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Layer 2 VPN Architectures: Understanding Any Transport over MPLS

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: May 12, 2005.


  1. Introducing the Label Distribution Protocol
  2. Understanding AToM Operations
  3. Summary

Chapter Description

This chapter provides an overview of LDP, including LDP components and operations that are related to pseudowire emulation over MPLS along with an explanation of the control signaling and data switching details of AToM.

From the Book

Layer 2 VPN Architectures

Layer 2 VPN Architectures


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 8

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



Frame Relay data-link connection identifier (DLCI)


ATM AAL5 service data unit (SDU) virtual channel connection (VCC)


ATM Transparent Cell


Ethernet VLAN




High-Level Data Link Control (HDLC)



Figure 9

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.

Establishing AToM Pseudowires

Typically, two types of LDP sessions are involved in establishing AToM pseudowires. They are the nontargeted LDP session and the targeted LDP session.

The nontargeted LDP session that is established through LDP basic discovery between a PE router and its directly connected P routers is used to distribute tunnel labels. The label distri-bution and management of tunnel labels pertains to the deployment model of the underlying MPLS network. It can be some combination of downstream on-demand or unsolicited label advertisement, independent or ordered control, and conservative or liberal label retention. Neither pseudowire emulation nor AToM dictates any particular label distribution and management mode for tunnel labels.

The other type of LDP sessions are established through LDP extended discovery between PE routers. These sessions are known as targeted LDP sessions because they send periodic Tar-geted Hello messages to each other. Targeted LDP sessions in the context of pseudowire emulation distribute pseudowire labels. IETF documents on pseudowire emulation over MPLS specify the use of downstream unsolicited label advertisement. In Cisco IOS Software, AToM uses independent label control and liberal label retention to improve performance and convergence time on pseudowire signaling.

Figure 6-10 illustrates an example of AToM deployment.

Figure 10

Figure 6-10 AToM Deployment Model

The following steps explain the procedures of establishing an AToM pseudowire:

  1. A pseudowire is provisioned with an attachment circuit on PE1.

  2. PE1 initiates a targeted LDP session to PE2 if none already exists. Both PE routers receive LDP Keepalive messages from each other and complete the session establishment. They are ready to exchange pseudowire label bindings.

  3. When the attachment circuit state on PE1 transitions to up, PE1 allocates a local pseudo-wire label corresponding to the pseudowire ID that is provisioned for the pseudowire.

  4. PE1 encodes the local pseudowire label into the Label TLV and the pseudowire ID into the FEC TLV. Then it sends this label binding to PE2 in a Label Mapping message.

  5. PE1 receives a Label Mapping message from PE2 and decodes the pseudowire label and pseudowire ID from the Label TLV and FEC TLV.

  6. PE2 performs Steps 1 through 5 independently.

  7. After PE1 and PE2 exchange the pseudowire labels and validate interface parameters for a particular pseudowire ID, the pseudowire with that pseudowire ID is considered estab-lished.

If one attachment circuit on one PE router goes down, a Label Withdraw message is sent to the peering PE router to withdraw the pseudowire label that it previously advertised.

Control Word Negotiation

During pseudowire establishment, Label Mapping messages are sent in both directions. To enable the pseudowire, you need to set some interface parameters to certain values that the peering PE router expects. When a mismatch occurs, fixing the problem requires manual intervention or configuration changes. The protocol cannot correct the mismatch automatically. For example, when the interface MTUs of the peering PE routers are different, the pseudowire is not established.

You can negotiate the presence of the control word through protocol signaling. The control word has 32 bits, as shown in Figure 6-11. If it is present, the control word is encapsulated in every pseudowire packet and carries per-packet information, such as sequence number, padding length, and control flags.

