Network Working Group K. Kompella (Editor) Internet Draft Y. Rekhter (Editor) Category: Standards Track Juniper Networks Expires:
JulyNovember 2004 JanuaryMay 2004 draft-ietf-l2vpn-vpls-bgp-01.txtdraft-ietf-l2vpn-vpls-bgp-02.txt Virtual Private LAN Service Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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 offered is 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, and describes a mechanism for signaling a VPLS, as well as for forwarding VPLS frames across a packet switched network. 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. 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 scenarios.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 here (section 3)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. Section 3 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. 2. Functional Model This will be described with reference to Figure 1. Figure 1: Example of a VPLS ----- / A1 \ ---- ____CE1 | / \ -------- -------- / | | | A2 CE2- / \ / PE1 \ / \ / \ / \___/ | \ ----- ---- ---PE2 | \ | | \ ----- | Service Provider Network | \ / \ | | CE5 A5 | | ___ | / \ / |----| \ / \ PE4_/ ----- |L2PE|--PE3|u-PE|--PE3 / \ / |----| -------- ------- ---- / | ---- / \/ \ / \ CE = Customer Edge Device | A3 CE3 --CE4 A4 | PE = Provider Edge Router \ / \ / L2PEu-PE = Layer 2 Aggregation ---- ---- A<n> = Customer site n 2.1. Terminology Terminology similar to that in  is used, with the addition of "L2PE","u- PE", a Layer 2 PE device used for Layer 2 aggregation. An L2PEA u-PE is owned and operated by the Service Provider (as is the PE). PE and L2PEu- 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 L2PE,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 an L2PEa 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, an L2PEa 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 the section below on "Split Horizon" Flooding). This assumption reduces (but does not eliminate) the need to run Spanning Tree Protocol among the PEs. 2.3. Interactions VPLS is a successful "LAN Service" if CE devices that belong to VPLS V can interact through the SP network as if they were connected by a LAN. VPLS is "private" if CE devices that belong to different VPLSs cannot interact. VPLS is "virtual" if 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 (i.e., that are attached to CE devices that belong to the same VPLS), and to exchange demultiplexors. These interactions are control-driven, not data-driven. L2PEsU-PEs interact with PEs to establish connections with remote PEs or L2PEsu-PEs in the same VPLS. Again, this interaction is control-driven. 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 subsections describe these functions. The next subsection describes the setting up of pseudowires that span Autonomous Systems. The last subsection details how multi-homing is handled. 3.1. Autodiscovery AutodiscoveryDiscovery 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 autodiscoveruse some protocol to discover the other PEs. The latter is called autodiscovery. The former approach is fairly configuration-intensive, especially since it is required (in this and other VPLS approaches) that the PEs participating in a given VPLS are fully meshed (i.e., every pair of PEs in a given VPLS establish pseudowires to each other). 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 that each customer belongs to, not the identity of every other PE in that VPLS. 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. L2PEU-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 an L2PEa u-PE is connected gives the L2PEu-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 initiate "relearning", and to transmit certain characteristics of the PE regarding 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 though the PE-to-PE tunnels may not be MPLS tunnels. 3.2.1. Setup and Teardown The VPLS BGP NLRI described below, with a new AFI and SAFI (see ) is used to exchange demultiplexors. A PE advertises a VPLS NLRI for each VPLS that it participates in. If the PE is doing learning and flooding, i.e., it is the VE, it announces a single set of VPLS NLRIs for each VPLS that it is in. If the PE is connected to several L2PEs,u-PEs, it announces one set of VPLS NLRIs for each L2PE.