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    Example: Multiple ATM Virtual Circuits over a Single Pseudowire

    Figure 1 shows a Martini circuit deployment in which the CE-side devices on either side of the network send and receive ATM cells. An MPLS tunnel that connects two E Series provider edge routers, PE1 and PE2, and ATM cross-connects provide a pseudowire between the ATM VCs on the two routers. PE2 is the remote router located at the other side of the service provider core. A customer edge ATM switch, CE1, and the provider edge router, PE1, on one side of the core are connected by an ATM port. Similarly, PE2 and CE2 are connected by an ATM port. The necessary MPLS Martini circuit configuration and VPI/VCI range configuration are added to the ATM ports on PE1 and PE2. The LDP signaling protocol performed the signaling operation and set up the pseudowire (a pair of unidirectional LSPs, from PE1 to PE2 and from PE2 to PE1) for transporting the ATM cells between CE1 and CE2.

    Figure 1: Martini Circuit Deployment for Transmission of Multiple ATM VCs over a Single Pseudowire

    Martini
Circuit Deployment for Transmission of Multiple ATM VCs over a Single
Pseudowire

    CE1 transmits ATM cells on the ATM port connected to PE1. The transmitted cells contain VPI/VCI values that are within the range specified as part of the MPLS Martini configuration on the ATM port of PE1. If cell concatenation is configured on that ATM port of PE1, PE1 accumulates the received ATM cells. If cell concatenation is not specified, cell concatenation count is reached, or the concatenation timer expired, PE1 encapsulates the ATM cells with a control word, adds the pseudowire label and transport label, and forwards the resulting MPLS labeled packet to be delivered to PE2.

    PE2 receives the MPLS-labeled packet and strips the labels on the packet. The bottommost label indicates to PE2 that the packet contains ATM cells to be forwarded on the ATM port connected to CE2. PE2 extracts the ATM cells from the packet, checks whether the VPI/VCI value on the ATM cells fall within the configured VPI/VCI range on the ATM port, and forwards those ATM cells whose VPI/VCI values fall within the configured range to CE2. PE2 discards the ATM cells that do not fall within the configured VPI/VCI range. For data traffic traversing from CE2 to CE1, the same workflow is followed with the roles reserved—PE2 as the transmitter of MPLS labeled packet and PE2 as the receiver of MPLS labeled packet.

    Note: The support for multiple VCs over a single pseudowire uses the ATM n-to-one VCC cell transport (0x0009) pseudowire (PW) type in LDP signaling messages. This PW type is also used in the signaling messages by the ATM Martini circuit feature, which enables cell relay encapsulation on an ATM subinterface, that transports ATM cells associated with an ATM subinterface on a single pseudowire.

    Because the same PW type is used for both the cell relay encapsulation for a single VC and multiple VCs, you can successfully configure an ATM subinterface Martini circuit (cell relay for a single VC) on one PE router and an ATM port Martini circuit (cell relay for multiple VCs) on another PE router, and bring up the pseudowires for this configuration. In such a scenario, traffic black holes might occur and rewriting of the ATM cell header takes place. In this topology, on the PE router configured with an ATM subinterface Martini circuit, the VPI/VCI values of all the ATM cells received on the pseudowire are rewritten to match with the configuration on the ATM subinterface. On the PE router configured with an ATM port Martini circuit, all the ATM cells received on the pseudowire are examined to determine whether they fall within the configured VPI/VCI range. If the ATM cells do not fall within the configured VPI/VCI range, they are discarded.

    Published: 2014-08-18