Figure 11

Figure 6-11 AToM Control Word

For certain Layer 2 payload types that are carried over pseudowires, such as Frame Relay DLCI and ATM AAL5, the control word must be present in the pseudowire encapsulation. That means you must set the C-bit in the pseudowire ID FEC element to 1 in both Label Mapping messages. When you receive a Label Mapping message that requires the mandatory control word but has a C-bit of 0, a Label Release message is sent with an Illegal C-bit status code. In this case, the pseudowire is not enabled.

For other Layer 2 payload types, the control word is optional. If a PE router cannot send and receive the optional control word, or if it is capable of doing that but prefers not to do so, the C-bit in the Label Mapping message that the PE router sends is set to 0. If a PE router is capable of and prefers sending and receiving the optional control word, the C-bit in the Label Mapping message it sends is set to 1. When two PE routers exchange Label Mapping messages, one of the following scenarios could happen when the control word is optional:

  • Both C-bits are set to the same value—that is, either 0 or 1. In this case, the pseudowire establishment is complete. The control word is used if the common C-bit value is 1. Otherwise, the control word is not used.

  • A PE router receives a Label Mapping message but has not sent a Label Mapping message for the pseudowire, and the local C-bit setting is different from the remote C-bit setting. If the received Label Mapping message has the C-bit set to 1, in this case, the PE router ignores the received Label Mapping message and continues to wait for the next Label message for the pseudowire. If the received Label Mapping message has the C-bit set to 0, the PE router changes the local C-bit setting to 0 for the Label Mapping message to be sent. If the attachment circuit comes up, the PE router sends a Label Mapping message with the latest local C-bit setting.

  • A PE router has already sent a Label Mapping message, and it receives a Label Mapping message from a remote PE router. However, the local C-bit setting is different from the remote C-bit setting. If the received Label Mapping message has the C-bit set to 1, in this case, the PE router ignores the received Label Mapping message and continues to wait for the next label message for the pseudowire. If the received Label Mapping message has the C-bit set to 0, the PE router sends a Label Withdraw message with a Wrong C-bit status code, followed by a Label Mapping message with the C-bit set to 0. The pseudowire establishment is now complete, and the control word is not used.

To summarize the previous two scenarios, when the C-bit settings in the two Label Mapping messages do not match, the PE router that prefers the use of the option control word surrenders to the PE router that does not prefer it, and the control word is not used.

Configuring whether the control word is to be used in an environment with many different platforms is sometimes a tedious process. AToM automates this task by detecting the hardware capability of the PE router. AToM always prefers the presence of the control word and utilizes the control word negotiation procedures to reach a common C-bit value between PE routers.

Using Sequence Numbers

Because Layer 2 packets are normally transported over Layer 1 physical media directly, most Layer 2 protocols assume that the underlying transport ensures in-order packet delivery. These protocols might not function correctly if out-of-order delivery occurs. For instance, if PPP LCP packets are reordered, the end-to-end PPP connection is unable to establish.

To avoid out-of-order packets, the best solution is to engineer a reordering-free packet network. Even though this goal is not always easy to achieve, you should make it a priority because no matter what kind of remedy you might use, network performance suffers significantly from out-of-order delivery.

Sequencing that is defined in pseudowire emulation mainly serves a detection mechanism for network operators to troubleshoot occasional out-of-order delivery problems. Implementations might choose to either discard or reorder out-of-order packets when they are detected. Because the latter requires huge packet buffer space for high-speed links and has significant performance overhead, AToM simply discards out-of-order packets and relies on the upper layer to retransmit these packets.

The first step in using sequencing is to signal the presence of the control word, as described in the previous section. The control word contains a 16-bit Sequence Number field. However, the presence of the control word does not mandate sequencing. When sequencing is not used, Sequence Number value is set to 0.

After negotiating the control word, the sequence number is set to 1 and increments by 1 for each subsequent packet that is being transmitted. If the transmitting sequence number reaches the maximum value 65535, it wraps around to 1 again.

