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Configuring Layer 2 Circuit Transport Mode

 

On ATM2 IQ interfaces only, you can configure Layer 2 circuit cell-relay, Layer 2 circuit AAL5, or Layer 2 circuit trunk transport mode.

Layer 2 circuit cell-relay and Layer 2 circuit AAL5 are defined in Internet draft draft-martini-l2circuit-encap-mpls-07.txt, Encapsulation Methods for Transport of Layer 2 Frames Over IP and MPLS Networks (expires December 2004).

Layer 2 circuit cell-relay and Layer 2 circuit AAL5 transport modes allow you to send ATM cells between ATM2 IQ interfaces across a Layer 2 circuit-enabled network. Layer 2 circuits are designed to transport Layer 2 frames between PE routers across an LDP-signaled MPLS backbone. You use Layer 2 circuit AAL5 transport mode to send AAL5 segmentation and reassembly protocol data units (SAR-PDUs) over the Layer 2 circuit.

A trunk is a collection of ATM VPs. Layer 2 circuit trunk transport mode allows you to send ATM cells over MPLS trunking.

By default, ATM2 IQ PICs are in standard AAL5 transport mode. Standard AAL5 allows multiple applications to tunnel the protocol data units of their Layer 2 protocols over an ATM virtual circuit. Encapsulation of these Layer 2 protocol data units allows a number of these emulated virtual circuits to be carried in a single tunnel. Protocol data units are segmented at one end of the tunnel and reassembled at the other end. The ingress router reassembles the protocol data units received from the incoming VC and transports each PDU as a single packet.

In contrast, Layer 2 circuit cell-relay and Layer 2 circuit AAL5 transport modes accept a stream of ATM cells, convert these to an encapsulated Layer 2 format, then tunnel them over an MPLS or IP backbone, where a similarly configured router segments these packets back into a stream of ATM cells, to be forwarded to the virtual circuit configured for the far-end router.

In Layer 2 circuit cell-relay transport mode, ATM cells are bundled together and transported in packet form to the far-end router, where they are segmented back into individual ATM cells and forwarded to the ATM virtual circuit configured for the far-end router.

Note

When you configure the cell-bundle-size statement at the [edit interfaces at -fpc/pic/port atm-options] hierarchy level is 1 and the atm-ccc-cell-relay trunk statement is included at the [edit interfaces interface-name encapsulation] hierarchy level, ATM cells are not bundled. Each ATM cell is forwarded as a single MPLS packet.

The uses for the four transport modes are defined as follows:

  • To tunnel IP packets over an ATM backbone, use the default standard AAL5 transport mode.

  • To tunnel a stream of AAL5-encoded ATM SAR-PDUs over an MPLS or IP backbone, use Layer 2 circuit AAL5 transport mode.

  • To tunnel a stream of ATM cells over an MPLS or IP backbone, use Layer 2 circuit cell-relay transport mode.

  • To transport ATM cells over an MPLS core network that is implemented between other vendors’ switches or routers, use Layer 2 circuit trunk transport mode.

    Note

    You can transport AAL5-encoded traffic with Layer 2 circuit cell-relay transport mode, because Layer 2 circuit cell-relay transport mode ignores the encoding of the cell data presented to the ingress interface.

    When you configure AAL5 mode Layer 2 circuits, the control word carries cell loss priority (CLP) information by default.

    The Layer 2 circuit trunk transport mode is not supported on the ATM2 IQ OC48c/STM16 PIC.

To configure Layer 2 circuit AAL5, Layer 2 circuit cell-relay, or Layer 2 circuit trunk mode, you must perform the following tasks:

  1. Identify the interface as an ATM2 IQ interface by including the pic-type atm2 statement at the [edit interfaces at-fpc/pic/port atm-options] hierarchy level:
  2. Include the atm-l2circuit-mode statement at the [edit chassis fpc slot-number pic pic-number] hierarchy level, specifying aal5, cell, or trunk:

    By default, the trunk mode uses user-to-network interface (UNI) mode. The trunk option can be UNI or network-to-network interface (NNI). For more information about UNI and NNI, see the Junos OS VPNs Library for Routing Devices.

