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rfc:rfc6074

Internet Engineering Task Force (IETF) E. Rosen Request for Comments: 6074 B. Davie Category: Standards Track Cisco Systems, Inc. ISSN: 2070-1721 V. Radoaca

                                                        Alcatel-Lucent
                                                                W. Luo
                                                          January 2011
            Provisioning, Auto-Discovery, and Signaling
            in Layer 2 Virtual Private Networks (L2VPNs)

Abstract

 Provider Provisioned Layer 2 Virtual Private Networks (L2VPNs) may
 have different "provisioning models", i.e., models for what
 information needs to be configured in what entities.  Once
 configured, the provisioning information is distributed by a
 "discovery process".  When the discovery process is complete, a
 signaling protocol is automatically invoked to set up the mesh of
 pseudowires (PWs) that form the (virtual) backbone of the L2VPN.
 This document specifies a number of L2VPN provisioning models, and
 further specifies the semantic structure of the endpoint identifiers
 required by each model.  It discusses the distribution of these
 identifiers by the discovery process, especially when discovery is
 based on the Border Gateway Protocol (BGP).  It then specifies how
 the endpoint identifiers are carried in the two signaling protocols
 that are used to set up PWs, the Label Distribution Protocol (LDP),
 and the Layer 2 Tunneling Protocol version 3 (L2TPv3).

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6074.

Rosen, et al. Standards Track [Page 1] RFC 6074 L2VPN Signaling January 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Rosen, et al. Standards Track [Page 2] RFC 6074 L2VPN Signaling January 2011

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
 2.  Signaling Protocol Framework . . . . . . . . . . . . . . . . .  5
   2.1.  Endpoint Identification  . . . . . . . . . . . . . . . . .  5
   2.2.  Creating a Single Bidirectional Pseudowire . . . . . . . .  7
   2.3.  Attachment Identifiers and Forwarders  . . . . . . . . . .  7
 3.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.1.  Individual Point-to-Point Pseudowires  . . . . . . . . . .  9
     3.1.1.  Provisioning Models  . . . . . . . . . . . . . . . . .  9
       3.1.1.1.  Double-Sided Provisioning  . . . . . . . . . . . .  9
       3.1.1.2.  Single-Sided Provisioning with Discovery . . . . .  9
     3.1.2.  Signaling  . . . . . . . . . . . . . . . . . . . . . . 10
   3.2.  Virtual Private LAN Service  . . . . . . . . . . . . . . . 11
     3.2.1.  Provisioning . . . . . . . . . . . . . . . . . . . . . 11
     3.2.2.  Auto-Discovery . . . . . . . . . . . . . . . . . . . . 12
       3.2.2.1.  BGP-Based Auto-Discovery . . . . . . . . . . . . . 12
     3.2.3.  Signaling  . . . . . . . . . . . . . . . . . . . . . . 14
     3.2.4.  Pseudowires as VPLS Attachment Circuits  . . . . . . . 15
   3.3.  Colored Pools: Full Mesh of Point-to-Point Pseudowires . . 15
     3.3.1.  Provisioning . . . . . . . . . . . . . . . . . . . . . 15
     3.3.2.  Auto-Discovery . . . . . . . . . . . . . . . . . . . . 16
       3.3.2.1.  BGP-Based Auto-Discovery . . . . . . . . . . . . . 16
     3.3.3.  Signaling  . . . . . . . . . . . . . . . . . . . . . . 18
   3.4.  Colored Pools: Partial Mesh  . . . . . . . . . . . . . . . 19
   3.5.  Distributed VPLS . . . . . . . . . . . . . . . . . . . . . 19
     3.5.1.  Signaling  . . . . . . . . . . . . . . . . . . . . . . 21
     3.5.2.  Provisioning and Discovery . . . . . . . . . . . . . . 23
     3.5.3.  Non-Distributed VPLS as a Sub-Case . . . . . . . . . . 23
     3.5.4.  Splicing and the Data Plane  . . . . . . . . . . . . . 24
 4.  Inter-AS Operation . . . . . . . . . . . . . . . . . . . . . . 24
   4.1.  Multihop EBGP Redistribution of L2VPN NLRIs  . . . . . . . 24
   4.2.  EBGP Redistribution of L2VPN NLRIs with Multi-Segment
         Pseudowires  . . . . . . . . . . . . . . . . . . . . . . . 25
   4.3.  Inter-Provider Application of Distributed VPLS
         Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 26
   4.4.  RT and RD Assignment Considerations  . . . . . . . . . . . 27
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
 6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
 7.  BGP-AD and VPLS-BGP Interoperability . . . . . . . . . . . . . 29
 8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 30
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 31

Rosen, et al. Standards Track [Page 3] RFC 6074 L2VPN Signaling January 2011

1. Introduction

 [RFC4664] describes a number of different ways in which sets of
 pseudowires may be combined together into "Provider Provisioned Layer
 2 VPNs" (L2 PPVPNs, or L2VPNs), resulting in a number of different
 kinds of L2VPN.  Different kinds of L2VPN may have different
 "provisioning models", i.e., different models for what information
 needs to be configured in what entities.  Once configured, the
 provisioning information is distributed by a "discovery process", and
 once the information is discovered, the signaling protocol is
 automatically invoked to set up the required pseudowires.  The
 semantics of the endpoint identifiers that the signaling protocol
 uses for a particular type of L2VPN are determined by the
 provisioning model.  That is, different kinds of L2VPN, with
 different provisioning models, require different kinds of endpoint
 identifiers.  This document specifies a number of L2VPN provisioning
 models and specifies the semantic structure of the endpoint
 identifiers required for each provisioning model.
 Either LDP (as specified in [RFC5036] and extended in [RFC4447]) or
 L2TP version 3 (as specified in [RFC3931] and extended in [RFC4667])
 can be used as signaling protocols to set up and maintain PWs
 [RFC3985].  Any protocol that sets up connections must provide a way
 for each endpoint of the connection to identify the other; each PW
 signaling protocol thus provides a way to identify the PW endpoints.
 Since each signaling protocol needs to support all the different
 kinds of L2VPN and provisioning models, the signaling protocol must
 have a very general way of representing endpoint identifiers, and it
 is necessary to specify rules for encoding each particular kind of
 endpoint identifier into the relevant fields of each signaling
 protocol.  This document specifies how to encode the endpoint
 identifiers of each provisioning model into the LDP and L2TPv3
 signaling protocols.
 We make free use of terminology from [RFC3985], [RFC4026], [RFC4664],
 and [RFC5659] -- in particular, the terms "Attachment Circuit",
 "pseudowire", "PE" (provider edge), "CE" (customer edge), and "multi-
 segment pseudowire".
 Section 2 provides an overview of the relevant aspects of [RFC4447]
 and [RFC4667].
 Section 3 details various provisioning models and relates them to the
 signaling process and to the discovery process.  The way in which the
 signaling mechanisms can be integrated with BGP-based auto-discovery
 is covered in some detail.

Rosen, et al. Standards Track [Page 4] RFC 6074 L2VPN Signaling January 2011

 Section 4 explains how the procedures for discovery and signaling can
 be applied in a multi-AS environment and outlines several options for
 the establishment of multi-AS L2VPNs.
 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 [RFC2119]