To detect an out-of-order packet, the receiving PE router calculates the expected sequence number for the next packet by using the last receiving sequence number (which has an initial value of 0) plus 1, and then mod (modulus) by 216 (216 = 65536). If the result is 0, the expected sequence number is set to 1. A packet that is received over a pseudowire is considered in-order if one of the following conditions is met:

  • The receiving sequence number is 0.

  • The receiving sequence number is no less than the expected sequence number and the result of the receiving sequence number minus the expected sequence number is less than 32768.

  • The receiving sequence number is less than the expected sequence number and the result of the expected sequence number minus the receiving sequence number is no less than 32768.

If none of these conditions is satisfied, the packet is considered out-of-order and is discarded.

Sometimes the sending or the receiving PE router might lose the last transmitting or receiving sequence number because of transient system problems. This router might want to restart the sequence number from the initial value. AToM implements a set of signaling procedures to reliably resynchronize the sequence number. Although the IETF documents do not specify these procedures, the procedures are interoperable with any standard-compliant implementation. The resynchronization procedures in AToM are as follows:

  • If the transmitting PE router needs to reset the transmitting sequence number, it must inform the receiving PE router to reset the receiving sequence number. AToM accomplishes this by letting the transmitting PE router send a Label Release message to the receiving PE router, followed by a Label Request message. Because the receiving PE router interprets this as a pseudowire flapping, it resets the receiving sequence number.

  • If the receiving PE router needs to reset the receiving sequence number, it must inform the receiving PE router to reset the transmitting sequence number. AToM does so by letting the receiving PE router send a Label Withdraw message to the transmitting PE router, followed by a Label Mapping message. Because the transmitting PE router perceives this as a pseudowire flapping, it resets the transmitting sequence number.

Pseudowire Encapsulation

To properly emulate Layer 2 protocols over pseudowires, you need to encapsulate each Layer 2 payload in such a way that Layer 2 characteristics are preserved as close to what they are in the native form as possible.

Aside from the MPLS label stack, pseudowire encapsulation also contains payload-specific information that varies on a per-transport and per-packet basis. This section discusses the payload-specific part of the encapsulation, which includes the control word and the Layer 2 payload.

The next few sections explain how the following Layer 2 protocols are encapsulated and processed on PE routers:

  • ATM

  • Frame Relay

  • HDLC

  • PPP

  • Ethernet


AToM supports two types of encapsulation for ATM transport: ATM AAL5 common part convergence sublayer service data unit (CPCS-SDU) and ATM Cell.

The ATM AAL5 CPCS-SDU encapsulation includes a mandatory control word. The ATM AAL5 CPCS-SDU encapsulation requires segmentation and reassembly (SAR) on the CE-PE ATM interface. When an ingress PE router receives ATM cells from a CE router, it reassembles them into an AAL5 CPCS-SDU and copies ATM control flags from the cell header into the control word before sending it over a pseudowire. The AAL5 CPCS-SDU is segmented into ATM cells with proper cell headers on the egress PE router. Figure 6-12 illustrates the AAL5 CPCS-SDU pseudowire encapsulation.

Figure 12

Figure 6-12 AAL5 CPCS-SDU Pseudowire Encapsulation

The control flags in the control word are described as follows:

  • Transport Type (T-Bit)—This bit indicates whether the packet contains an ATM Operation, Administration, and Maintenance (OAM) cell or an AAL5 CPCS-SDU. If T = 1, the packet contains an ATM OAM cell. Otherwise, it contains an AAL5 CPCS-SDU. Being able to transport an ATM OAM cell in the AAL5 mode provides a way to enable administrative functionality over AAL5 VC.

  • EFCI (E-Bit)—The E-bit stores the value of the EFCI bit of the last cell to be reassembled when the payload contains an AAL5 CPCS-SDU or that of the ATM OAM cell when the payload is an ATM OAM cell on the ingress PE router. The egress PE router then sets the EFCI bit of all cells to the value of the E-bit.

