Network Working Group K. Kompella, Ed. Internet-Draft Y. Rekhter, Ed. Expires:
August 23,October 10, 2005 Juniper Networks February 19,April 8, 2005 Virtual Private LAN Service draft-ietf-l2vpn-vpls-bgp-04draft-ietf-l2vpn-vpls-bgp-05 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667.By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomebecomes aware will be disclosed, in accordance with RFC 3668.Section 6 of BCP 79. 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. 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. This Internet-Draft will expire on August 23,October 10, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract Virtual Private LAN Service (VPLS), also known as Transparent LAN Service, and Virtual Private Switched Network service, is a useful Service Provider offering. The service offers a Layer 2 Virtual Private Network (VPN); however, in the case of VPLS, the customers in the VPN are connected by a multipoint network, in contrast to the usual Layer 2 VPNs, which are point-to-point in nature. This document describes the functions required to offer VPLS, a mechanism for signaling a VPLS, and rules for forwarding VPLS frames across a packet switched network. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Scope of this Document . . . . . . . . . . . . . . . . . . 3 1.2 Conventions used in this document . . . . . . . . . . . . 4 1.3 Changes from lastversion 04 to 05 . . . . . . . . . . . . . . 4 1.4 Changes from version 03 to 04 . . . . . . . 4. . . . . . . 5 2. Functional Model . . . . . . . . . . . . . . . . . . . . . . . 56 2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 56 2.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 67 2.3 Interactions . . . . . . . . . . . . . . . . . . . . . . . 67 3. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 89 3.1 Autodiscovery . . . . . . . . . . . . . . . . . . . . . . 89 3.1.1 Functions . . . . . . . . . . . . . . . . . . . . . . 89 3.1.2 Protocol Specification . . . . . . . . . . . . . . . . 910 3.2 Signaling . . . . . . . . . . . . . . . . . . . . . . . . 910 3.2.1 Setup and TeardownConcepts . . . . . . . . . . . . . . . . . . 9. . . . . 10 3.2.2 PW Setup and Teardown . . . . . . . . . . . . . . . . 11 3.2.3 Signaling PE Capabilities . . . . . . . . . . . . . . 1112 3.3 BGP VPLS Operation . . . . . . . . . . . . . . . . . . . . 13 3.4 Multi-AS VPLS . . . . . . . . . . . . . . . . . . . . . . 12 3.3.114 3.4.1 a) VPLS-to-VPLS connections at the AS border routers. . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.215 3.4.2 b) EBGP redistribution of VPLS information between ASBRs. . . . . . . . . . . . . . . . . . . . . . . . . 13 3.3.315 3.4.3 c) Multi-hop EBGP redistribution of VPLS information between ASes. . . . . . . . . . . . . . . 14 3.416 3.4.4 Allocation of VE IDs Across Multiple ASes . . . . . . 17 3.5 Multi-homing and Path Selection . . . . . . . . . . . . . 1517 4. Data Plane . . . . . . . . . . . . . . . . . . . . . . . . . . 1719 4.1 Encapsulation . . . . . . . . . . . . . . . . . . . . . . 1719 4.2 Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 1719 4.2.1 MAC address learning . . . . . . . . . . . . . . . . . 1719 4.2.2 Flooding . . . . . . . . . . . . . . . . . . . . . . . 1719 4.2.3 "Split Horizon" Forwarding . . . . . . . . . . . . . . 1820 5. Deployment Options . . . . . . . . . . . . . . . . . . . . . . 1921 6. Security Considerations . . . . . . . . . . . . . . . . . . . 2022 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 2123 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 2224 8.1 Normative References . . . . . . . . . . . . . . . . . . . 2224 8.2 Informative References . . . . . . . . . . . . . . . . . . 2224 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 2325 A. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 2426 B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 2527 Intellectual Property and Copyright Statements . . . . . . . . 2628 1. Introduction Virtual Private LAN Service (VPLS), also known as Transparent LAN Service, and Virtual Private Switched Network service, is a useful service offering. A Virtual Private LAN appears in (almost) all respects as a LAN to customers of a Service Provider. However, in a VPLS, the customers are not all connected to a single LAN; the customers may be spread across a metro or wide area. In essence, a VPLS glues several individual LANs across a packet-switched network to appear and function as a single LAN (). This document describes the functions needed to offer VPLS, and goes on to describe a mechanism for signaling a VPLS, as well as a mechanism for transport of VPLS frames over tunnels across a packet switched network. The signaling mechanism uses BGP as the control plane protocol. This document also briefly discusses deployment options, in particular, the notion of decoupling functions across devices. Alternative approaches include: , which allows one to build a Layer 2 VPN with Ethernet as the interconnect; and ), which allows one to set up an Ethernet connection across a packet-switched network. Both of these, however, offer point-to-point Ethernet services. What distinguishes VPLS from the above two is that a VPLS offers a multipoint service. A mechanism for setting up pseudowires for VPLS using the Label Distribution Protocol (LDP) is defined in . 