2. Signaling Protocol Framework

2.1. Endpoint Identification

 Per [RFC4664], a pseudowire can be thought of as a relationship
 between a pair of "Forwarders".  In simple instances of Virtual
 Private Wire Service (VPWS), a Forwarder binds a pseudowire to a
 single Attachment Circuit, such that frames received on the one are
 sent on the other, and vice versa.  In Virtual Private LAN Service
 (VPLS), a Forwarder binds a set of pseudowires to a set of Attachment
 Circuits; when a frame is received from any member of that set, a MAC
 (Media Access Control) address table is consulted (and various 802.1d
 procedures executed) to determine the member or members of that set
 on which the frame is to be transmitted.  In more complex scenarios,
 Forwarders may bind PWs to PWs, thereby "splicing" two PWs together;
 this is needed, e.g., to support distributed VPLS and some inter-AS
 scenarios.
 In simple VPWS, where a Forwarder binds exactly one PW to exactly one
 Attachment Circuit, a Forwarder can be identified by identifying its
 Attachment Circuit.  In simple VPLS, a Forwarder can be identified by
 identifying its PE device and its VPN.
 To set up a PW between a pair of Forwarders, the signaling protocol
 must allow the Forwarder at one endpoint to identify the Forwarder at
 the other.  In [RFC4447], the term "Attachment Identifier", or "AI",
 is used to refer to a quantity whose purpose is to identify a
 Forwarder.  In [RFC4667], the term "Forwarder Identifier" is used for
 the same purpose.  In the context of this document, "Attachment
 Identifier" and "Forwarder Identifier" are used interchangeably.
 [RFC4447] specifies two Forwarding Equivalence Class (FEC) elements
 that can be used when setting up pseudowires, the PWid FEC element,
 and the Generalized ID FEC element.  The PWid FEC element carries
 only one Forwarder identifier; it can be thus be used only when both
 forwarders have the same identifier, and when that identifier can be
 coded as a 32-bit quantity.  The Generalized ID FEC element carries
 two Forwarder identifiers, one for each of the two Forwarders being

Rosen, et al. Standards Track [Page 5] RFC 6074 L2VPN Signaling January 2011

 connected.  Each identifier is known as an Attachment Identifier, and
 a signaling message carries both a "Source Attachment Identifier"
 (SAI) and a "Target Attachment Identifier" (TAI).
 The Generalized ID FEC element also provides some additional
 structuring of the identifiers.  It is assumed that the SAI and TAI
 will sometimes have a common part, called the "Attachment Group
 Identifier" (AGI), such that the SAI and TAI can each be thought of
 as the concatenation of the AGI with an "Attachment Individual
 Identifier" (AII).  So the pair of identifiers is encoded into three
 fields: AGI, Source AII (SAII), and Target AII (TAII).  The SAI is
 the concatenation of the AGI and the SAII, while the TAI is the
 concatenation of the AGI and the TAII.
 Similarly, [RFC4667] allows using one or two Forwarder Identifiers to
 set up pseudowires.  If only the target Forwarder Identifier is used
 in L2TP signaling messages, both the source and target Forwarders are
 assumed to have the same value.  If both the source and target
 Forwarder Identifiers are carried in L2TP signaling messages, each
 Forwarder uses a locally significant identifier value.
 The Forwarder Identifier in [RFC4667] is an equivalent term to
 Attachment Identifier in [RFC4447].  A Forwarder Identifier also
 consists of an Attachment Group Identifier and an Attachment
 Individual Identifier.  Unlike the Generalized ID FEC element, the
 AGI and AII are carried in distinct L2TP Attribute-Value Pairs
 (AVPs).  The AGI is encoded in the AGI AVP, and the SAII and TAII are
 encoded in the Local End ID AVP and the Remote End ID AVP,
 respectively.  The source Forwarder Identifier is the concatenation
 of the AGI and SAII, while the target Forwarder Identifier is the
 concatenation of the AGI and TAII.
 In applications that group sets of PWs into "Layer 2 Virtual Private
 Networks", the AGI can be thought of as a "VPN Identifier".
 It should be noted that while different forwarders support different
 applications, the type of application (e.g., VPLS vs. VPWS) cannot
 necessarily be inferred from the forwarders' identifiers.  A router
 receiving a signaling message with a particular TAI will have to be
 able to determine which of its local forwarders is identified by that
 TAI, and to determine the application provided by that forwarder.
 But other nodes may not be able to infer the application simply by
 inspection of the signaling messages.
 In this document, some further structure of the AGI and AII is
 proposed for certain L2VPN applications.  We note that [RFC4447]
 defines a TLV structure for AGI and AII fields.  Thus, an operator
 who chooses to use the AII structure defined here could also make use

Rosen, et al. Standards Track [Page 6] RFC 6074 L2VPN Signaling January 2011

 of different AGI or AII types if he also wanted to use a different
 structure for these identifiers for some other application.  For
 example, the long prefix type of [RFC5003] could be used to enable
 the communication of administrative information, perhaps combined
 with information learned during auto-discovery.

2.2. Creating a Single Bidirectional Pseudowire

 In any form of LDP-based signaling, each PW endpoint must initiate
 the creation of a unidirectional LSP.  A PW is a pair of such LSPs.
 In most of the L2VPN provisioning models, the two endpoints of a
 given PW can simultaneously initiate the signaling for it.  They must
 therefore have some way of determining when a given pair of LSPs are
 intended to be associated together as a single PW.
 The way in which this association is done is different for the
 various different L2VPN services and provisioning models.  The
 details appear in later sections.
 L2TP signaling inherently establishes a bidirectional session that
 carries a PW between two PW endpoints.  The two endpoints can also
 simultaneously initiate the signaling for a given PW.  It is possible
 that two PWs can be established for a pair of Forwarders.
 In order to avoid setting up duplicated pseudowires between two
 Forwarders, each PE must be able to independently detect such a
 pseudowire tie.  The procedures of detecting a pseudowire tie are
 described in [RFC4667].

2.3. Attachment Identifiers and Forwarders

 Every Forwarder in a PE must be associated with an Attachment
 Identifier (AI), either through configuration or through some
 algorithm.  The Attachment Identifier must be unique in the context
 of the PE router in which the Forwarder resides.  The combination
 <PE router, AI> must be globally unique.
 As specified in [RFC4447], the Attachment Identifier may consist of
 an Attachment Group Identifier (AGI) plus an Attachment Individual
 Identifier (AII).  In the context of this document, an AGI may be
 thought of as a VPN-ID, or some attribute that is shared by all the
 Attachment Circuits that are allowed to be connected.
 It is sometimes helpful to consider a set of attachment circuits at a
 single PE to belong to a common "pool".  For example, a set of
 attachment circuits that connect a single CE to a given PE may be
 considered a pool.  The use of pools is described in detail in
 Section 3.3.

Rosen, et al. Standards Track [Page 7] RFC 6074 L2VPN Signaling January 2011

 The details for how to construct the AGI and AII fields identifying
 the pseudowire endpoints in particular provisioning models are
 discussed later in this document.
 We can now consider an LSP for one direction of a pseudowire to be
 identified by:
 o  <PE1, <AGI, AII1>, PE2, <AGI, AII2>>
 and the LSP in the opposite direction of the pseudowire will be
 identified by:
 o  <PE2, <AGI, AII2>, PE1, <AGI, AII1>>
 A pseudowire is a pair of such LSPs.  In the case of using L2TP
 signaling, these refer to the two directions of an L2TP session.
 When a signaling message is sent from PE1 to PE2, and PE1 needs to
 refer to an Attachment Identifier that has been configured on one of
 its own Attachment Circuits (or pools), the Attachment Identifier is
 called a "Source Attachment Identifier".  If PE1 needs to refer to an
 Attachment Identifier that has been configured on one of PE2's
 Attachment Circuits (or pools), the Attachment Identifier is called a
 "Target Attachment Identifier".  (So an SAI at one endpoint is a TAI
 at the remote endpoint, and vice versa.)
 In the signaling protocol, we define encodings for the following
 three fields:
 o  Attachment Group Identifier (AGI)
 o  Source Attachment Individual Identifier (SAII)
 o  Target Attachment Individual Identifier (TAII)
 If the AGI is non-null, then the SAI consists of the AGI together
 with the SAII, and the TAI consists of the TAII together with the
 AGI.  If the AGI is null, then the SAII and TAII are the SAI and TAI,
 respectively.
 The intention is that the PE that receives an LDP Label Mapping
 message or an L2TP Incoming Call Request (ICRQ) message containing a
 TAI will be able to map that TAI uniquely to one of its Attachment
 Circuits (or pools).  The way in which a PE maps a TAI to an
 Attachment Circuit (or pool) should be a local matter (including the
 choice of whether to use some or all of the bytes in the TAI for the
 mapping).  So as far as the signaling procedures are concerned, the
 TAI is really just an arbitrary string of bytes, a "cookie".

Rosen, et al. Standards Track [Page 8] RFC 6074 L2VPN Signaling January 2011

3. Applications

 In this section, we specify the way in which the pseudowire signaling
 using the notion of source and target Forwarder is applied for a
 number of different applications.  For some of the applications, we
 specify the way in which different provisioning models can be used.
 However, this is not meant to be an exhaustive list of the
 applications, or an exhaustive list of the provisioning models that
 can be applied to each application.

