Technical Overview of AToM

A good understanding of AToM is essential to allow fast and efficient troubleshooting. This section, therefore, contains an examination of the operation of AToM.

Note that it is a good idea to read the section entitled "MPLS Architecture" in Chapter 6, "Troubleshooting Multiprotocol Label Switching Layer 3 VPNs," if you do not already have a good understanding of MPLS.

Layer 2 PDU Transport

Layer 2 PDUs are transported over the MPLS backbone between attachment circuits (circuits between PE and CE devices). This transportation occurs by prepending a control word, a VC label, and one or more tunnel labels (assuming PE routers are not in a back-to-back configuration) to the Layer 2 PDU itself. Figure 7-2 illustrates this transport.

The function of the tunnel label is to transport the Layer 2 PDU from the ingress PE router to the egress PE router. This label can be signaled by the Label Distribution Protocol (LDP), the Resource Reservation Protocol (RSVP, if using traffic engineering), or the Tag Distribution Protocol (TDP). Note that LDP is assumed throughout this chapter.

The VC label is a demultiplexer field and serves to identify the correct attachment circuit on the egress PE router. When a Layer 2 PDU arrives at the egress PE router, the PE router examines the VC label and forwards the Layer 2 PDU on the correct attachment circuit. Note that the VC label is locally significant on the egress PE router and is advertised to the ingress PE router using LDP.

The control word carries control information such as sequence numbering (if used), padding, and control bits. For more information, see the section "Control Word."

Finally, the Layer 2 PDU itself is carried in the payload of the packet. Information that is easily replicable by the egress PE router, such as Frame Check Sequence (FCS), is stripped off the Layer 2 PDU before transmission. The exact information that is removed, as well as the information copied into the control word, is dependent on the Layer 2 PDU type.

Figure 7-3 illustrates transport of a Layer 2 PDU (in this example, a Frame Relay PDU) across an AToM pseudowire.

Figure 7-3. Transport of a Frame Relay PDU Across an AToM Pseudowire

[View full size image]

In Figure 7-3, mjlnet_CE1 transmits a Frame Relay PDU to London_PE. London_PE prepends an optional control word, as well as VC and tunnel labels, and forwards the packet to London_P.

Note that London_PE removes extraneous information such as frame header and FCS, and copies appropriate portions of this information, such as Backward Explicit Congestion Notification (BECN) and Forward Explicit Congestion Notification (FECN) flags, into the control word.

London_P swaps the tunnel label and forwards the packet to Paris_P. Note that the VC label remains unchanged. Paris_P (being the penultimate hop) pops the tunnel label and forwards the packet to Paris_PE. Again, the VC label remains unchanged.

Finally, Paris_PE removes the VC label, reconstitutes any information removed by London_PE, and forwards the Frame Relay PDU on the attachment circuit to mjlnet_CE2.

Control Word

As previously mentioned, a control word can be carried as an option when Layer 2 PDUs are transported across the MPLS backbone. Figure 7-4 shows the control word format.

Figure 7-4. Control Word Format

[View full size image]

The contents of the control word are as follows:

• The first four bits in the control word are reserved (Rsvd) and must be set to zero.

• The 4-bit Flags field contains information specific to the Layer 2 protocol being transported.

• Following the Flags are two bits that must be set to zero.

• Next is the 6-bit Length field. If the length of the control word itself plus the Layer 2 PDU payload is less than 64 bytes, this field is set to the length of the packet. If the length of the control word plus the Layer 2 PDU payload is equal to or greater than 64 bytes, this field is set to zero. The function of this field is to allow the egress PE router to remove any padding.

• The final field is the Sequence Number field. This can be used to carry sequence numbering that can allow the egress PE router to ensure ordered packet delivery.

Frame Relay Control Word

Frame Relay PDUs are transported over the AToM pseudowire without their header and FCS. The control word is, therefore, required.

The control word used when transporting Frame Relay PDUs takes the form shown in Figure 7-5.

The Flags field consists of a B (BECN) bit, an F (FECN) bit, a D (Discard Eligible, DE) bit, and a C (Command/Response, C/R) bit.

The ingress PE router can copy the BECN, FECN, DE, and C/R bit settings from the header of the Frame Relay PDU received on the attachment circuit into the control word when transmitting the Frame Relay PDU across the pseudowire.

The egress PE router copies the BECN, FECN, DE, and C/R bit settings from the control word back into the reconstructed Frame Relay header as it transmits the Frame Relay PDU out onto the attachment circuit.

ATM AAL5 CPCS-SDU Control Word

If ATM AAL5 is being used, the ingress PE router reassembles the Common Part Convergence Sublayer-Service Data Units (CPCS-SDUs) received on the attachment circuit. Each CPCS-SDU is then transported over the AToM pseudowire as a single packet, without the AAL5 trailer.

The control word is required and takes the form shown in Figure 7-6.

The Flags field now consists of the following bits:

• The T (transport type) bit is used to indicate whether the packet contains an ATM cell (1) or AAL5 CPCS-SDU (0). Transport of individual ATM cells can be used to enable Operations, Administration, and Maintenance (OAM) over the AAL5 VC.

• The E (Explicit Forward Congestion Indication, EFCI) bit is set by the ingress PE router if the EFCI bit is set in the final cell of the AAL5 CPCS-SDU, or if the EFCI bit is set in any single ATM cell sent over the pseudowire.

• The L (Cell Loss Priority, CLP) bit is set if the CLP bit is set in any of the cells that make up the AAL5 CPCS-SDU, or in any single cell sent over the pseudowire.