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 L2PEs).u-PEs). In this case, the PE would announce one set of VPLS NLRIs for each L2PEu-PE that has customer ports in a given VPLS, and one set for itself, if it has customer ports in that VPLS. Each set of NLRIs defines the demultiplexors for a range of other PEs in the VPLS. Ideally, a single NLRI suffices 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 serves to add labels for the new PEs without disrupting service to the pre-existing PEs. If service disruption is acceptable (or when the PE restarts its BGP process), a PE MAY consider coalescing all NLRIs for a VPLS into a single NLRI. If a PE X is part of VPLS V, and X receives a VPLS NLRI for V from PE Y that includes a demultiplexor that X can use, X sets up its ends of a pair of pseudowires between X and Y. X may also have 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 configuration is changed to remove it from VPLS V, orthen Y MUST withdraw all its NLRIs for V. If all Y's links to CEs in V go down, then Y SHOULD either withdraw all its NLRIs for V.V, or let other PEs in the VPLS V know in some way that Y is no longer connected to its CEs. If Y withdraws an NLRI for V that X was using, then X tearsMUST tear down its ends of the pseudowires between X and Y. The format of the VPLS NLRI is given below. The AFI and SAFI are the same as for the L2 VPN NLRI . Figure 2: BGP NLRI for VPLS Information +------------------------------------+ | 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) | +------------------------------------+ 3.2.2. Signaling PE Capabilities The Encaps Type and Control Flags are encoded in an extended attribute. The community type also is used in L2 VPNs . There is a newThe Encaps Type for VPLS (TBD).is 19. Figure 3: layer2-info extended community +------------------------------------+ | Extended community type (2 octets) | +------------------------------------+ | Encaps Type (1 octet) | +------------------------------------+ | Control Flags (1 octet) | +------------------------------------+ | Layer-2 MTU (2 octet) | +------------------------------------+ | Reserved (2 octets) | +------------------------------------+ There are three new control flags, Q, F and P defined for VPLS. Q says whether qualified learning occurs (1) or not (0); F which says whether the PE is capable of flooding (1) or not (0). P indicates that the PE will strip the outermost VLAN from the layer 2 customer frame on ingress, and push a VLAN on egress.Figure 4: Control Flags Bit Vector 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | MBZ |P|Q|F|C|S| (MBZ = MUST Be Zero) +-+-+-+-+-+-+-+-+ With reference to Figure 4, the following bits are defined; the MBZ bits MUST be set to zero. Name Meaning P If set to 1, then the PE will strip the outermost VLAN tag from the customer frame on ingress, and push a VLAN tag on egress. If set to 0, the customer frame is left unchanged. Q Reserved. F If set to 1 (0), the PE is (not) capable of flooding. C If set to 1 (0), Control word is (not) required when encapsulating Layer 2 frames . S If set to 1 (0), Sequenced delivery of frames is (not) required. 3.3. 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 | +-----+ +-------+ | +-------+ +-----+ ___ / \ ___ / \ / \ / \ \__________/ \____________/ \____________/ \__________/ 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 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 the Spanning Tree Protocol for each such VPLS.. 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). 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. 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 is no overlap between VE ID ranges among ASes. The exception to this rule is multi-homing, which is dealt with below. 3.4. 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. 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 MUST 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 L2PEsu- 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 Forwarding of VPLS packets is based on the interface over which the packet is received, which determines which VPLS the packet belongs to, and 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 connections from the SP edge, be it a PE or an L2PE,a u-PE, to the CE. 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; callthis 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. There are two modes of learning: qualified and unqualified learning. In qualified learning, the learning decisions at the VE are based on the customer ethernet packet's MAC address and VLAN tag, if one exists. If noThis VLAN tag exists,is often called the default"service delimiting VLAN". Each VLAN on a given port is assumed. Effectively, within one VPLS, there are multiple logical FIBs, one formapped to a different service (VPLS, IP VPN, point-to-point Layer 2 VPN, etc.); each customerVLAN tag identified inthat is mapped to a customer packet.VPLS service has its own VPLS FIB. In unqualified learning, learning is based on a customer ethernet packet's MAC address only. In other words, at any VE, thereThis is only one FIB per VPLS. Every VE must have at least one FIB for each VPLS that it participates in.also called "port-mode VPLS". 