3.1. Individual Point-to-Point Pseudowires

 The signaling specified in this document can be used to set up
 individually provisioned point-to-point pseudowires.  In this
 application, each Forwarder binds a single PW to a single Attachment
 Circuit.  Each PE must be provisioned with the necessary set of
 Attachment Circuits, and then certain parameters must be provisioned
 for each Attachment Circuit.

3.1.1. Provisioning Models

3.1.1.1. Double-Sided Provisioning

 In this model, the Attachment Circuit must be provisioned with a
 local name, a remote PE address, and a remote name.  During
 signaling, the local name is sent as the SAII, the remote name as the
 TAII, and the AGI is null.  If two Attachment Circuits are to be
 connected by a PW, the local name of each must be the remote name of
 the other.
 Note that if the local name and the remote name are the same, the
 PWid FEC element can be used instead of the Generalized ID FEC
 element in the LDP-based signaling.
 With L2TP signaling, the local name is sent in Local End ID AVP, and
 the remote name in Remote End ID AVP.  The AGI AVP is optional.  If
 present, it contains a zero-length AGI value.  If the local name and
 the remote name are the same, Local End ID AVP can be omitted from
 L2TP signaling messages.

3.1.1.2. Single-Sided Provisioning with Discovery

 In this model, each Attachment Circuit must be provisioned with a
 local name.  The local name consists of a VPN-ID (signaled as the
 AGI) and an Attachment Individual Identifier that is unique relative
 to the AGI.  If two Attachment Circuits are to be connected by a PW,
 only one of them needs to be provisioned with a remote name (which of

Rosen, et al. Standards Track [Page 9] RFC 6074 L2VPN Signaling January 2011

 course is the local name of the other Attachment Circuit).  Neither
 needs to be provisioned with the address of the remote PE, but both
 must have the same VPN-ID.
 As part of an auto-discovery procedure, each PE advertises its
 <VPN-id, local AII> pairs.  Each PE compares its local <VPN-id,
 remote AII> pairs with the <VPN-id, local AII> pairs advertised by
 the other PEs.  If PE1 has a local <VPN-id, remote AII> pair with
 value <V, fred>, and PE2 has a local <VPN-id, local AII> pair with
 value <V, fred>, PE1 will thus be able to discover that it needs to
 connect to PE2.  When signaling, it will use "fred" as the TAII, and
 will use V as the AGI.  PE1's local name for the Attachment Circuit
 is sent as the SAII.
 The primary benefit of this provisioning model when compared to
 Double-Sided Provisioning is that it enables one to move an
 Attachment Circuit from one PE to another without having to
 reconfigure the remote endpoint.  However, compared to the approach
 described in Section 3.3 below, it imposes a greater burden on the
 discovery mechanism, because each Attachment Circuit's name must be
 advertised individually (i.e., there is no aggregation of Attachment
 Circuit names in this simple scheme).

3.1.2. Signaling

 The LDP-based signaling follows the procedures specified in
 [RFC4447].  That is, one PE (PE1) sends a Label Mapping message to
 another PE (PE2) to establish an LSP in one direction.  If that
 message is processed successfully, and there is not yet an LSP for
 the pseudowire in the opposite (PE1->PE2) direction, then PE2 sends a
 Label Mapping message to PE1.
 In addition to the procedures of [RFC4447], when a PE receives a
 Label Mapping message, and the TAI identifies a particular Attachment
 Circuit that is configured to be bound to a point-to-point PW, then
 the following checks must be made.
 If the Attachment Circuit is already bound to a pseudowire (including
 the case where only one of the two LSPs currently exists), and the
 remote endpoint is not PE1, then PE2 sends a Label Release message to
 PE1, with a Status Code meaning "Attachment Circuit bound to
 different PE", and the processing of the Mapping message is complete.
 If the Attachment Circuit is already bound to a pseudowire (including
 the case where only one of the two LSPs currently exists), but the AI
 at PE1 is different than that specified in the AGI/SAII fields of the
 Mapping message then PE2 sends a Label Release message to PE1, with a

Rosen, et al. Standards Track [Page 10] RFC 6074 L2VPN Signaling January 2011

 Status Code meaning "Attachment Circuit bound to different remote
 Attachment Circuit", and the processing of the Mapping message is
 complete.
 Similarly, with the L2TP-based signaling, when a PE receives an ICRQ
 message, and the TAI identifies a particular Attachment Circuit that
 is configured to be bound to a point-to-point PW, it performs the
 following checks.
 If the Attachment Circuit is already bound to a pseudowire, and the
 remote endpoint is not PE1, then PE2 sends a Call Disconnect Notify
 (CDN) message to PE1, with a Status Code meaning "Attachment Circuit
 bound to different PE", and the processing of the ICRQ message is
 complete.
 If the Attachment Circuit is already bound to a pseudowire, but the
 pseudowire is bound to a Forwarder on PE1 with the AI different than
 that specified in the SAI fields of the ICRQ message, then PE2 sends
 a CDN message to PE1, with a Status Code meaning "Attachment Circuit
 bound to different remote Attachment Circuit", and the processing of
 the ICRQ message is complete.
 These errors could occur as the result of misconfigurations.

3.2. Virtual Private LAN Service

 In the VPLS application [RFC4762], the Attachment Circuits can be
 thought of as LAN interfaces that attach to "virtual LAN switches",
 or, in the terminology of [RFC4664], "Virtual Switching Instances"
 (VSIs).  Each Forwarder is a VSI that attaches to a number of PWs and
 a number of Attachment Circuits.  The VPLS service requires that a
 single pseudowire be created between each pair of VSIs that are in
 the same VPLS.  Each PE device may have multiple VSIs, where each VSI
 belongs to a different VPLS.

3.2.1. Provisioning

 Each VPLS must have a globally unique identifier, which in [RFC4762]
 is referred to as the VPLS identifier (or VPLS-id).  Every VSI must
 be configured with the VPLS-id of the VPLS to which it belongs.
 Each VSI must also have a unique identifier, which we call a VSI-ID.
 This can be formed automatically by concatenating its VPLS-id with an
 IP address of its PE router.  (Note that the PE address here is used
 only as a form of unique identifier; a service provider could choose
 to use some other numbering scheme if that was desired, as long as

Rosen, et al. Standards Track [Page 11] RFC 6074 L2VPN Signaling January 2011

 each VSI is assigned an identifier that is unique within the VPLS
 instance.  See Section 4.4 for a discussion of the assignment of
 identifiers in the case of multiple providers.)

3.2.2. Auto-Discovery

3.2.2.1. BGP-Based Auto-Discovery

 This section specifies how BGP can be used to discover the
 information necessary to build VPLS instances.
 When BGP-based auto-discovery is used for VPLS, the AFI/SAFI (Address
 Family Identifier / Subsequent Address Family Identifier) [RFC4760]
 will be:
 o  An AFI (25) for L2VPN.  (This is the same for all L2VPN schemes.)
 o  A SAFI (65) specifically for an L2VPN service whose pseudowires
    are set up using the procedures described in the current document.
 See Section 6 for further discussion of AFI/SAFI assignment.
 In order to use BGP-based auto-discovery, there must be at least one
 globally unique identifier associated with a VPLS, and each such
 identifier must be encodable as an 8-byte Route Distinguisher (RD).
 Any method of assigning one or more unique identifiers to a VPLS and
 encoding each of them as an RD (using the encoding techniques of
 [RFC4364]) will do.
 Each VSI needs to have a unique identifier that is encodable as a BGP
 Network Layer Reachability Information (NLRI).  This is formed by
 prepending the RD (from the previous paragraph) to an IP address of
 the PE containing the VSI.  Note that the role of this address is
 simply as a readily available unique identifier for the VSIs within a
 VPN; it does not need to be globally routable, but it must be unique
 within the VPLS instance.  An alternate scheme to assign unique
 identifiers to each VSI within a VPLS instance (e.g., numbering the
 VSIs of a single VPN from 1 to n) could be used if desired.
 When using the procedures described in this document, it is necessary
 to assign a single, globally unique VPLS-id to each VPLS instance
 [RFC4762].  This VPLS-id must be encodable as a BGP Extended
 Community [RFC4360].  As described in Section 6, two Extended
 Community subtypes are defined by this document for this purpose.
 The Extended Community MUST be transitive.