1.1 Scope of this Document This document has four major parts: defining a VPLS functional model; defining a control plane for setting up VPLS; defining the data plane for VPLS (encapsulation and forwarding of data); and defining various deployment options. The functional model underlying VPLS is laid out in Section 2. This describes the service being offered, the network components that interact to provide the service, and at a high level their interactions. The control plane described in this document uses Multiprotocol BGP  to establish VPLS service, i.e., for the autodiscovery of VPLS members and for the setup and teardown of the pseudowires that constitute a given VPLS instance. Section 3 focuses on this, and also describes how a VPLS that spans Autonomous System boundaries is set up, as well as how multi-homing is handled. Using BGP as the control plane for VPNs is not new (see ,  and ): what is described here is based on the mechanisms proposed in . The forwarding plane and the actions that a participating PE must take is described in Section 4. In Section 5, the notion of 'decoupled' operation is defined, and the interaction of decoupled and non-decoupled PEs is described. Decoupling allows for more flexible deployment of VPLS. 1.2 Conventions used in this document 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 (). 1.3 Changes from lastversion 04 to 05 [NOTE to RFC Editor: this section is to be removed before publication.] Incorporated IDR reviewUpdated IANA section to reflect agreement with authors of  that the two docs should use the same AFI for L2VPN information. Addressed comments received from Eric Ji, Chaitanya Kodeboyina, and Mike Loomis. Most changes are clarifications and rewording for better readability. The substantive changes areAlex Zinin. No technical changes, but a more complete description to remove several flags fromcover the control field.issues that Alex raised: 1. encoding of BGP NEXT_HOP for the new AFI/SAFI is not described 2. Functional Model This will beVE ID, Block offset, Block size, Label base are not described with reference toanywhere 3. no information on how the following figure. ----- / A1 \receiving PE choose the PW label 4. section 3.2.2 talks about PE capabilities all of a sudden and introduces a L2 Info Community, whose fields and use are not described Changes to address these: 1. Broke up section 3.2.1 into "Concepts" and "PW Setup". 2. Expanded section on "Signaling PE Capabilities". 3. Added a new section 3.3 "BGP VPLS Operation". 4. Minor tweaking, e.g. to fix section number references. 1.4 Changes from version 03 to 04 [NOTE to RFC Editor: this section is to be removed before publication.] Incorporated IDR review comments from Eric Ji, Chaitanya Kodeboyina, and Mike Loomis. Most changes are clarifications and rewording for better readability. The substantive changes are to remove several flags from the control field. 2. Functional Model This will be described with reference to the following figure. ----- / A1 \ ---- ____CE1 | / \ -------- -------- / | | | A2 CE2- / \ / PE1 \ / \ / \ / \___/ | \ ----- ---- ---PE2 | \ | | \ ----- | Service Provider Network | \ / \ | | CE5 A5 | | ___ | / \ / |----| \ / \ PE4_/ ----- |u-PE|--PE3 / \ / |----| -------- ------- ---- / | ---- / \/ \ / \ CE = Customer Edge Device | A3 CE3 --CE4 A4 | PE = Provider Edge Router \ / \ / u-PE = Layer 2 Aggregation ---- ---- A<n> = Customer site n Figure 1: Example of a VPLS 2.1 Terminology Terminology similar to that in  is used, with the addition of "u-PE", a Layer 2 PE device used for Layer 2 aggregation. The notion of u-PE is described further in Section 5. PE and u-PE devices are "VPLS-aware", which means that they know that a VPLS service is being offered. We will call these VPLS edge devices, which could be either a PE or an u-PE, a VE. In contrast, the CE device (which may be owned and operated by either the SP or the customer) is VPLS-unaware; as far as the CE is concerned, it is connected to the other CEs in the VPLS via a Layer 2 switched network. This means that there should be no changes to a CE device, either to the hardware or the software, in order to offer VPLS. A CE device may be connected to a PE or a u-PE via Layer 2 switches that are VPLS-unaware. From a VPLS point of view, such Layer 2 switches are invisible, and hence will not be discussed further. Furthermore, a u-PE may be connected to a PE via Layer 2 and Layer 3 devices; this will be discussed further in a later section. The term "demultiplexor" refers to an identifier in a data packet that identifies both the VPLS to which the packet belongs as well as the ingress PE. In this document, the demultiplexor is an MPLS label. The term "VPLS" will refer to the service as well as a particular instantiation of the service (i.e., an emulated LAN); it should be clear from the context which usage is intended. 2.2 Assumptions The Service Provider Network is a packet switched network. The PEs are assumed to be (logically) full-meshed with tunnels over which packets that belong to a service (such as VPLS) are encapsulated and forwarded. These tunnels can be IP tunnels, such as GRE, or MPLS tunnels, established by RSVP-TE or LDP. These tunnels are established independently of the services offered over them; the signaling and establishment of these tunnels are not discussed in this document. "Flooding" and MAC address "learning" (see Section 4) are an integral part of VPLS. However, these activities are private to an SP device, i.e., in the VPLS described below, no SP device requests another SP device to flood packets or learn MAC addresses on its behalf. All the PEs participating in a VPLS are assumed to be fully meshed, i.e., every (ingress) PE can send a VPLS packet to the egress PE(s) directly, without the need for an intermediate PE (see Section 4.2.3.) 