• The C (Command / Response) bit may contain the C/R bit when transporting FRF 8.1 (Frame Relay/ATM service internetworking) traffic over the pseudowire.

ATM Cell Relay, Ethernet, HDLC, and PPP Control Words

If ATM cell relay (individual ATM cells), Ethernet, HDLC, or PPP PDUs are being transported, the control may be carried, but all four bits in the control word Flags field are set to zero.

Note that when transporting ATM cells with cell-relay, the entire cell consisting of the 4-byte header and 48-byte payload is transported.

If Ethernet frames are being transported, the entire frame is transmitted, with the exception of the preamble and the FCS. Cisco HDLC frames are transported in their entirety with the exception of the flags (7E) and FCS fields, which are removed. PPP frames are transported in their entirety, with the exception of the flags (7E), HDLC Address, Control, and FCS fields. These are removed.

VC Label Exchange

Before VC labels can be exchanged between peer PE routers, LDP discovery and session establishment must take place.

LDP Discovery

LDP discovery consists of an exchange of hello messages and allows Label Switch Routers (LSRs) to discover each other, forming a hello adjacency. Discovery must be successfully completed before session establishment can begin.

LDP offers two discovery modes:

• Basic discovery This mode is used between directly connected peers.

• Extended discovery Extended discovery is used between non-directly connected peers. Peer PE routers use extended discovery. Figure 7-7 illustrates the extended discovery mode.

  Figure 7-7. LDP Extended Discovery

  [View full size image]
  
  
  

  Note that LDP extended discovery, unlike basic discovery, is asymmetric and uses targeted hello messages with unicast transmission on UDP port 646.

  In Figure 7-7, the LDP discovery takes place between interface loopback 0 (the LDP IDs) of London_PE and Paris_PE.

Note that LDP discovery (and session establishment) between PE routers is initiated on Cisco routers when the xconnect command is configured.

LDP Session Establishment

Once LDP discovery has been successful, session establishment can begin.

Session establishment is a two-stage process:

Note that the method of label distribution used by AToM PE routers is unsolicited downstream.

Figure 7-8 illustrates an LDP session.

Figure 7-8. LDP Session

[View full size image]

Label Mapping Messages

Once peer PE routers have established an LDP session, they can exchange VC label bindings.

VC labels are assigned to local attachment circuits, and the bindings are advertised to the peer (ingress) PE router in an LDP Label Mapping message.

The VC label binding consists of the VC label itself and associated VC information. This VC information is carried within the Label Mapping message using a new type (type 128) of Forwarding Equivalence Class (FEC) element, which is defined in Internet draft draft-martini-l2circuit-trans-mpls.

Figure 7-9 shows the VC FEC element.

The contents of the VC FEC element depicted in Figure 7-9 are as follows:

• The first 8-bit field indicates the FEC element type itself. In this case, it is the VC TLV (Type, Length, Value) type, so a value of 128 is contained in this field.

• The C bit, if set to 1, signals the presence of the control word in packets sent across the pseudowire (see Figure 7-2).

• The VC Type is a 15-bit field that, appropriately, indicates the VC type.

  Table 7-1 shows the possible VC types.

  Table 7-1. Possible VC Types

  VC Type	Description
  0x0001	Frame Relay DLCI
  0x0002	ATM AAL5 VCC (virtual channel connection) transport
  0x0003	ATM transparent cell transport
  0x0004	Ethernet VLAN
  0x0005	Ethernet
  0x0006	HDLC
  0x0007	PPP
  0x8008	CEM (circuit emulation)
  0x0009	ATM VCC cell transport
  0x000A	ATM VPC (virtual path connection) cell transport

  
  

• Next is the 8-bit VC info Length field. This indicates the length of the VC ID and interface parameters fields in octets.

• The 32-bit Group ID field is a port or virtual tunnel index and can be used to group VCs associated with the same interface.

  The Group ID field can be used to withdraw the labels associated with a number of VCs at the same time.

• The 32-bit VC ID field is a globally unique (between PE routers) value that, together with VC type, uniquely identifies a virtual circuit. Note that the VC ID must be a nonzero value.

• The variable length Interface Parameters field is used to specify parameters associated with the attachment circuit interface.

  Figure 7-10 shows the Interface Parameters field.

  Figure 7-10. Interface Parameters Field

  

  
  

The contents of the Interface Parameters field are as follows:

- The 8-bit Parameter ID field is used to specify the interface parameter type.

Table 7-2 shows the possible Parameter ID values.

Table 7-2. Parameter ID values

Parameter	ID Length	Description
0x01	4	Interface MTU in octets
0x02	4	Maximum number of concatenated ATM cells
0x03	up to 82	Optional interface description string
0x04	4	CEM payload bytes
0x05	4	CEM options

- The Length field specifies the length of the interface parameter starting from the Parameter ID field.

- The parameter value itself is contained in the Variable Length Value field.

Note that VC labels are assigned and advertised to peer PE routers when the local attachment circuit changes state to up.

Figure 7-11 illustrates VC label bindings exchange.

Figure 7-11. VC Label Bindings Exchange

[View full size image]

Label Withdraw Messages

If the attachment circuit changes state to down, or there is another service affecting condition, the ingress PE router signals this to its peer using an LDP Label Withdraw message.

The egress PE router receiving a Label Withdraw message can signal the circuit down state to its attached customer device. If the circuit type is Frame Relay, for example, the circuit state may be signaled to the CE device using the Local Management Interface (LMI).

Figure 7-12 illustrates transmission of the Label Withdraw message.

Figure 7-12. Transmission of the Label Withdraw Message

[View full size image]