Rosen, et al. Standards Track [Page 12] RFC 6074 L2VPN Signaling January 2011

 The first Extended Community subtype is a Two-octet AS Specific
 Extended Community.  The second Extended Community subtype is an IPv4
 Address Specific Extended Community.  The encoding of such
 Communities is defined in [RFC4360].  These encodings ensure that a
 service provider can allocate a VPLS-id without risk of collision
 with another provider.  However, note that coordination of VPLS-ids
 among providers is necessary for inter-provider L2VPNs, as described
 in Section 4.4.
 Each VSI also needs to be associated with one or more Route Target
 (RT) Extended Communities.  These control the distribution of the
 NLRI, and hence will control the formation of the overlay topology of
 pseudowires that constitutes a particular VPLS.
 Auto-discovery proceeds by having each PE distribute, via BGP, the
 NLRI for each of its VSIs, with itself as the BGP next hop, and with
 the appropriate RT for each such NLRI.  Typically, each PE would be a
 client of a small set of BGP route reflectors, which would
 redistribute this information to the other clients.
 If a PE receives a BGP update from which any of the elements
 specified above is absent, the update should be ignored.
 If a PE has a VSI with a particular RT, it can then import all the
 NLRIs that have that same RT, and from the BGP next hop attribute of
 these NLRI it will learn the IP addresses of the other PE routers
 which have VSIs with the same RT.  The considerations in Section
 4.3.3 of [RFC4364] on the use of route reflectors apply.
 If a particular VPLS is meant to be a single fully connected LAN, all
 its VSIs will have the same RT, in which case the RT could be (though
 it need not be) an encoding of the VPN-id.  A VSI can be placed in
 multiple VPLSes by assigning it multiple RTs.
 Note that hierarchical VPLS can be set up by assigning multiple RTs
 to some of the VSIs; the RT mechanism allows one to have complete
 control over the pseudowire overlay that constitutes the VPLS
 topology.
 If Distributed VPLS (described in Section 3.5) is deployed, only the
 Network-facing PEs (N-PEs) participate in BGP-based auto-discovery.
 This means that an N-PE would need to advertise reachability to each
 of the VSIs that it supports, including those located in User-facing
 PEs (U-PEs) to which it is connected.  To create a unique identifier
 for each such VSI, an IP address of each U-PE combined with the RD
 for the VPLS instance could be used.

Rosen, et al. Standards Track [Page 13] RFC 6074 L2VPN Signaling January 2011

 In summary, the BGP advertisement for a particular VSI at a given PE
 will contain:
 o  an NLRI of AFI = L2VPN, SAFI = VPLS, encoded as RD:PE_addr
 o  a BGP next hop equal to the loopback address of the PE
 o  an Extended Community Attribute containing the VPLS-id
 o  an Extended Community Attribute containing one or more RTs.
 See Section 6 for discussion of the AFI and SAFI values.  The format
 for the NLRI encoding is:
      +------------------------------------+
      |  Length (2 octets)                 |
      +------------------------------------+
      |  Route Distinguisher (8 octets)    |
      +------------------------------------+
      |  PE_addr (4 octets)                |
      +------------------------------------+
 Note that this advertisement is quite similar to the NLRI format
 defined in [RFC4761], the main difference being that [RFC4761] also
 includes a label block in the NLRI.  Interoperability between the
 VPLS scheme defined here and that defined in [RFC4761] is beyond the
 scope of this document.

3.2.3. Signaling

 It is necessary to create Attachment Identifiers that identify the
 VSIs.  In the preceding section, a VSI-ID was encoded as RD:PE_addr,
 and the VPLS-id was carried in a BGP Extended Community.  For
 signaling purposes, this information is encoded as follows.  We
 encode the VPLS-id in the AGI field, and place the PE_addr (or, more
 precisely, the VSI-ID that was contained in the NLRI in BGP, minus
 the RD) in the TAII field.  The combination of AGI and TAII is
 sufficient to fully specify the VSI to which this pseudowire is to be
 connected, in both single AS and inter-AS environments.  The SAII
 MUST be set to the PE_addr of the sending PE (or, more precisely, the
 VSI-ID, without the RD, of the VSI associated with this VPLS in the
 sending PE) to enable signaling of the reverse half of the PW if
 needed.
 The structure of the AGI and AII fields for the Generalized ID FEC in
 LDP is defined in [RFC4447].  The AGI field in this case consists of
 a Type of 1, a length field of value 8, and the 8 bytes of the

Rosen, et al. Standards Track [Page 14] RFC 6074 L2VPN Signaling January 2011

 VPLS-id.  The AIIs consist of a Type of 1, a length field of value 4,
 followed by the 4-byte PE address (or other 4-byte identifier).  See
 Section 6 for discussion of the AGI and AII Type assignment.
 The encoding of the AGI and AII in L2TP is specified in [RFC4667].
 Note that it is not possible using this technique to set up more than
 one PW per pair of VSIs.

3.2.4. Pseudowires as VPLS Attachment Circuits

 It is also possible using this technique to set up a PW that attaches
 at one endpoint to a VSI, but at the other endpoint only to an
 Attachment Circuit.  There may be more than one PW terminating on a
 given VSI, which must somehow be distinguished, so each PW must have
 an SAII that is unique relative to the VSI-ID.

3.3. Colored Pools: Full Mesh of Point-to-Point Pseudowires

 The "Colored Pools" model of operation provides an automated way to
 deliver VPWS.  In this model, each PE may contain several pools of
 Attachment Circuits, each pool associated with a particular VPN.  A
 PE may contain multiple pools per VPN, as each pool may correspond to
 a particular CE device.  It may be desired to create one pseudowire
 between each pair of pools that are in the same VPN; the result would
 be to create a full mesh of CE-CE Virtual Circuits for each VPN.

3.3.1. Provisioning

 Each pool is configured, and associated with:
 o  a set of Attachment Circuits;
 o  a "color", which can be thought of as a VPN-id of some sort;
 o  a relative pool identifier, which is unique relative to the color.
 [Note: depending on the technology used for Attachment Circuits
 (ACs), it may or may not be necessary to provision these circuits as
 well.  For example, if the ACs are frame relay circuits, there may be
 some separate provisioning system to set up such circuits.
 Alternatively, "provisioning" an AC may be as simple as allocating an
 unused VLAN ID on an interface and communicating the choice to the
 customer.  These issues are independent of the procedures described
 in this document.]

Rosen, et al. Standards Track [Page 15] RFC 6074 L2VPN Signaling January 2011

 The pool identifier and color, taken together, constitute a globally
 unique identifier for the pool.  Thus, if there are n pools of a
 given color, their pool identifiers can be (though they do not need
 to be) the numbers 1-n.
 The semantics are that a pseudowire will be created between every
 pair of pools that have the same color, where each such pseudowire
 will be bound to one Attachment Circuit from each of the two pools.
 If each pool is a set of Attachment Circuits leading to a single CE
 device, then the Layer 2 connectivity among the CEs is controlled by
 the way the colors are assigned to the pools.  To create a full mesh,
 the "color" would just be a VPN-id.
 Optionally, a particular Attachment Circuit may be configured with
 the relative pool identifier of a remote pool.  Then, that Attachment
 Circuit would be bound to a particular pseudowire only if that
 pseudowire's remote endpoint is the pool with that relative pool
 identifier.  With this option, the same pairs of Attachment Circuits
 will always be bound via pseudowires.

