Setting Up WAN over MPLS PseudowiresIn the most general sense, the transport and tunneling of Layer 2 WAN protocols is no different from the transport of Ethernet over MPLS. As discussed in detail in previous chapters, draft-martinibased technologies allow the transport of Layer 2 packets over an MPLS network over point-to-point pseudowires. Although the underlying architecture does not change, several Layer 2-specific customizations from the architectural model allow the transport of specific Layer 2 WAN protocols. This section covers some of these similarities and differences. Control PlaneThe setup and maintenance of AToM pseudowires is based on the targeted (also referred to as directed) Label Distribution Protocol (LDP) session between a pair of provider edge (PE) routers. You can bind the Layer 2 attachment circuit (AC) to the label by using the LDP Label Mapping message. Several pseudowires that are signaled by the targeted LDP session between PEs use one packet-switched network (PSN) tunnel label-switched path (LSP) signaled by Link LDP (IGP) or another label distribution protocol, such as Resource Reservation Protocol Traffic Engineering (RSVP-TE).Chapter 6. For example, whereas some interface parameters are applicable to multiple VC types or emulated services (such as maximum transmission unit [MTU] and Interface Description), others are valid only for specific VC types.The maximum number of concatenated ATM cells interface parameter is applicable only to the different ATM cell transport modes. The Frame Relay DLCI length interface parameter indicates the length of the DLCI field in the Frame Relay header and pertains only to Frame Relay over MPLS.The following subsections explain more about the transport of WAN protocols over MPLS PSNs:Pseudowire types usedData plane encapsulationUsage of the control wordMTU size requirements Pseudowire Types UsedThe 15-bit pseudowire type (or VC type) field identifies the type of pseudowire. The different VC types used in the transport of WAN protocols over MPLS are shown in Table 8-1.
The case study sections later in this chapter reference these pseudowire types. Data Plane EncapsulationThe encapsulation of Layer 2 WAN protocol data units (PDU) is specified in the encapsulation martini draft and subsequent Pseudowire Emulation Edge-to-Edge (PWE3) working group derivative drafts spawned from it (see Figure 8-1). Essentially, the Layer 2 PDU is encapsulated in an MPLS stack where the inner or bottom label (the VC label contained in the VC MPLS shim header) identifies the Layer 2 attachment circuit and is advertised in the LDP targeted session. The tunnel header is in turn comprised by 0 or more MPLS headers from the PSN tunnel control plane. Figure 8-1. Encapsulation of WAN Protocols over MPLSChapter 6. The upcoming section discusses the control word usage in the context of WAN protocols over MPLS. Usage of the Control WordDuring pseudowire setup, the usage of a control word is negotiated by setting the C bit in the pseudowire ID FEC element. Figure 8-2 compares the control word format for different WAN protocols over MPLS. Figure 8-2. Control Word Format for Different WAN Protocols Although the control word is optional for some encapsulations such as PPP, HDLC, and cell relay (ATM cell mode transport), it is required for Frame Relay and ATM AAL5 over MPLS. This requirement for Frame Relay and ATM AAL5 transport modes is because the control word carries control information. This information is specific for the Layer 2 that is being emulated that is not carried in the AToM payload. For example, you will see in the upcoming section "Frame Relay over MPLS" that the Frame Relay Q.922 header is stripped at the ingress PE at MPLS imposition, so the control word carries the FECN, BECN, and DE bits that were present in the now-stripped header. MTU RequirementsEvery time you encapsulate a PDU with a new protocol header, you need to take into account maximum transmission unit (MTU) considerations. In a Layer 2 VPN, PEs during imposition are encapsulating a customer edge's (CE) Layer 2 PDU to be switched across an MPLS network. You need to calculate a series of associated overheads to properly set up the core MTU. You can subdivide these overheads into three categories:Transport overhead This is the overhead that is associated with the specific Layer 2 that is being transported. Table 8-2 lists transport overheads for different WAN protocols.
ATM Cell transport is deliberately left out of Encapsulations and Packet Format for Cell Transport" covers this topic.Frame Relay with IETF encapsulation refers to RFC 2427, "Multiprotocol Interconnect over Frame Relay," which makes RFC 1490 obsolete. For Frame Relay with IETF encapsulation DLCI transport, the overhead is considered variable; many packets have a transport overhead of 2 bytes: the control byte of 0x03 and the Network Layer Protocol Identifier (NLPID). This is the minimum overhead in Frame Relay IETF. However, in some other cases (such as when a protocol does not have an NLPID assigned), the NLPID value of 0x80 indicates that a Subnet-work Attachment Point (SNAP) header follows. The Organizationally Unique Identifier (OUI) of 0x000000 indicates that a 2-byte Ethertype follows. In this case, in which the upper layer protocol has no NLPID, the transport overhead is 8 bytes, and you need to use the worst case when setting the core MTU.On the other hand, for Frame Relay Cisco encapsulation, a 2-byte Ethertype is always used instead of control and NLPID, making the transport overhead permanently 2 bytes.TipWhen you are studying the packet formats for the different WAN protocols in the upcoming sections, a good exercise is to come back to Table 8-2 and identify the different fields that make up the transport overhead.You can use the following generic formula to calculate the core MTU from the edge MTU. The edge MTU is the MTU configured in the interface of the CE-facing PE:
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Edge MTU + Transport Header + AToM Header + (MPLS Label Stack *