been received in an OSPF
   type 3 LSA. This means that the PE will report the route in a type 3
   LSA to the CE. (Note that this case is possible even if the VPN-IP
   route carries an area number identical to that of the CE router.
   This means that if an area is "partitioned" such that the two pieces
   are connected only via the VPN backbone, it appears to be two areas,
   with inter-area routes between them.)  If the link from the PE to the
   CE is an area 0 link, the DN bit MUST be set in this LSA; if the link
   is not an area 0 link, the DN bit MAY be set.

   Note that this way of handling AS-external routes makes every PE
   appear to be an ASBR attached to all the AS-external routes. In a
   multihomed site, this can result in a number of type 5 LSAs
   containing the same information.




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5. ID Summary for Sub-IP Area

5.1. Summary

   The base specification for RFC2547 VPNs, i.e., [VPN], allows for
   several methods by which a VPN customer may distribute VPN routes to
   and receive VPN routes from a service provider.  One such method is
   by the use of OSPF on the link connecting the customer to the service
   provider.  However, the base specification does not describe the full
   set of procedures needed in order to use OSPF in this manner.  This
   draft provides the specification of those procedures.


5.2. Related Documents

    draft-ietf-ppvpn-requirements-00.txt
    draft-ietf-ppvpn-framework-00.txt
    draft-rosen-rfc2547bis-03.txt
    draft-rosen-vpn-mcast-00.txt
    draft-rosen-ppvpn-ipsec-2547-00.txt


5.3. Targeted at PPVPN WG

   This work addresses the following work item from the PPVPN WG
   charter:

           "The working group is expected to consider at least three
           specific approaches, including BGP-VPNs (e.g. RFC 2547)."

   It  extends the  base specification  for RFC2547  VPNs by  adding
   procedures governing a fairly common case.

   The WG should consider this document as it specifically addresses one
   of the work items called out in the charter.


6. Acknowledgments

   Significant contributions to this work have been made by Derek Yeung
   and Yakov Rekhter.

   Thanks to Ross Callon and Ajay Singhal for their comments.








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7. Authors' Address


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

   E-mail: erosen@cisco.com



   Peter Psenak
   Parc Pegasus,
   De Kleetlaan 6A
   1831 Diegem
   Belgium

   E-mail: ppsenak@cisco.com



   Padma Pillay-Esnault
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089

   E-mail: padma@juniper.net



8. Bibliography

   [EXT] "BGP Extended Communities Attribute", draft-ramachandra-bgp-
   ext-communities-09.txt, Ramachandra, S. and Tappan, D.  June 2001.

   [OSPF] "OSPF Version 2", RFC 2328, Moy, J., April 1998.

   [VPN] "BGP/MPLS VPNs", draft-ietf-ppvpn-rfc2547bis-00.txt, Rosen, E.,
   et. al., July 2001.











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