3.3.2. Auto-Discovery

3.3.2.1. BGP-Based Auto-Discovery

 This section specifies how BGP can be used to discover the
 information necessary to build VPWS instances.
 When BGP-based auto-discovery is used for VPWS, the AFI/SAFI will be:
 o  An AFI specified by IANA for L2VPN.  (This is the same for all
    L2VPN schemes.)
 o  A SAFI specified by IANA specifically for an L2VPN service whose
    pseudowires are set up using the procedures described in the
    current document.
 See Section 6 for further discussion of AFI/SAFI assignment.
 In order to use BGP-based auto-discovery, there must be one or more
 unique identifiers associated with a particular VPWS instance.  Each
 identifier must be encodable as an RD (Route Distinguisher).  The
 globally unique identifier of a pool must be encodable as NLRI; the
 pool identifier, which we define to be a 4-byte quantity, is appended
 to the RD to create the NLRI.
 When using the procedures described in this document, it is necessary
 to assign a single, globally unique identifier to each VPWS instance.

Rosen, et al. Standards Track [Page 16] RFC 6074 L2VPN Signaling January 2011

 This identifier must be encodable as a BGP Extended Community
 [RFC4360].  As described in Section 6, two Extended Community
 subtypes are defined by this document for this purpose.  The Extended
 Community MUST be transitive.
 The first Extended Community subtype is a Two-octet AS Specific
 Extended Community.  The second Extended Community subtype is an IPv4
 Address Specific Extended Community.  The encoding of such
 Communities is defined in [RFC4360].  These encodings ensure that a
 service provider can allocate a VPWS identifier without risk of
 collision with another provider.  However, note that co-ordination of
 VPWS identifiers among providers is necessary for inter-provider
 L2VPNs, as described in Section 4.4.
 Each pool must also be associated with an RT (route target), which
 may also be an encoding of the color.  If the desired topology is a
 full mesh of pseudowires, all pools may have the same RT.  See
 Section 3.4 for a discussion of other topologies.
 Auto-discovery proceeds by having each PE distribute, via BGP, the
 NLRI for each of its pools, with itself as the BGP next hop, and with
 the RT that encodes the pool's color.  If a given PE has a pool with
 a particular color (RT), it must receive, via BGP, all NLRI with that
 same color (RT).  Typically, each PE would be a client of a small set
 of BGP route reflectors, which would redistribute this information to
 the other clients.
 If a PE receives a BGP update from which any of the elements
 specified above is absent, the update should be ignored.
 If a PE has a pool with a particular color, it can then receive all
 the NLRI that have that same color, and from the BGP next hop
 attribute of these NLRI will learn the IP addresses of the other PE
 routers that have pools switches with the same color.  It also learns
 the unique identifier of each such remote pool, as this is encoded in
 the NLRI.  The remote pool's relative identifier can be extracted
 from the NLRI and used in the signaling, as specified below.
 In summary, the BGP advertisement for a particular pool of attachment
 circuits at a given PE will contain:
 o  an NLRI of AFI = L2VPN, SAFI = VPLS, encoded as RD:pool_num;
 o  a BGP next hop equal to the loopback address of the PE;
 o  an Extended Community Attribute containing the VPWS identifier;
 o  an Extended Community Attribute containing one or more RTs.

Rosen, et al. Standards Track [Page 17] RFC 6074 L2VPN Signaling January 2011

 See Section 6 for discussion of the AFI and SAFI values.

3.3.3. Signaling

 The LDP-based signaling follows the procedures specified in
 [RFC4447].  That is, one PE (PE1) sends a Label Mapping message to
 another PE (PE2) to establish an LSP in one direction.  The address
 of PE2 is the next-hop address learned via BGP as described above.
 If the message is processed successfully, and there is not yet an LSP
 for the pseudowire in the opposite (PE1->PE2) direction, then PE2
 sends a Label Mapping message to PE1.  Similarly, the L2TPv3-based
 signaling follows the procedures of [RFC4667].  Additional details on
 the use of these signaling protocols follow.
 When a PE sends a Label Mapping message or an ICRQ message to set up
 a PW between two pools, it encodes the VPWS identifier (as
 distributed in the Extended Community Attribute by BGP) as the AGI,
 the local pool's relative identifier as the SAII, and the remote
 pool's relative identifier as the TAII.
 The structure of the AGI and AII fields for the Generalized ID FEC in
 LDP is defined in [RFC4447].  The AGI field in this case consists of
 a Type of 1, a length field of value 8, and the 8 bytes of the VPWS
 identifier.  The TAII consists of a Type of 1, a length field of
 value 4, followed by the 4-byte remote pool number.  The SAII
 consists of a Type of 1, a length field of value 4, followed by the
 4-byte local pool number.  See Section 6 for discussion of the AGI
 and AII Type assignment.  Note that the VPLS and VPWS procedures
 defined in this document can make use of the same AGI Type (1) and
 the same AII Type (1).
 The encoding of the AGI and AII in L2TP is specified in [RFC4667].
 When PE2 receives a Label Mapping message or an ICRQ message from
 PE1, and the TAI identifies a pool, and there is already a pseudowire
 connecting an Attachment Circuit in that pool to an Attachment
 Circuit at PE1, and the AI at PE1 of that pseudowire is the same as
 the SAI of the Label Mapping or ICRQ message, then PE2 sends a Label
 Release or CDN message to PE1, with a Status Code meaning "Attachment
 Circuit already bound to remote Attachment Circuit".  This prevents
 the creation of multiple pseudowires between a given pair of pools.
 Note that the signaling itself only identifies the remote pool to
 which the pseudowire is to lead, not the remote Attachment Circuit
 that is to be bound to the pseudowire.  However, the remote PE may
 examine the SAII field to determine which Attachment Circuit should
 be bound to the pseudowire.

Rosen, et al. Standards Track [Page 18] RFC 6074 L2VPN Signaling January 2011

3.4. Colored Pools: Partial Mesh

 The procedures for creating a partial mesh of pseudowires among a set
 of colored pools are substantially the same as those for creating a
 full mesh, with the following exceptions:
 o  Each pool is optionally configured with a set of "import RTs" and
    "export RTs";
 o  During BGP-based auto-discovery, the pool color is still encoded
    in the RD, but if the pool is configured with a set of "export
    RTs", these are encoded in the RTs of the BGP Update messages
    INSTEAD of the color;
 o  If a pool has a particular "import RT" value X, it will create a
    PW to every other pool that has X as one of its "export RTs".  The
    signaling messages and procedures themselves are as in
    Section 3.3.3.
 As a simple example, consider the task of building a hub-and-spoke
 topology with a single hub.  One pool, the "hub" pool, is configured
 with an export RT of RT_hub and an import RT of RT_spoke.  All other
 pools (the spokes) are configured with an export RT of RT_spoke and
 an import RT of RT_hub.  Thus, the hub pool will connect to the
 spokes, and vice-versa, but the spoke pools will not connect to each
 other.

3.5. Distributed VPLS

 In Distributed VPLS ([RFC4664]), the VPLS functionality of a PE
 router is divided among two systems: a U-PE and an N-PE.  The U-PE
 sits between the user and the N-PE.  VSI functionality (e.g., MAC
 address learning and bridging) is performed on the U-PE.  A number of
 U-PEs attach to an N-PE.  For each VPLS supported by a U-PE, the U-PE
 maintains a pseudowire to each of the other U-PEs in the same VPLS.
 However, the U-PEs do not maintain signaling control connections with
 each other.  Rather, each U-PE has only a single signaling
 connection, to its N-PE.  In essence, each U-PE-to-U-PE pseudowire is
 composed of three pseudowires spliced together: one from U-PE to
 N-PE, one from N-PE to N-PE, and one from N-PE to U-PE.  In the
 terminology of [RFC5659], the N-PEs perform the pseudowire switching
 function to establish multi-segment PWs from U-PE to U-PE.