  • CLP (C-Bit)—This is set to 1 if the CLP bit of any cell is set to 1 regardless of whether the cell is part of an AAL5 CPCS-SDU or is an ATM OAM cell on the ingress PE router. The egress PE router sets the CLP bit of all cells to the value of the C-bit.

  • Command/Response Field (U-Bit)—When FRF.8.1 Frame Relay/ATM PVC Service Interworking traffic is being transmitted, the CPCS-UU Least Significant Bit of the AAL5 CPCS-SDU might contain the Frame Relay C/R bit. This flag carries that bit from the ingress PE router to the egress PE router.

With the ATM Cell encapsulation, ATM cells are transported individually without SAR. The ATM Cell encapsulation consists of the optional control word and one or more ATM cells. Each ATM cell has a 4-byte ATM cell header and a 48-byte ATM cell payload. Figure 6-13 illustrates the ATM cell pseudowire encapsulation.

Figure 13

Figure 6-13 ATM Cell Pseudowire Encapsulation

The maximum number of ATM cells that an ingress PE router can fit into a single pseudowire packet is constrained by the network MTU and the number of ATM cells that the egress PE router is willing to receive. This is signaled to the ingress PE router through the interface parameter "maximum number of concatenated ATM cells" in the Label Mapping message.

Frame Relay

Frame Relay DLCIs are locally significant, and it is likely that two Frame Relay attachment circuits that are connected through a pseudowire have different DLCIs. Therefore, you do not need to include DLCI as part of the Frame Relay pseudowire encapsulation. The control word is mandatory. Control flags in the Frame Relay header are mapped to the corresponding flag fields in the control word. Frame Relay payloads that are carried over pseudowires do not include the Frame Relay header or the FCS. Figure 6-14 illustrates the Frame Relay pseudowire encapsulation.

Figure 14

Figure 6-14 Frame Relay Pseudowire Encapsulation

The Frame Relay control flags in the control word are described as follows:

  • Backward Explicit Congestion Notification (B-Bit)—The ingress PE router copies the BECN field of an incoming Frame Relay packet into the B-bit. The B-bit value is copied to the BECN field of the outgoing Frame Relay packet on the egress PE router.

  • Forward Explicit Congestion Notification (F-Bit)—The ingress PE router copies the FECN field of an incoming Frame Relay packet into the F-bit. The F-bit value is copied to the FECN field of the outgoing Frame Relay packet on the egress PE router.

  • Discard Eligibility (D-Bit)—The ingress PE router copies the DE field of an incoming Frame Relay packet into the D-bit. The D-bit value is copied to the DE field of the outgoing Frame Relay packet on the egress PE router.

  • Command/Response (C-Bit)—The ingress PE router copies the C/R field of an incoming Frame Relay packet into the C-bit. The C-bit value is copied to the C/R field of the outgoing Frame Relay packet on the egress PE router.


HDLC mode provides port-to-port transport of HDLC encapsulated frames. The pseudowire HDLC encapsulation consists of the optional control word, HDLC address, control and pro-tocol fields without HDLC flags, and the FCS.

You can also use the HDLC mode to transport Frame Relay User-to-Network Interface (UNI) or Network-to-Network Interface (NNI) traffic port-to-port transparently because they use HDLC framing.


PPP mode provides port-to-port transport of PPP encapsulated frames. The PPP pseudowire encapsulation consists of the optional control word and the protocol field without media-specific framing information, such as HDLC address and control fields or FCS.

When you enable the Protocol Field Compression (PFC) option in PPP, the Protocol field is compressed from two octets into a single octet. PFC occurs between CE routers and is trans-parent to PE routers. PE routers transmit the protocol field in its entirety as it is received from CE routers.

If the CE-PE interface uses HDLC-like framing, the ingress PE router always strips off HDLC address and control fields from the PPP frames before transporting them over pseudowires. Perhaps two CE routers negotiate Address and Control Field Compression (ACFC). The egress PE router has no way of knowing that unless it snoops into the PPP LCP negotiation between the CE routers, and that is normally undesirable because of system complexities and perfor-mance overhead. Therefore, the egress PE router cannot determine whether it should add HDLC address and control fields for PPP frames that are being sent to the CE router.

