Layer 2 VPN Architectures: Understanding Any Transport over MPLS


Understanding AToM Operations

In Chapter 3, you learned how AToM achieves a high degree of scalability by using the MPLS encoding method. You also read an overview of LDP in the previous section. Reading through this section, you will develop a further understanding of how MPLS encapsulation, LDP sig-naling, and pseudowire emulation work together.

The primary tasks of AToM include establishing pseudowires between provider edge (PE) routers and carrying Layer 2 packets over these pseudowires. The next sections cover the operations of AToM from the perspectives of both the control plane and the data plane as follows:

  • Pseudowire label binding
  • Establishing AToM pseudowires
  • Control word negotiation
  • Using sequence numbers
  • Pseudowire encapsulation

Pseudowire Label Binding
An AToM pseudowire essentially consists of two unidirectional LSPs. Each is represented by a pseudowire label, also known as a VC label. The pseudowire label is part of the label stack encoding that encapsulates Layer 2 packets going over AToM pseudowires. Refer to Chapter 3 for an overview of an AToM packet.

The label distribution procedures that are defined in LDP specifications distribute and manage the pseudowire labels. To associate a pseudowire label with a particular Layer 2 connection, you need a way to represent such a Layer 2 connection. The baseline LDP specification only defines Layer 3 FECs. Therefore, the pseudowire emulation over MPLS application defines a new LDP extension—the Pseudowire ID FEC element—that contains a pseudowire identifier shared by the pseudowire endpoints. Figure 6-8 depicts the Pseudowire ID FEC element en-coding.

Figure 6-8 Pseudowire ID FEC Element

The Pseudowire ID FEC element has the following components:

  • Pseudowire ID FEC—The first octet has a value of 128 that identifies it as a Pseudowire ID FEC element.
  • Control Word Bit (C-Bit)—The C-bit indicates whether the advertising PE expects the control word to be present for pseudowire packets. A control word is an optional 4-byte field located between the MPLS label stack and the Layer 2 payload in the pseudowire packet. The control word carries generic and Layer 2 payload-specific information. If the C-bit is set to 1, the advertising PE expects the control word to be present in every pseudowire packet on the pseudowire that is being signaled. If the C-bit is set to 0, no control word is expected to be present.
  • Pseudowire Type—PW Type is a 15-bit field that represents the type of pseudowire. Examples of pseudowire types are shown in Table 6-1.
  • Pseudowire Information Length—Pseudowire Information Length is the length of the Pseudowire ID field and the interface parameters in octets. When the length is set to 0, this FEC element stands for all pseudowires using the specified Group ID. The Pseudowire ID and Interface Parameters fields are not present.
  • Group ID—The Group ID field is a 32-bit arbitrary value that is assigned to a group of pseudowires.
  • Pseudowire ID—The Pseudowire ID, also known as VC ID, is a non-zero, 32-bit identifier that distinguishes one pseudowire from another. To connect two attachment circuits through a pseudowire, you need to associate each one with the same Pseudowire ID.
  • Interface Parameters—The variable-length Interface Parameters field provides attachment circuit-specific information, such as interface MTU, maximum number of concatenated ATM cells, interface description, and so on. Each interface parameter uses a generic TLV encoding, as shown in Figure 6-9.

Table 6-1 Pseudowire Types
Pseudowire Type Description
0x0001 Frame Relay data-link connection identifier (DLCI)
0x0002 ATM AAL5 service data unit (SDU) virtual channel connection (VCC)
0x0003 ATM Transparent Cell
0x0004 Ethernet VLAN
0x0005 Ethernet
0x0006 High-Level Data Link Control (HDLC)
0x0007 PPP

Figure 6-9 Interface Parameter Encoding

Even though LDP allows multiple FEC elements encoded into an FEC TLV, only one FEC element—the Pseudowire ID FEC element—exists in each FEC TLV for the pseudowire emulation over MPLS application.

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