Rosen, et al. Standards Track [Page 19] RFC 6074 L2VPN Signaling January 2011

 Consider, for example, the following topology:
         U-PE A-----|             |----U-PE C
                    |             |
                    |             |
                  N-PE E--------N-PE F
                    |             |
                    |             |
         U-PE B-----|             |-----U-PE D
 where the four U-PEs are in a common VPLS.  We now illustrate how PWs
 get spliced together in the above topology in order to establish the
 necessary PWs from U-PE A to the other U-PEs.
 There are three PWs from A to E.  Call these A-E/1, A-E/2, and A-E/3.
 In order to connect A properly to the other U-PEs, there must be two
 PWs from E to F (call these E-F/1 and E-F/2), one PW from E to B
 (E-B/1), one from F to C (F-C/1), and one from F to D (F-D/1).
 The N-PEs must then splice these pseudowires together to get the
 equivalent of what the non-distributed VPLS signaling mechanism would
 provide:
 o  PW from A to B: A-E/1 gets spliced to E-B/1.
 o  PW from A to C: A-E/2 gets spliced to E-F/1 gets spliced to F-C/1.
 o  PW from A to D: A-E/3 gets spliced to E-F/2 gets spliced to F-D/1.
 It doesn't matter which PWs get spliced together, as long as the
 result is one from A to each of B, C, and D.
 Similarly, there are additional PWs that must get spliced together to
 properly interconnect U-PE B with U-PEs C and D, and to interconnect
 U-PE C with U-PE D.
 The following figure illustrates the PWs from A to C and from B to D.
 For clarity of the figure, the other four PWs are not shown.

Rosen, et al. Standards Track [Page 20] RFC 6074 L2VPN Signaling January 2011

                    splicing points
                     |           |
                     V           V
    A-C PW    <-----><-----------><------>
         U-PE A-----|             |----U-PE C
                    |             |
                    |             |
                  N-PE E--------N-PE F
                    |             |
                    |             |
         U-PE B-----|             |-----U-PE D
    B-D PW    <-----><-----------><------>
                     ^           ^
                     |           |
                    splicing points
 One can see that distributed VPLS does not reduce the number of
 pseudowires per U-PE, but it does reduce the number of control
 connections per U-PE.  Whether this is worthwhile depends, of course,
 on what the bottleneck is.

3.5.1. Signaling

 The signaling to support Distributed VPLS can be done with the
 mechanisms described in this document.  However, the procedures for
 VPLS (Section 3.2.3) need some additional machinery to ensure that
 the appropriate number of PWs are established between the various
 N-PEs and U-PEs, and among the N-PEs.
 At a given N-PE, the directly attached U-PEs in a given VPLS can be
 numbered from 1 to n.  This number identifies the U-PE relative to a
 particular VPN-id and a particular N-PE.  (That is, to uniquely
 identify the U-PE, the N-PE, the VPN-id, and the U-PE number must be
 known.)
 As a result of configuration/discovery, each U-PE must be given a
 list of <j, IP address> pairs.  Each element in this list tells the
 U-PE to set up j PWs to the specified IP address.  When the U-PE
 signals to the N-PE, it sets the AGI to the proper-VPN-id, and sets
 the SAII to the PW number, and sets the TAII to null.

Rosen, et al. Standards Track [Page 21] RFC 6074 L2VPN Signaling January 2011

 In the above example, U-PE A would be told <3, E>, telling it to set
 up 3 PWs to E.  When signaling, A would set the AGI to the proper
 VPN-id, and would set the SAII to 1, 2, or 3, depending on which of
 the three PWs it is signaling.
 As a result of configuration/discovery, each N-PE must be given the
 following information for each VPLS:
 o  A "Local" list: {<j, IP address>}, where each element tells it to
    set up j PWs to the locally attached U-PE at the specified
    address.  The number of elements in this list will be n, the
    number of locally attached U-PEs in this VPLS.  In the above
    example, E would be given the local list: {<3, A>, <3, B>},
    telling it to set up 3 PWs to A and 3 to B.
 o  A local numbering, relative to the particular VPLS and the
    particular N-PE, of its U-PEs.  In the above example, E could be
    told that U-PE A is 1, and U-PE B is 2.
 o  A "Remote" list: {<IP address, k>}, telling it to set up k PWs,
    for each U-PE, to the specified IP address.  Each of these IP
    addresses identifies an N-PE, and k specifies the number of U-PEs
    at the N-PE that are in the VPLS.  In the above example, E would
    be given the remote list: {<2, F>}.  Since N-PE E has 2 U-PEs,
    this tells it to set up 4 PWs to N-PE F, 2 for each of its E's
    U-PEs.
 The signaling of a PW from N-PE to U-PE is based on the local list
 and the local numbering of U-PEs.  When signaling a particular PW
 from an N-PE to a U-PE, the AGI is set to the proper VPN-id, and SAII
 is set to null, and the TAII is set to the PW number (relative to
 that particular VPLS and U-PE).  In the above example, when E signals
 to A, it would set the TAII to be 1, 2, or 3, respectively, for the 3
 PWs it must set up to A.  It would similarly signal 3 PWs to B.
 The LSP signaled from U-PE to N-PE is associated with an LSP from
 N-PE to U-PE in the usual manner.  A PW between a U-PE and an N-PE is
 known as a "U-PW".
 The signaling of the appropriate set of PWs from N-PE to N-PE is
 based on the remote list.  The PWs between the N-PEs can all be
 considered equivalent.  As long as the correct total number of PWs
 are established, the N-PEs can splice these PWs to appropriate U-PWs.
 The signaling of the correct number of PWs from N-PE to N-PE is based
 on the remote list.  The remote list specifies the number of PWs to
 set up, per local U-PE, to a particular remote N-PE.

Rosen, et al. Standards Track [Page 22] RFC 6074 L2VPN Signaling January 2011

 When signaling a particular PW from an N-PE to an N-PE, the AGI is
 set to the appropriate VPN-id.  The TAII identifies the remote N-PE,
 as in the non-distributed case, i.e., it contains an IP address of
 the remote N-PE.  If there are n such PWs, they are distinguished by
 the setting of the SAII.  In order to allow multiple different SAII
 values in a single VPLS, the sending N-PE needs to have as many VSI-
 IDs as it has U-PEs.  As noted above in Section 3.2.2, this may be
 achieved by using an IP address of each attached U-PE, for example.
 A PW between two N-PEs is known as an "N-PW".
 Each U-PW must be "spliced" to an N-PW.  This is based on the remote
 list.  If the remote list contains an element <i, F>, then i U-PWs
 from each local U-PE must be spliced to i N-PWs from the remote N-PE
 F.  It does not matter which U-PWs are spliced to which N-PWs, as
 long as this constraint is met.
 If an N-PE has more than one local U-PE for a given VPLS, it must
 also ensure that a U-PW from each such U-PE is spliced to a U-PW from
 each of the other U-PEs.

3.5.2. Provisioning and Discovery

 Every N-PE must be provisioned with the set of VPLS instances it
 supports, a VPN-id for each one, and a list of local U-PEs for each
 such VPLS.  As part of the discovery procedure, the N-PE advertises
 the number of U-PEs for each VPLS.  See Section 3.2.2 for details.
 Auto-discovery (e.g., BGP-based) can be used to discover all the
 other N-PEs in the VPLS, and for each, the number of U-PEs local to
 that N-PE.  From this, one can compute the total number of U-PEs in
 the VPLS.  This information is sufficient to enable one to compute
 the local list and the remote list for each N-PE.

3.5.3. Non-Distributed VPLS as a Sub-Case

 A PE that is providing "non-distributed VPLS" (i.e., a PE that
 performs both the U-PE and N-PE functions) can interoperate with
 N-PE/U-PE pairs that are providing distributed VPLS.  The "non-
 distributed PE" simply advertises, in the discovery procedure, that
 it has one local U-PE per VPLS.  And of course, the non-distributed
 PE does no PW switching.
 If every PE in a VPLS is providing non-distributed VPLS, and thus
 every PE is advertising itself as an N-PE with one local U-PE, the
 resultant signaling is exactly the same as that specified in
 Section 3.2.3 above.

Rosen, et al. Standards Track [Page 23] RFC 6074 L2VPN Signaling January 2011

3.5.4. Splicing and the Data Plane

 Splicing two PWs together is quite straightforward in the MPLS data
 plane, as moving a packet from one PW directly to another is just a
 'label replace' operation on the PW label.  When a PW consists of two
 or more PWs spliced together, it is assumed that the data will go to
 the node where the splicing is being done, i.e., that the data path
 will pass through the nodes that participate in PW signaling.
 Further details on splicing are discussed in [RFC6073].

4. Inter-AS Operation

 The provisioning, auto-discovery, and signaling mechanisms described
 above can all be applied in an inter-AS environment.  As in
 [RFC4364], there are a number of options for inter-AS operation.