2.3 Interactions VPLS is a "LAN Service" in that CE devices that belong to VPLS V can interact through the SP network as if they were connected by a LAN. VPLS is "private" in that CE devices that belong to different VPLSs cannot interact. VPLS is "virtual" in that multiple VPLSs can be offered over a common packet switched network. PE devices interact to "discover" all the other PEs participating in the same VPLS, and to exchange demultiplexors. These interactions are control-driven, not data-driven. u-PEs interact with PEs to establish connections with remote PEs or u-PEs in the same VPLS. This interaction is control-driven. PE devices can participate simultaneously in both VPLS and IP VPNs (). These are independent services, and the information exchanged for each type of service is kept separate as the Network Layer Reachability Information (NLRI) used for this exchange have different Address Family Identifiers (AFI) and Subsequent Address Family Identifiers (SAFI). Consequently, an implementation MUST maintain a separate routing storage for each service. However, multiple services can use the same underlying tunnels; the VPLS or VPN label is used to demultiplex the packets belonging to different services. 3. Control Plane There are two primary functions of the VPLS control plane: autodiscovery, and setup and teardown of the pseudowires that constitute the VPLS, often called signaling. The first two subsectionsSection 3.1 and Section 3.2 describe these functions. The third subsectionBoth of these functions are accomplished with a single BGP Update advertisement; Section 3.3 describes how this is done by detailing BGP protocol operation for VPLS. Section 3.4 describes the setting up of pseudowires that span Autonomous Systems. The last subsection detailsSection 3.5 describes how multi-homing is handled. 3.1 Autodiscovery Discovery refers to the process of finding all the PEs that participate in a given VPLS. A PE can either be configured with the identities of all the other PEs in a given VPLS, or the PE can use some protocol to discover the other PEs. The latter is called autodiscovery. The former approach is fairly configuration-intensive, especially since it is required that the PEs participating in a given VPLS are fully meshed (i.e., that every PE in a given VPLS establish pseudowires to every other PE in that VPLS). Furthermore, when the topology of a VPLS changes (i.e., a PE is added to, or removed from the VPLS), the VPLS configuration on all PEs in that VPLS must be changed. In the autodiscovery approach, each PE "discovers" which other PEs are part of a given VPLS by means of some protocol, in this case BGP. This allows each PE's configuration to consist only of the identity of the VPLS instance established on this PE, not the identity of every other PE in that VPLS instance -- that is auto-discovered. Moreover, when the topology of a VPLS changes, only the affected PE's configuration changes; other PEs automatically find out about the change and adapt. 3.1.1 Functions A PE that participates in a given VPLS V must be able to tell all other PEs in VPLS V that it is also a member of V. A PE must also have a means of declaring that it no longer participates in a VPLS. To do both of these, the PE must have a means of identifying a VPLS and a means by which to communicate to all other PEs. U-PE devices also need to know what constitutes a given VPLS; however, they don't need the same level of detail. The PE (or PEs) to which a u-PE is connected gives the u-PE an abstraction of the VPLS; this is described in section 5. 3.1.2 Protocol Specification The specific mechanism for autodiscovery described here is based on  and ; it uses BGP extended communities  to identify members of a VPLS. A more generic autodiscovery mechanism is described in . The specific extended community used is the Route Target, whose format is described in . The semantics of the use of Route Targets is described in ; their use in VPLS is identical. As it has been assumed that VPLSs are fully meshed, a single Route Target RT suffices for a given VPLS V, and in effect that RT is the identifier for VPLS V. A PE announces (typically via I-BGP) that it belongs to VPLS V by annotating its NLRIs for V (see next subsection) with Route Target RT, and acts on this by accepting NLRIs from other PEs that have Route Target RT. A PE announces that it no longer participates in V by withdrawing all NLRIs that it had advertised with Route Target RT. 3.2 Signaling Once discovery is done, each pair of PEs in a VPLS must be able to establish (and tear down) pseudowires to each other, i.e., exchange (and withdraw) demultiplexors. This process is known as signaling. Signaling is also used to transmit certain characteristics of the pseudowires that a PE regardingsets up for a given VPLS. Recall that a demultiplexor is used to distinguish among several different streams of traffic carried over a tunnel, each stream possibly representing a different service. In the case of VPLS, the demultiplexor not only says to which specific VPLS a packet belongs, but also identifies the ingress PE. The former information is used for forwarding the packet; the latter information is used for learning MAC addresses. The demultiplexor described here is an MPLS label, even thoughlabel. However, note that the PE-to-PE tunnels mayneed not be MPLS tunnels. 