In Cisco IOS, AToM uses a simple solution to solve this problem without snooping. Basically, the PPP specification says that a PPP implementation that supports HDLC-like framing must prepare to receive PPP frames with uncompressed address and control fields at all times regard-less of ACFC. So with AToM, the egress PE router always adds HDLC address and control fields back to the PPP packet if the egress CE-PE interface uses HDLC-like framing. For interfaces that do not use HDLC-like framing, such as PPP over Ethernet, PPP over Frame Relay, and PPP over ATM AAL5, the egress PE router does not add HDLC address and control fields to the PPP packet.


With the Ethernet pseudowire encapsulation, the preamble and FCS are removed from the Ethernet frames on the ingress PE router before sending them over pseudowires, and they are regenerated on the egress PE router. The control word is optional.

Ethernet pseudowires have two modes of operations:

  • Raw mode—In raw mode, an Ethernet frame might or might not have an IEEE 802.1q VLAN tag. If the frame does have this tag, the tag is not meaningful to both the ingress and egress PE routers.

  • Tagged mode—In tagged mode, each frame must contain an IEEE 802.1q VLAN tag. The tag value is meaningful to both the ingress and egress PE routers.

To explain how ingress and egress PE routers process a VLAN tag, it is necessary to define the semantics for the VLAN tag first. For example, when the ingress PE receives an Ethernet frame from a CE router and the frame contains a VLAN tag, there are two possible scenarios:

  • The VLAN tag is a service delimiter. The provider uses a service delimiter to distinguish one type of customer traffic from another. For example, each service-delimiting VLAN tag can represent a different customer who the provider is serving or a particular network service that the provider wants to offer. Some equipment that the provider operates usually places this VLAN tag onto the Ethernet frame.

  • The VLAN tag is not a service delimiter. A CE router or some equipment that the customer operates usually places this VLAN tag. The VLAN tag is not meaningful to the ingress PE router.

If an Ethernet pseudowire operates in raw mode, a service-delimiting VLAN tag, if present, is removed from the Ethernet frame that is received from a CE router before the frame is sent over the pseudowire. If the VLAN tag is not a service delimiter, it is passed across the pseudowire transparently.

If an Ethernet pseudowire operates in tagged mode, each Ethernet frame that is sent over the pseudowire must have a VLAN tag, regardless of whether it is a service-delimiting VLAN tag.

In both modes, the service-delimiting VLAN tags have only local significance. That is, these tags are meaningful only at a particular CE-PE interface. When the egress PE router receives an Ethernet frame from the pseudowire, it references the operation mode and its local configu-ration to determine how to process this frame before transmitting it to the CE router. If the egress PE is using raw mode, it might add a service-delimiting VLAN tag, but it will not rewrite or remove a VLAN tag that is already present in the frame. If the egress PE is using tagged mode, it can rewrite, remove, or keep the VLAN tag that is present in the frame.

In Metro Ethernet deployment, in which CE routers and PE routers are connected through an Ethernet switched access network, packets that arrive at PE routers can contain two IEEE 802.1q VLAN tags. This type of packet is commonly known as a QinQ packet. When the outer VLAN tag is the service-delimiting VLAN tag, QinQ packets are processed exactly like the ones with a single VLAN tag in both raw mode and tagged mode. When the combination of the outer and inner VLAN tags is used for service-delimiting, it is processed as if it were a single VLAN tag but with an extended range of values.

If you need to take QoS into consideration, the ingress PE router can map the user priority bits in the VLAN header to the MPLS EXP bits in the MPLS label stack. In this way, transit LSRs in the MPLS network can apply QoS policies to the Ethernet frames that are carried over pseudowires.

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We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020