4.1. Multihop EBGP Redistribution of L2VPN NLRIs

 This option is most like option (c) in [RFC4364].  That is, we use
 multihop External BGP (EBGP) redistribution of L2VPN NLRIs between
 source and destination ASes, with EBGP redistribution of labeled IPv4
 or IPv6 routes from AS to neighboring AS.
 An Autonomous System Border Router (ASBR) must maintain labeled IPv4
 /32 (or IPv6 /128) routes to the PE routers within its AS.  It uses
 EBGP to distribute these routes to other ASes, and sets itself as the
 BGP next hop for these routes.  ASBRs in any transit ASes will also
 have to use EBGP to pass along the labeled /32 (or /128) routes.
 This results in the creation of a set of label switched paths from
 all ingress PE routers to all egress PE routers.  Now, PE routers in
 different ASes can establish multi-hop EBGP connections to each other
 and can exchange L2VPN NLRIs over those connections.  Following such
 exchanges, a pair of PEs in different ASes could establish an LDP
 session to signal PWs between each other.
 For VPLS, the BGP advertisement and PW signaling are exactly as
 described in Section 3.2.  As a result of the multihop EBGP session
 that exists between source and destination AS, the PEs in one AS that
 have VSIs of a certain VPLS will discover the PEs in another AS that
 have VSIs of the same VPLS.  These PEs will then be able to establish
 the appropriate PW signaling protocol session and establish the full
 mesh of VSI-VSI pseudowires to build the VPLS as described in
 Section 3.2.3.
 For VPWS, the BGP advertisement and PW signaling are exactly as
 described in Section 3.3.  As a result of the multihop EBGP session
 that exists between source and destination AS, the PEs in one AS that

Rosen, et al. Standards Track [Page 24] RFC 6074 L2VPN Signaling January 2011

 have pools of a certain color (VPN) will discover PEs in another AS
 that have pools of the same color.  These PEs will then be able to
 establish the appropriate PW signaling protocol session and establish
 the full mesh of pseudowires as described in Section 3.2.3.  A
 partial mesh can similarly be established using the procedures of
 Section 3.4.
 As in Layer 3 VPNs, building an L2VPN that spans the networks of more
 than one provider requires some co-ordination in the use of RTs and
 RDs.  This subject is discussed in more detail in Section 4.4.

4.2. EBGP Redistribution of L2VPN NLRIs with Multi-Segment Pseudowires

 A possible drawback of the approach of the previous section is that
 it creates PW signaling sessions among all the PEs of a given L2VPN
 (VPLS or VPWS).  This means a potentially large number of LDP or
 L2TPv3 sessions will cross the AS boundary and that these sessions
 connect to many devices within an AS.  In the case where the ASes
 belong to different providers, one might imagine that providers would
 like to have fewer signaling sessions crossing the AS boundary and
 that the entities that terminate the sessions could be restricted to
 a smaller set of devices.  Furthermore, by forcing the LDP or L2TPv3
 signaling sessions to terminate on a small set of ASBRs, a provider
 could use standard authentication procedures on a small set of inter-
 provider sessions.  These concerns motivate the approach described
 here.
 [RFC6073] describes an approach to "switching" packets from one
 pseudowire to another at a particular node.  This approach allows an
 end-to-end, multi-segment pseudowire to be constructed out of several
 pseudowire segments, without maintaining an end-to-end control
 connection.  We can use this approach to produce an inter-AS solution
 that more closely resembles option (b) in [RFC4364].
 In this model, we use EBGP redistribution of L2VPN NLRI from AS to
 neighboring AS.  First, the PE routers use Internal BGP (IBGP) to
 redistribute L2VPN NLRI either to an ASBR, or to a route reflector of
 which an ASBR is a client.  The ASBR then uses EBGP to redistribute
 those L2VPN NLRI to an ASBR in another AS, which in turn distributes
 them to the PE routers in that AS, or perhaps to another ASBR which
 in turn distributes them, and so on.
 In this case, a PE can learn the address of an ASBR through which it
 could reach another PE to which it wishes to establish a PW.  That
 is, a local PE will receive a BGP advertisement containing L2VPN NLRI
 corresponding to an L2VPN instance in which the local PE has some
 attached members.  The BGP next-hop for that L2VPN NLRI will be an
 ASBR of the local AS.  Then, rather than building a control

Rosen, et al. Standards Track [Page 25] RFC 6074 L2VPN Signaling January 2011

 connection all the way to the remote PE, it builds one only to the
 ASBR.  A pseudowire segment can now be established from the PE to the
 ASBR.  The ASBR in turn can establish a PW to the ASBR of the next
 AS, and splice that PW to the PW from the PE as described in
 Section 3.5.4 and [RFC6073].  Repeating the process at each ASBR
 leads to a sequence of PW segments that, when spliced together,
 connect the two PEs.
 Note that in the approach just described, the local PE may never
 learn the IP address of the remote PE.  It learns the L2VPN NLRI
 advertised by the remote PE, which need not contain the remote PE
 address, and it learns the IP address of the ASBR that is the BGP
 next hop for that NLRI.
 When this approach is used for VPLS, or for full-mesh VPWS, it leads
 to a full mesh of pseudowires among the PEs, just as in the previous
 section, but it does not require a full mesh of control connections
 (LDP or L2TPv3 sessions).  Instead, the control connections within a
 single AS run among all the PEs of that AS and the ASBRs of the AS.
 A single control connection between the ASBRs of adjacent ASes can be
 used to support however many AS-to-AS pseudowire segments are needed.
 Note that the procedures described here will result in the splicing
 points (PW Switching PEs (S-PEs) in the terminology of [RFC5659])
 being co-located with the ASBRs.  It is of course possible to have
 multiple ASBR-ASBR connections between a given pair of ASes.  In this
 case, a given PE could choose among the available ASBRs based on a
 range of criteria, such as IGP metric, local configuration, etc.,
 analogous to choosing an exit point in normal IP routing.  The use of
 multiple ASBRs would lead to greater resiliency (at the timescale of
 BGP routing convergence) since a PE could select a new ASBR in the
 event of the failure of the one currently in use.
 As in layer 3 VPNs, building an L2VPN that spans the networks of more
 than one provider requires some co-ordination in the use of RTs and
 RDs.  This subject is discussed in more detail in Section 4.4.

4.3. Inter-Provider Application of Distributed VPLS Signaling

 An alternative approach to inter-provider VPLS can be derived from
 the Distributed VPLS approach described above.  Consider the
 following topology:
 PE A --- Network 1 ----- Border ----- Border ----- Network 2 --- PE B
                          Router 12    Router 21       |
                                                       |
                                                      PE C

Rosen, et al. Standards Track [Page 26] RFC 6074 L2VPN Signaling January 2011

 where A, B, and C are PEs in a common VPLS, but Networks 1 and 2 are
 networks of different service providers.  Border Router 12 is Network
 1's border router to network 2, and Border Router 21 is Network 2's
 border router to Network 1.  We suppose further that the PEs are not
 "distributed", i.e, that each provides both the U-PE and N-PE
 functions.
 In this topology, one needs two inter-provider pseudowires: A-B and
 A-C.
 Suppose a service provider decides, for whatever reason, that it does
 not want each of its PEs to have a control connection to any PEs in
 the other network.  Rather, it wants the inter-provider control
 connections to run only between the two border routers.
 This can be achieved using the techniques of Section 3.5, where the
 PEs behave like U-PEs, and the BRs behave like N-PEs.  In the example
 topology, PE A would behave like a U-PE that is locally attached to
 BR12; PEs B and C would be have like U-PEs that are locally attached
 to BR21; and the two BRs would behave like N-PEs.
 As a result, the PW from A to B would consist of three segments:
 A-BR12, BR12-BR21, and BR21-B.  The border routers would have to
 splice the corresponding segments together.
 This requires the PEs within a VPLS to be numbered from 1-n (relative
 to that VPLS) within a given network.