3.2.1 Setup and TeardownConcepts The VPLS BGP NLRI described below, with a new AFI and SAFI (see ) is used to exchange VPLS membership and demultiplexors. A PE advertises aVPLS BGP NLRI for eachhas the following information elements: a VE ID, a VE Block Offset, a VE Block Size and a label base. The exact format is given below. A PE participating in a VPLS that it participates in.must have at least one VE ID. If the PE is doing learning and flooding, i.e., it isthe VE, it announces a single set of VPLS NLRIs for each VPLS that it is in.typically has one VE ID. If the PE is connected to several u-PEs, it announces one set of VPLS NLRIshas a distinct VE ID for each u-PE. A hybrid scheme is also possible, where the PE learns MAC addresses on some interfaces (over which it is directly connected to CEs) and delegates learning on other interfaces (over which it is connected to u-PEs). In this case, the PE would announce one set of VPLS NLRIs for each u-PE that has customer ports inIt may additionally have a given VPLS, and one setVE ID for itself, if it has customer ports initself acts as a VE for that VPLS. Each set of NLRIs definesIn what follows, we will call the demultiplexors for a range of other PEs inPE announcing the VPLS. Ideally, a singleVPLS NLRI sufficesPE-a, and we will assume that PE-a owns VE ID V (either belonging to cover all PEs in a VPLS; however, there are cases (such as a newly added PE) where the pre-existing NLRI does not have enough labels. In such cases, advertising an additional NLRI for the same VPLS servesPE-a itself, or to add labels for the new PEs without disrupting servicea u-PE connected to PE-a). VE IDs are typically assigned by the pre-existing PEs. If service disruptionnetwork administrator. Their scope is acceptable (or when the PE restarts its BGP process), a PE MAY consider coalescing all NLRIs for a VPLS intolocal to a single NLRI. IfVPLS. A given VE ID should belong to only one PE, unless a PE XCE is partmulti-homed (see Section 3.5). A label block is a set of VPLS V, and X receivesdemultiplexor labels used to reach a given VE ID. A VPLS BGP NLRI with VE ID V, VE Block Offset VBO, VE Block Size VBS and label base LB implicitly announces label block for VV: labels from PE Y that includes a demultiplexor that X can use, X sets up its ends ofLB to (LB + VBS - 1), and remote VE set for V: from VBO to (VBO + VBS - 1). There is a pseudowireone-to-one correspondance between Xthe remote VE set and Y. X may also havethe label block: VE ID (VBO + n) corresponds to advertise a new NLRI for V that includes a demultiplexor that Y can use, if its pre-existing NLRI for V did not include a demultiplexor for Y. If Y's configurationlabel (LB + n). 3.2.2 PW Setup and Teardown Suppose PE-a is changed to remove it frompart of VPLS foo, and makes an announcement with VE ID V, VE Block Offset VBO, VE Block Size VBS and label base LB. If PE-b is also part of VPLS foo, and has VE ID W, PE-b does the following: 1. is W part of PE-a's 'remote VE set': if VBO <= W < VBO + VBS, then Y MUST withdraw all its NLRIs for V.W is part of PE-a's remote VE set. If all Y's linksnot, PE-b ignores this message, and skips the rest of this procedure. 2. set up a PW to CEs in V go down, then Y SHOULD either withdraw all its NLRIs for V, or let other PEs inPE-a: the VPLSdemultiplexor label to send traffic from PE-b to PE-a is computed as (LB + W - VBO). 3. is V know inpart of any 'remote VE set' that PE-b announced: PE-b checks if V belongs to some wayremote VE set that YPE-b announced, say with VE Block Offset VBO', VE Block Size VBS' and label base LB'. If not, PE-b MUST make a new announcement as described in Section 3.3. 4. set up a PW from PE-a: the demultiplexor label over which PE-b should expect traffic from PE-a is no longer connected to its CEs.computed as: (LB' + V - VBO'). If Y withdraws an NLRI for V that X was using, then X MUST tear down its ends of the pseudowire between X and Y. The format of the VPLS NLRI is given below. The AFI is tothe L2VPN AFI (to be assigned by IANA,IANA), and the SAFI is the VPLS SAFI (65). +------------------------------------+ | Length (2 octets) | +------------------------------------+ | Route Distinguisher (8 octets) | +------------------------------------+ | VE ID (2 octets) | +------------------------------------+ | VE Block Offset (2 octets) | +------------------------------------+ | VE Block Size (2 octets) | +------------------------------------+ | Label Base (3 octets) | +------------------------------------+ Figure 2: BGP NLRI for VPLS Information 220.127.116.11.3 Signaling PE Capabilities The following extended attribute, the "Layer2 Info Extended Community", is used to signal control information about the pseudowires to be setup for a given VPLS. This information includes the Encaps Type and(type of encapsulation on the pseudowires), Control Flags are encoded in an extended attribute.