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 L2PE.u-PE. If PE3 was connected to 2 L2PEs,u-PEs, it would announce that it has two L2PEs.u-PEs. PE3 could either announce that it is incapable of flooding, in which case it would receive two frames, one for each L2PE,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 L2PEs.u-PEs. 4.2.3. "Split Horizon" Flooding 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" flooding, 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. 5. Deployment ScenariosOptions In deploying a network that supports VPLS, the SP must decide whether the VPLS-aware device closest to the customer (the VE) is an L2PEa u-PE or a PE. 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 an L2PE,a u- PE, i.e., a device that does layer 2 functions such as MAC address learning and flooding, and some limited layer 3 functions such as communicating to its PE, but doesn't do full-fledged discovery and PE-to-PE signaling. 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. PE1 may be directly connected to CE devices; PE2 may be connected to L2PEsu-PEs that are connected to CEs; and PE3 may be connected directly to a customer over some interfaces and to L2PEsu-PEs over others. All these PEs do discovery and signaling in the same manner. How they do learning and forwarding depends on whether or not there is an L2PE;a u-PE; however, this is a local matter, and is not signaled. 6. Normative References [ 1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 [ 6] Bates, T., Rekhter, Y., Chandra, R., and Katz, D., "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000 [ 7] Rosen, E., and Rekhter, Y., Editors, "BGP/MPLS VPNs", draft- ietf-l3vpn-rfc2547bis-01.txt, work in progress.  Sangli, S., D. Tappan, and Y. Rekhter, "BGP Extended Communities Attribute", draft-ietf-idr-bgp-ext-communities-06.txt, workdraft-ietf-idr-bgp-ext-communities-07.txt (work in progress.progress)  Martini, L., et al, "Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS Networks", draft-ietf- pwe3-ethernet-encap-05.txt, workpwe3-ethernet-encap-06.txt (work in progress.progress)  Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option," RFC 2385, August 1998 7. Informative References [ 2] Augustyn, W. (Editor), "RequirementsAndersson, L., and Rosen, E., "Framework for Layer 2 Virtual Private LAN Services (VPLS)", draft-ietf-l2vpn-vpls-requirements-00.txt, workNetworks (L2VPNs)", draft-ietf-l2vpn-l2-framework-04.txt (work in progress.progress) [ 3] Kompella, K., (Editor), "Layer 2 VPNs Over Tunnels", draft- kompella-l2vpn-l2vpn-00.txt, workkompella-l2vpn-l2vpn-00.txt (work in progress.progress) [ 4] Martini, L., et al, "Pseudowire Setup and Maintenance using LDP" draft-ietf-pwe3-control-protocol-05.txt, workdraft-ietf-pwe3-control-protocol-06.txt (work in progress.progress) [ 5] Kompella, V., et al, "Virtual Private LAN Services over MPLS", draft-ietf-ppvpn-vpls-ldp-01.txt, workdraft-ietf-ppvpn-vpls-ldp-03.txt (work in progress.progress) [ 7] Rosen, E., and Rekhter, Y., Editors, "BGP/MPLS VPNs", draft- ietf-l3vpn-rfc2547bis-01.txt (work in progress) [ 8] Ould-Brahim, H. et al,H., Rosen, E., and Rekhter, Y., "Using BGP as an Auto-Discovery Mechanism for Network-basedLayer-3 and Layer-2 VPNs", workdraft- ietf-l3vpn-bgpvpn-auto-04.txt (work in progress.progress) 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. IANA Considerations IANA is asked to allocate an AFI for Layer 2 information (suggested value: 25). IANA is also asked to register a SAFI for VPLS of 65 from the First-Come-First-Served space for SAFIs.Contributors The following contributed to this document: Javier Achirica, Telefonica Loa Andersson, TLA Chaitanya Kodeboyina, Juniper Giles Heron, Consultant Sunil Khandekar, Alcatel Vach Kompella, Alcatel Marc Lasserre, Riverstone Pierre Lin, Yipes Pascal Menezes, Terabeam Ashwin Moranganti, Appian Hamid Ould-Brahim, Nortel Seo Yeong-il, Korea Tel Acknowledgments Thanks to Joe Regan and Alfred Nothaft for their contributions. Authors' Addresses Kireeti Kompella Juniper Networks 1194 N. Mathilda Ave Sunnyvale, CA 94089 firstname.lastname@example.org Yakov Rekhter Juniper Networks 1194 N. 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