4.4. RT and RD Assignment Considerations

 We note that, in order for any of the inter-AS procedures described
 above to work correctly, the two ASes must use RTs and RDs
 consistently, just as in Layer 3 VPNs [RFC4364].  The structure of
 RTs and RDs is such that there is not a great risk of accidental
 collisions.  The main challenge is that it is necessary for the
 operator of one AS to know what RT or RTs have been chosen in another
 AS for any VPN that has sites in both ASes.  As in Layer 3 VPNs,
 there are many ways to make this work, but all require some co-
 operation among the providers.  For example, provider A may tag all
 the NLRI for a given VPN with a single RT, say RT_A, and provider B
 can then configure the PEs that connect to sites of that VPN to
 import NLRI that contains that RT.  Provider B can choose a different
 RT, RT_B, tag all NLRI for this VPN with that RT, and then provider A
 can import NLRI with that RT at the appropriate PEs.  However, this
 does require both providers to communicate their choice of RTs for
 each VPN.  Alternatively, both providers could agree to use a common
 RT for a given VPN.  In any case, communication of RTs between the

Rosen, et al. Standards Track [Page 27] RFC 6074 L2VPN Signaling January 2011

 providers is essential.  As in Layer 3 VPNs, providers may configure
 RT filtering to ensure that only coordinated RT values are allowed
 across the AS boundary.
 Note that a single VPN identifier (carried in a BGP Extended
 Community) is required for each VPLS or VPWS instance.  The encoding
 rules for these identifiers [RFC4360] ensure that collisions do not
 occur with other providers.  However, for a single VPLS or VPWS
 instance that spans the networks of two or more providers, one
 provider will need to allocate the identifier and communicate this
 choice to the other provider(s), who must use the same value for
 sites in the same VPLS or VPWS instance.

5. Security Considerations

 This document describes a number of different L2VPN provisioning
 models, and specifies the endpoint identifiers that are required to
 support each of the provisioning models.  It also specifies how those
 endpoint identifiers are mapped into fields of auto-discovery
 protocols and signaling protocols.
 The security considerations related to the signaling protocols are
 discussed in the relevant protocol specifications ([RFC5036],
 [RFC4447], [RFC3931], and [RFC4667]).
 The security considerations related to BGP-based auto-discovery,
 including inter-AS issues, are discussed in [RFC4364].  L2VPNs that
 use BGP-based auto-discovery may automate setup of security
 mechanisms as well.  Specification of automated security mechanisms
 are outside the scope of this document, but are recommended as a
 future work item.
 The security considerations related to the particular kind of L2VPN
 service being supported are discussed in [RFC4664], [RFC4665], and
 [RFC4762].
 The way in which endpoint identifiers are mapped into protocol fields
 does not create any additional security issues.

6. IANA Considerations

 IANA has assigned an AFI and a SAFI for L2VPN NLRI.  Both the AFI and
 SAFI are the same as the values assigned for [RFC4761].  That is, the
 AFI is 25 (L2VPN) and the SAFI is 65 (already allocated for VPLS).
 The same AFI and SAFI are used for both VPLS and VPWS auto-discovery
 as described in this document.

Rosen, et al. Standards Track [Page 28] RFC 6074 L2VPN Signaling January 2011

 [RFC4446] defines registries for "Attachment Group Identifier (AGI)
 Type" and "Attachment Individual Identifier (AII) Type".  Type 1 in
 each registry has been assigned to the AGI and AII formats defined in
 this document.
 IANA has assigned two new LDP status codes.  IANA already maintains a
 registry of name "STATUS CODE NAME SPACE" defined by [RFC5036].  The
 following values have been assigned:
 0x00000030 Attachment Circuit bound to different PE
 0x0000002D Attachment Circuit bound to different remote Attachment
 Circuit
 Two new L2TP Result Codes have been registered for the CDN message.
 IANA already maintains a registry of L2TP Result Code Values for the
 CDN message, defined by [RFC3438].  The following values have been
 assigned:
 27: Attachment Circuit bound to different PE
 28: Attachment Circuit bound to different remote Attachment Circuit
 [RFC4360] defines a registry entitled "Two-octet AS Specific Extended
 Community".  IANA has assigned a value in this registry from the
 "transitive" range (0x0000-0x00FF).  The value is as follows:
 o  0x000A Two-octet AS specific Layer 2 VPN Identifier
 [RFC4360] defines a registry entitled "IPv4 Address Specific Extended
 Community".  IANA has assigned a value in this registry from the
 "transitive" range (0x0100-0x01FF).  The value is as follows:
 o  0x010A Layer 2 VPN Identifier

7. BGP-AD and VPLS-BGP Interoperability

 Both BGP-AD and VPLS-BGP [RFC4761] use the same AFI/SAFI.  In order
 for both BGP-AD and VPLS-BGP to co-exist, the NLRI length must be
 used as a demultiplexer.
 The BGP-AD NLRI has an NLRI length of 12 bytes, containing only an
 8-byte RD and a 4-byte VSI-ID.  VPLS-BGP [RFC4761] uses a 17-byte
 NLRI length.  Therefore, implementations of BGP-AD must ignore NLRI
 that are greater than 12 bytes.

Rosen, et al. Standards Track [Page 29] RFC 6074 L2VPN Signaling January 2011

8. Acknowledgments

 Thanks to Dan Tappan, Ted Qian, Ali Sajassi, Skip Booth, Luca
 Martini, Dave McDysan, Francois Le Faucheur, Russ Gardo, Keyur Patel,
 Sam Henderson, and Matthew Bocci for their comments, criticisms, and
 helpful suggestions.
 Thanks to Tissa Senevirathne, Hamid Ould-Brahim, and Yakov Rekhter
 for discussing the auto-discovery issues.
 Thanks to Vach Kompella for a continuing discussion of the proper
 semantics of the generalized identifiers.

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3438]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
            Internet Assigned Numbers Authority (IANA) Considerations
            Update", BCP 68, RFC 3438, December 2002.
 [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
            Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
 [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
            Communities Attribute", RFC 4360, February 2006.
 [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
            Networks (VPNs)", RFC 4364, February 2006.
 [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
            Heron, "Pseudowire Setup and Maintenance Using the Label
            Distribution Protocol (LDP)", RFC 4447, April 2006.
 [RFC4667]  Luo, W., "Layer 2 Virtual Private Network (L2VPN)
            Extensions for Layer 2 Tunneling Protocol (L2TP)",
            RFC 4667, September 2006.
 [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
            "Multiprotocol Extensions for BGP-4", RFC 4760,
            January 2007.
 [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
            Specification", RFC 5036, October 2007.

Rosen, et al. Standards Track [Page 30] RFC 6074 L2VPN Signaling January 2011

 [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
            Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.

9.2. Informative References

 [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
            Edge (PWE3) Architecture", RFC 3985, March 2005.
 [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
            Private Network (VPN) Terminology", RFC 4026, March 2005.
 [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
            Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [RFC4664]  Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual
            Private Networks (L2VPNs)", RFC 4664, September 2006.
 [RFC4665]  Augustyn, W. and Y. Serbest, "Service Requirements for
            Layer 2 Provider-Provisioned Virtual Private Networks",
            RFC 4665, September 2006.
 [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
            (VPLS) Using BGP for Auto-Discovery and Signaling",
            RFC 4761, January 2007.
 [RFC4762]  Lasserre, M. and V. Kompella, "Virtual Private LAN Service
            (VPLS) Using Label Distribution Protocol (LDP) Signaling",
            RFC 4762, January 2007.
 [RFC5003]  Metz, C., Martini, L., Balus, F., and J. Sugimoto,
            "Attachment Individual Identifier (AII) Types for
            Aggregation", RFC 5003, September 2007.
 [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
            Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
            October 2009.

Rosen, et al. Standards Track [Page 31] RFC 6074 L2VPN Signaling January 2011

Authors' Addresses

 Eric Rosen
 Cisco Systems, Inc.
 1414 Mass. Ave.
 Boxborough, MA  01719
 USA
 EMail: erosen@cisco.com
 Bruce Davie
 Cisco Systems, Inc.
 1414 Mass. Ave.
 Boxborough, MA  01719
 USA
 EMail: bsd@cisco.com
 Vasile Radoaca
 Alcatel-Lucent
 Think Park Tower 6F
 2-1-1 Osaki, Tokyo, 141-6006
 Japan
 EMail: vasile.radoaca@alcatel-lucent.com
 Wei Luo
 EMail: luo@weiluo.net

Rosen, et al. Standards Track [Page 32]

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