(control information regarding the pseudowires) and the Maximum Transmission Unit (MTU) to be used on the pseudowires. The Encaps Type for VPLS is 19. +------------------------------------+ | Extended community type (2 octets) | +------------------------------------+ | Encaps Type (1 octet) | +------------------------------------+ | Control Flags (1 octet) | +------------------------------------+ | Layer-2 MTU (2 octet) | +------------------------------------+ | Reserved (2 octets) | +------------------------------------+ Figure 3: Layer2 Info Extended Community 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | MBZ |C|S| (MBZ = MUST Be Zero) +-+-+-+-+-+-+-+-+ Figure 4: Control Flags Bit Vector With reference to Figure 4, the following bits are defined;following bits in the Control Flags are defined; the remaining bits, designated MBZ, MUST be set to zero when sending and MUST be ignored when receiving this community. Name Meaning C If set to 1 (0), Control word MUST (NOT) be present when sending VPLS packets to this PE . S If set to 1 (0), Sequenced delivery of frames is (not) required when sending VPLS packets to this PE. 3.3 BGP VPLS Operation To create a new VPLS, say VPLS foo, a network administrator must pick a RT for VPLS foo, say RT-foo. This will be used by all PEs that serve VPLS foo. To configure a given PE, say PE-a, to be part of VPLS foo, the network administrator only has to choose a VE ID V for PE-a. (If PE-a is connected to u-PEs, PE-a may be configured with more than one VE ID; in that case, the following is done for each VE ID). The PE may also be configured with a Route Distinguisher (RD); if not, it generates a unique RD for VPLS foo. Say the RD is RD-foo-a. PE-a then generates an initial label block and a remote VE set for V, defined by VE Block Offset VBO, VE Block Size VBS and label base LB. These may be empty. PE-a then creates a VPLS BGP NLRI with RD RD-foo-a, VE ID V, VE Block Offset VBO, VE Block Size VBS and label base LB. To this, it attaches a Layer2 Info Extended Community and a RT, RT-foo. It sets the BGP Next Hop for this NLRI as itself, and announces this NLRI to its peers. The Network Layer protocol associated with the Network Address of the Next Hop for the combination <AFI=L2VPN AFI, SAFI=VPLS SAFI> is IP; this association is required by , Section 5. If the value of the Length of the Next Hop field is 4, then the Next Hop contains an IPv4 address. If this value is 16, then the Next Hop contains an IPv6 address. If PE-a hears from another PE, say PE-b, a VPLS BGP announcement with RT-foo and VE ID W, then PE-a knows that PE-b is a member of the same VPLS (auto-discovery). PE-a then has to set up its part of a VPLS pseudowire between PE-a and PE-b, using the mechanisms in Section 3.2. Similarly, PE-b will have discovered that PE-a is in the same VPLS, and PE-b must set up its part of the VPLS pseudowire. Thus, signaling and pseudowire setup is also achieved with the MBZ bits MUST besame Update message. If W is not in any remote VE set that PE-a announced for VE ID V in VPLS foo, PE-b will not be able to zero. Name Meaning C Ifset up its part of the pseudowire to 1 (0), Control wordPE-a. To address this, PE-a can choose to withdraw the old announcement(s) it made for VPLS foo, and announce a new Update with a larger remote VE set and corresponding label block that covers all VE IDs that are in VPLS foo. This however, may cause some service disruption. An alternative for PE-a is (not) required when encapsulating Layer 2 frames . S Ifto create a new remote VE set and corresponding label block, and announce them in a new Update, without withdrawing previous announcements. If PE-a's configuration is changed to 1 (0), Sequenced deliveryremove VE ID V from VPLS foo, then PE-a MUST withdraw all its announcements for VPLS foo that contain VE ID V. If all of framesPE-a's links to its CEs in VPLS foo go down, then PE-a SHOULD either withdraw all its NLRIs for VPLS foo, or let other PEs in the VPLS foo know in some way that PE-a is (not) required. 3.3no longer connected to its CEs. 3.4 Multi-AS VPLS As in  and , the above autodiscovery and signaling functions are typically announced via I-BGP. This assumes that all the sites in a VPLS are connected to PEs in a single Autonomous System (AS). However, sites in a VPLS may connect to PEs in different ASes. This leads to two issues: 1) there would not be an I-BGP connection between those PEs, so some means of signaling across ASes may be needed; and 2) there may not be PE-to-PE tunnels between the ASes. A similar problem is solved in , Section 10. Three methods are suggested to address issue (1); all these methods have analogs in multi-AS VPLS. Here is a diagram for reference: __________ ____________ ____________ __________ / \ / \ / \ / \ \___/ AS 1 \ / AS 2 \___/ \ / +-----+ +-------+ | +-------+ +-----+ | PE1 | ---...--- | ASBR1 | ======= | ASBR2 | ---...--- | PE2 | +-----+ +-------+ | +-------+ +-----+ ___ / \ ___ / \ / \ / \ \__________/ \____________/ \____________/ \__________/ Figure 6: Inter-AS VPLS 18.104.22.168.1 a) VPLS-to-VPLS connections at the AS border routers. In this method, an AS Border Router (ASBR1) acts as a PE for all VPLSs that span AS1 and an AS to which ASBR1 is connected, such as AS2 here. The ASBR on the neighboring AS (ASBR2) is viewed by ASBR1 as a CE for the VPLSs that span AS1 and AS2; similarly, ASBR2 acts as a PE for this VPLS from AS2's point of view, and views ASBR1 as a CE. This method does not require MPLS on the ASBR1-ASBR2 link, but does require that this link carry Ethernet traffic, and that there be a separate VLAN sub-interface for each VPLS traversing this link. It further requires that ASBR1 does the PE operations (discovery, signaling, MAC address learning, flooding, encapsulation, etc.) for all VPLSs that traverse ASBR1. This imposes a significant burden on ASBR1, both on the control plane and the data plane, which limits the number of multi-AS VPLSs. Note that in general, there will be multiple connections between a pair of ASes, for redundancy. In this case, the Spanning Tree Protocol (STP), or some other means of loop detection and prevention, must be run on each VPLS that spans these ASes, so that a loop-free topology can be constructed in each VPLS. This imposes a further burden on the ASBRs and PEs participating in those VPLSs, as these devices would need to run a loop detection algorithm for each such VPLS. How this may be achieved is outside the scope of this document. 