    Transport mode is per PIC, not per port. If you do not include the atm-l2circuit-mode statement in the configuration, the ATM2 IQ PIC uses standard AAL5 transport mode. If you configure Layer 2 circuit cell-relay, Layer 2 circuit AAL5 transport mode, or Layer 2 circuit trunk mode, the entire ATM2 PIC uses the configured transport mode.

  3. For Layer 2 circuit trunk mode only, you must also configure a trunk identification number by including the trunk-id statement:

    You can include this statement at the following hierarchy levels:

    • [edit interfaces interface-name unit logical-unit-number]

    • [edit logical-systemslogical-system-name interfaces interface-name unit logical-unit-number]

    The trunk identification number can be from 0 through 31; each trunk on an interface must have a unique trunk ID. When you associate a trunk ID number with a logical interface, you are in effect specifying the interfaces that are allowed to send ATM traffic over an LSP. For UNI mode, the trunk ID range is from 0 through 7. For NNI mode, the trunk ID range is from 0 through 31. Trunk IDs on connecting trunks do not need to be the same.

    For information about proportional bandwidth sharing in trunk mode, see Configuring Layer 2 Circuit Trunk Mode Scheduling Overview.

    For example:

    Configuring Layer 2 Circuit Trunk Transport Mode

    In Figure 1, Router A is a local PE router. Router B is a remote PE router. Both Juniper Networks routers have Layer 2 circuit cell-relay capability. You configure an ATM physical interface on Router A in Layer 2 circuit trunk mode and specify trunks that are allowed to send traffic over the LSP. As a cell is received on this interface, it is classified using the CoS bits in the cell header, and encapsulated as a labeled packet. It is then queued on one of the outgoing queues according to its classification and sent over the LSP to Router B. At Router B, the packet label is removed and the raw cell is put on one of the queues of the ATM interface and forwarded to the second ATM switch. To carry the CoS information and CLP of the cell over the network, the CoS and CLP bits are copied into the EXP bits of the MPLS label. This CoS information is used to select the output queues. Using EPD profiles, the CLP is used to determine whether the cell should be dropped.

    For more information about ATM CoS capability, see ATM2 IQ VC Tunnel CoS Components Overview.

    Figure 1: Layer 2 Circuit Trunk Topology
    Layer 2 Circuit Trunk Topology

    On Router A

    On Router B

  4. For Layer 2 circuit AAL5 mode, configure logical interface encapsulation by including the encapsulation statement, specifying the atm-ccc-vc-mux encapsulation type:

    You can include this statement at the following hierarchy levels:

    • [edit interfaces interface-name unit logical-unit-number]

    • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

    For example:

    Configuring Layer 2 Circuit AAL5 Transport Mode

  5. For Layer 2 circuit cell-relay and Layer 2 circuit trunk modes, configure physical interface encapsulation by including the encapsulation statement at the [edit interfaces interface-name] hierarchy level, specifying the atm-ccc-cell-relay encapsulation type:

    You can also include the encapsulation atm-ccc-cell-relay statement at the [edit interface interface-name unit logical-unit-number] hierarchy level. When you use the configuration given in the preceding steps,, keep the following points in mind:

    • This configuration interoperates between Juniper routers running Junos OS Release 8.2 or earlier.

    • This configuration does not interoperate with other network equipment, including a Juniper router running Junos OS Release 8.3 or later.

    • For a Juniper router running Junos OS Relase 8.3 or later to interoperate with another Juniper router running Junos OS Release 8.2 or earlier, include the use-null-cw statement at the [edit interfaces interface-name atm-options] hierarchy level on the router running Junos OS Release 8.3 or later.

    • The use-null-cw statement inserts (for sending traffic) or strips (for receiving traffic) an extra null control word in the MPLS packet.

    • The use-null-cw statement is not supported on a router running Junos OS Release 8.2 or earlier.

    For more information about Layer 2 circuits, see the Junos OS VPNs Library for Routing Devices and the Junos OS Routing Protocols Library.

    For example:

    Configuring Layer 2 Circuit Cell-Relay Transport Mode