22.214.171.124.2 b) EBGP redistribution of VPLS information between ASBRs. This method requires I-BGP peerings between the PEs in AS1 and ASBR1 in AS1 (perhaps via route reflectors), an E-BGP peering between ASBR1 and ASBR2 in AS2, and I-BGP peerings between ASBR2 and the PEs in AS2. In the above example, PE1 sends a VPLS NLRI to ASBR1 with a label block and itself as the BGP nexthop; ASBR1 sends the NLRI to ASBR2 with new labels and itself as the BGP nexthop; and ASBR2 sends the NLRI to PE2 with new labels and itself as the nexthop. The VPLS NLRI that ASBR1 sends to ASBR2 (and the NLRI that ASBR2 sends to PE2) is identical to the VPLS NLRI that PE1 sends to ASBR1, except for the label block. To be precise, the Length, the Route Distinguisher, the VE ID, the VE Block Offset, and the VE Block Size MUST be the same; the Label Base may be different. Furthermore, ASBR1 must also update its forwarding path as follows: if the Label Base sent by PE1 is L1, the Label-block Size is N, the Label Base sent by ASBR1 is L2, and the tunnel label from ASBR1 to PE1 is T, then ASBR1 must install the following in the forwarding path: swap L2 with L1 and push T, swap L2+1 with L1+1 and push T, ... swap L2+N-1 with L1+N-1 and push T. ASBR2 must act similarly, except that it may not need a tunnel label if it is directly connected with ASBR1. When PE2 wants to send a VPLS packet to PE1, PE2 uses its VE ID to get the right VPLS label from ASBR2's label block for PE1, and uses a tunnel label to reach ASBR2. ASBR2 swaps the VPLS label with the label from ASBR1; ASBR1 then swaps the VPLS label with the label from PE1, and pushes a tunnel label to reach PE1. In this method, one needs MPLS on the ASBR1-ASBR2 interface, but there is no requirement that the link layer be Ethernet. Furthermore, the ASBRs take part in distributing VPLS information. However, the data plane requirements of the ASBRs is much simpler than in method (a), being limited to label operations. Finally, the construction of loop-free VPLS topologies is done by routing decisions, viz. BGP path and nexthop selection, so there is no need to run the Spanning Tree Protocol on a per-VPLS basis. Thus, this method is considerably more scalable than method (a). 126.96.36.199.3 c) Multi-hop EBGP redistribution of VPLS information between ASes. In this method, there is a multi-hop E-BGP peering between the PEs (or preferably, a Route Reflector) in AS1 and the PEs (or Route Reflector) in AS2. PE1 sends a VPLS NLRI with labels and nexthop self to PE2; if this is via route reflectors, the BGP nexthop is not changed. This requires that there be a tunnel LSP from PE1 to PE2. This tunnel LSP can be created exactly as in , section 10 (c), for example using E-BGP to exchange labeled IPv4 routes for the PE loopbacks. When PE1 wants to send a VPLS packet to PE2, it pushes the VPLS label corresponding to its own VE ID onto the packet. It then pushes the tunnel label(s) to reach PE2. This method requires no VPLS information (in either the control or the data plane) on the ASBRs. The ASBRs only need to set up PE-to-PE tunnel LSPs in the control plane, and do label operations in the data plane. Again, as in the case of method (b), the construction of loop-free VPLS topologies is done by routing decisions, i.e., BGP path and nexthop selection, so there is no need to run the Spanning Tree Protocol on a per-VPLS basis. This option is likely to be the most scalable of the three methods presented here. 3.4.4 Allocation of VE IDs Across Multiple ASes In order to ease the allocation of VE IDs for a VPLS that spans multiple ASes, one can allocate ranges for each AS. For example, AS1 uses VE IDs in the range 1 to 100, AS2 from 101 to 200, etc. If there are 10 sites attached to AS1 and 20 to AS2, the allocated VE IDs could be 1-10 and 101 to 120. This minimizes the number of VPLS NLRIs that are exchanged while ensuring that VE IDs are kept unique. In the above example, if AS1 needed more than 100 sites, then another range can be allocated to AS1. The only caveat is that there isbe no overlap between VE ID ranges among ASes. The exception to this rule is multi-homing, which is dealt with below. 3.43.5 Multi-homing and Path Selection It is often desired to multi-home a VPLS site, i.e., to connect it to multiple PEs, perhaps even in different ASes. In such a case, the PEs connected to the same site can either be configured with the same VE ID or with different VE IDs. In the latter case, it is mandatory to run STP on the CE device, and possibly on the PEs, to construct a loop-free VPLS topology. How this can be accomplished is outside the scope of this document; however, the rest of this section will describe in some detail the former case. In the case where the PEs connected to the same site are assigned the same VE ID, a loop-free topology is constructed by routing mechanisms, in particular, by BGP path selection. When a BGP speaker receives two equivalent NLRIs (see below for the definition), it applies standard path selection criteria such as Local Preference and AS Path Length to determine which NLRI to choose; it MUST pick only one. If the chosen NLRI is subsequently withdrawn, the BGP speaker applies path selection to the remaining equivalent VPLS NLRIs to pick another; if none remain, the forwarding information associated with that NLRI is removed. Two VPLS NLRIs are considered equivalent from a path selection point of view if the Route Distinguisher, the VE ID and the VE Block Offset are the same. If two PEs are assigned the same VE ID in a given VPLS, they MUST use the same Route Distinguisher, and they SHOULD announce the same VE Block Size for a given VE Offset. 4. Data Plane This section discusses two aspects of the data plane for PEs and u-PEs implementing VPLS: encapsulation and forwarding. 4.1 Encapsulation Ethernet frames received from CE devices are encapsulated for transmission over the packet switched network connecting the PEs. The encapsulation is as in , with one change: a PE that sets the P bit in the Control Flags strips the outermost VLAN from an Ethernet frame received from a CE before encapsulating it, and pushes a VLAN onto a decapsulated frame before sending it to a CE. 4.2 Forwarding VPLS packets are classified as belonging to a given service instance and associated forwarding table based on the interface over which the packet is received. Packets are forwarded in the context of the service instance based on the destination MAC address. The former mapping is determined by configuration. The latter is the focus of this section. 4.2.1 MAC address learning As was mentioned earlier, the key distinguishing feature of VPLS is that it is a multipoint service. This means that the entire Service Provider network should appear as a single logical learning bridge for each VPLS that the SP network supports. The logical ports for the SP "bridge" are the customer ports on all of the VE on a given service. Just as a learning bridge learns MAC addresses on its ports, the SP bridge must learn MAC addresses at its VEs. Learning consists of associating source MAC addresses of packets with the (logical) ports on which they arrive; this association is the Forwarding Information Base (FIB). The FIB is used for forwarding packets. For example, suppose the bridge receives a packet with source MAC address S on (logical) port P. If subsequently, the bridge receives a packet with destination MAC address S, it knows that it should send the packet out on port P. 4.2.2 Flooding When a bridge receives a packet to a destination that is not in its FIB, it floods the packet on all the other ports. Similarly, a VE will flood packets to an unknown destination to all other VEs in the VPLS. In Figure 1 above, if CE2 sent an Ethernet frame to PE2, and the destination MAC address on the frame was not in PE2's FIB (for that VPLS), then PE2 would be responsible for flooding that frame to every other PE in the same VPLS. On receiving that frame, PE1 would be responsible for further flooding the frame to CE1 and CE5 (unless PE1 knew which CE "owned" that MAC address). On the other hand, if PE3 received the frame, it could delegate further flooding of the frame to its u-PE. If PE3 was connected to 2 u-PEs, it would announce that it has two u-PEs. PE3 could either announce that it is incapable of flooding, in which case it would receive two frames, one for each u-PE, or it could announce that it is capable of flooding, in which case it would receive one copy of the frame, which it would then send to both u-PEs. 4.2.3 "Split Horizon" Forwarding When a PE capable of flooding receives a broadcast Ethernet frame, or one with an unknown destination MAC address, it must flood the frame. If the frame arrived from an attached CE, the PE must send a copy of the frame to every other attached CE, as well as to all PEs participating in the VPLS. If the frame arrived from another PE, however, the PE must only send a copy of the packet to attached CEs. The PE MUST NOT send the frame to other PEs. This notion has been termed "split horizon" forwarding, and is a consequence of the PEs being logically full-meshed -- if a broadcast frame is received from PEx, then PEx would have sent a copy to all other PEs. Split horizon forwarding rules also apply to multicast frames (i.e., those with a multicast destination MAC address). In this case, when a PE receives a multicast frame from another PE, the frame is replicated and sent to the relevant subset of attached CEs; however, it MUST NOT be sent to other PEs. 5. Deployment Options In deploying a network that supports VPLS, the SP must decide what functions the VPLS-aware device closest to the customer (the VE) supports. The default case described in this document is that the VE is a PE. However, there are a number of reasons that the VE might be a device that does all the Layer 2 functions (such as MAC address learning and flooding), and a limited set of Layer 3 functions (such as communicating to its PE), but, for example, doesn't do full-fledgedfull- fledged discovery and PE-to-PE signaling. Such a device is called a "u-PE". As both of these cases have benefits, one would like to be able to "mix and match" these scenarios. The signaling mechanism presented here allows this. For example, in a given provider network, one PE may be directly connected to CE devices; another may be connected to u-PEs that are connected to CEs; and a third may be connected directly to a customer over some interfaces and to u-PEs over others. All these PEs perform discovery and signaling in the same manner. How they do learning and forwarding depends on whether or not there is a u-PE; however, this is a local matter, and is not signaled. However, the details of the operation of a u-PE and its interactions with PEs and other u-PEs is beyond the scope of this document. 6. Security Considerations The focus in Virtual Private LAN Service is the privacy of data, i.e., that data in a VPLS is only distributed to other nodes in that VPLS and not to any external agent or other VPLS. Note that VPLS does not offer security or authentication: VPLS packets are sent in the clear in the packet-switched network, and a man-in-the-middle can eavesdrop, and may be able to inject packets into the data stream. If security is desired, the PE-to-PE tunnels can be IPsec tunnels. For more security, the end systems in the VPLS sites can use appropriate means of encryption to secure their data even before it enters the Service Provider network. There are two aspects to achieving data privacy in a VPLS: securing the control plane, and protecting the forwarding path. Compromise of the control plane could result in a PE sending data belonging to some VPLS to another VPLS, or blackholing VPLS data, or even sending it to an eavesdropper, none of which are acceptable from a data privacy point of view. Since all control plane exchanges are via BGP, techniques such as in  help authenticate BGP messages, making it harder to spoof updates (which can be used to divert VPLS traffic to the wrong VPLS), or withdraws (denial of service attacks). In the multi-AS options (b) and (c), this also means protecting the inter-AS BGP sessions, between the ASBRs, the PEs or the Route Reflectors. Note that  will not help in keeping VPLS labels private -- knowing the labels, one can eavesdrop on VPLS traffic. However, this requires access to the data path within a Service Provider network. Protecting the data plane requires ensuring that PE-to-PE tunnels are well-behaved (this is outside the scope of this document), and that VPLS labels are accepted only from valid interfaces. For a PE, valid interfaces comprise links from P routers. For an ASBR, a valid interface is a link from an ASBR in an AS that is part of a given VPLS. It is especially important in the case of multi-AS VPLSs that one accept VPLS packets only from valid interfaces. 7. IANA Considerations IANA is asked to allocate an AFI for Layer 2L2VPN information (suggested value: 25). This should be the same as the AFI requested by . 8. References 8.1 Normative References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.  Bates, T., Rekhter, Y.,Chandra, R. and D.R., Katz, D., and Y. Rekhter, "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.draft-ietf-idr-rfc2858bis-06 (work in progress), May 2004.  Sangli, S., Tappan, D.D., and Y. Rekhter, "BGP Extended Communities Attribute", Internet-Draft draft-ietf-idr-bgp-ext-communities-08,draft-ietf-idr-bgp-ext-communities-08 (work in progress), February 2005.  Martini, L., Rosen, E., El-Aawar, N. and G. Heron,"Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLSMPLS Networks", Internet-Draft draft-ietf-pwe3-ethernet-encap-08, September 2004.draft-ietf-pwe3-ethernet-encap-09 (work in progress), February 2005. 8.2 Informative References  Andersson, L. and E. Rosen, "Framework for Layer 2 Virtual Private Networks (L2VPNs)", Internet-Draft draft-ietf-l2vpn-l2-framework-05,draft-ietf-l2vpn-l2-framework-05 (work in progress), June 2004.  Lasserre, M. and V. Kompella, "Virtual Private LAN Services over MPLS", Internet-Draft draft-ietf-l2vpn-vpls-ldp-05, September 2004.draft-ietf-l2vpn-vpls-ldp-06 (work in progress), February 2005.  Ould-Brahim, H., Rosen, E.E., and Y. Rekhter, "Using BGP as an Auto-Discovery Mechanism for Layer-3 and Layer-2 VPNs", Internet-Draft draft-ietf-l3vpn-bgpvpn-auto-04, May 2004.draft-ietf-l3vpn-bgpvpn-auto-05 (work in progress), February 2005.  Rosen, E., "BGP/MPLS IP VPNs", Internet-Draft draft-ietf-l3vpn-rfc2547bis-03,draft-ietf-l3vpn-rfc2547bis-03 (work in progress), October 2004.  Martini, L., "Pseudowire Setup and Maintenance using LDP", Internet-Draft draft-ietf-pwe3-control-protocol-14, November 2004.draft-ietf-pwe3-control-protocol-16 (work in progress), March 2005.  Kompella, K., "Layer 2 VPNs Over Tunnels", Internet-Draft draft-kompella-l2vpn-l2vpn-00,draft-kompella-l2vpn-l2vpn-00 (work in progress), January 2004. Authors' Addresses Kireeti Kompella (editor) Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US Email: email@example.com Yakov Rekhter (editor) Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94089 US Email: firstname.lastname@example.org Appendix A. Contributors The following contributed to this document: Javier Achirica, Telefonica Loa Andersson, Acreo Chaitanya Kodeboyina, Juniper Giles Heron, Alcatel Sunil Khandekar, Alcatel Vach Kompella, Alcatel Marc Lasserre, Riverstone Pierre Lin Pascal Menezes Ashwin Moranganti, Appian Hamid Ould-Brahim, Nortel Seo Yeong-il, Korea Tel Appendix B. Acknowledgements Thanks to Joe Regan and Alfred Nothaft for their contributions. Many thanks too to Eric Ji, Chaitanya Kodeboyina, and Mike Loomis for their detailed reviews. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at email@example.com. 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