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    ATM Layer 2 Services over MPLS Overview

    ATM layer 2 services over MPLS provide ATM switch-like functionality for E Series routers. This feature is useful for customers who run IP in the majority of their network but still have to carry a small amount of non-IP traffic, as in the example shown in Figure 1.

    Figure 1: Common ISP Network

    Common ISP Network

    In this scenario, it is not economical to have special ATM switches in front of E Series routers just to accommodate the small percentage of non-IP traffic. The ATM layer 2 services over MPLS feature lets you replace the ATM traffic selector switch with an E Series router, as shown in Figure 2. The two routers pass traffic between two interfaces through an MPLS tunnel using Martini encapsulation, regardless of the contents of the packets.

    Figure 2: E Series Router Replacing Remote ATM Switch

    E Series Router Replacing Remote
ATM Switch

    ATM layer 2 services over MPLS supports two encapsulation methods on E Series routers:

    • AAL5 relay encapsulation
    • VCC cell relay encapsulation

    The following sections describe each of these encapsulation methods.

    AAL5 Encapsulation

    JunosE Software supports the AAL5 relay method of encapsulation that is specified in the Martini draft. This method is also referred to as AAL5 service data unit (SDU) encapsulation.

    ATM Martini encapsulation emulates ATM switch behavior by creating a pseudowire between pairs of ATM virtual circuits. When the router receives AAL5 packets on one of those circuits, it reassembles them, encapsulates them using Martini encapsulation, and forwards them to an MPLS tunnel. At the end of the tunnel, the packet is de-encapsulated, segmented back, and sent to a selected ATM VC.

    In Figure 3, an MPLS tunnel connects two E Series routers, and ATM cross-connects provide a pseudowire between the ATM VCs on the two routers. All AAL5 packets on the pseudowire are encapsulated. The egress VC does not need the same ATM address as the ingress circuit.

    Figure 3: AAL5 Pseudowire and MPLS Tunnel

    AAL5 Pseudowire and MPLS Tunnel

    To use AAL5 SDU encapsulation, you must use the aal5all encapsulation keyword when you configure ATM subinterfaces.

    OAM Cells

    The E Series router performs a similar operation for Operation, Administration, and Maintenance (OAM) cells, except that they do not need reassembly.

    The router passes the following OAM cells transparently:

    • F5 alarm indication signal (AIS) segment and end-to-end
    • F5 remote defect indication (RDI) segment and end-to-end
    • F5 loopback segment and end-to-end
    • Resource management
    • F5 continuity check segment and end-to-end

    In addition, F4 OAM cell forwarding is supported.

    JunosE Software does not allow for setting a segment endpoint on an ATM cross-connect interface. Segment OAM cells are forwarded to the egress interface in the same manner as end-to-end cells.

    QoS Classification

    Packets are subject to normal quality of service (QoS) classification according to service categories assigned to ATM virtual circuits that make up the connection.

    Limitations

    The JunosE implementation of the Martini draft has the following limitations:

    • Only AAL5 packets and OAM cells are forwarded.
    • There is no equivalent of VP switching.
    • Point-to-multipoint connections are not supported.
    • Automatic connection setup using user-to-network interface (UNI) signaling and private network-to-network interface (PNNI) is not supported.
    • The ATM MIB cross-connected table is not supported.
    • Connections between ATM circuits and non-ATM interfaces are not supported.

    Control Word Support

    Martini draft encapsulation includes a control word with the following fields:

    • T bit for transport type
    • E bit for explicit forward congestion indication (EFCI)
    • C bit for cell loss priority (CLP)=1 indication
    • U bit for command/response indication based on AAL5 common part convergence sublayer user-to-user indication (CPCS-UU)
    • Optional sequence number

    The current JunosE implementation supports the T bit and optional sequence number fields.

    VCC Cell Relay Encapsulation

    E Series routers support virtual channel connection (VCC) cell relay encapsulation for ATM layer 2 services over MPLS. VCC cell relay encapsulation enables a router to emulate ATM switch behavior by forwarding individual ATM cells over an MPLS pseudowire (also referred to as an MPLS tunnel) created between two ATM VCCs, or as part of a local ATM cross connect between two ATM 1483 subinterfaces on the same router. The JunosE implementation conforms to the required N-to-1 cell mode encapsulation method described in the Martini draft, with the provision that only a single ATM virtual circuit (VC) can be mapped to an MPLS pseudowire.

    VCC cell relay encapsulation is useful for voice-over-ATM applications that use ATM Adaptation Layer 2 (AAL2)–encapsulated voice transmission.

    AAL0 Raw Cell Mode

    VCC cell relay encapsulation supports ATM Adaptation Layer 0 (AAL0) encapsulation, also referred to as raw cell mode or null encapsulation. AAL0 is often used to carry signaling ATM cells, which the router treats as raw cells.

    When you create an ATM PVC as part of a VCC cell relay configuration, you must use the aal0 encapsulation.

    Cell Concatenation Parameters

    VCC cell relay encapsulation supports the concatenation (aggregation) of multiple ATM cells in a single encapsulated packet that is transmitted on the MPLS pseudowire.

    You can use the atm cell-packing and atm mcpt-timers commands to configure the following parameters that control how the router performs cell concatenation:

    • Maximum number of ATM cells that the router can concatenate in a single packet.
    • Values (in microseconds) for each of the three ATM Martini cell packing timers maintained on the router. These timers define the time threshold that the router uses to concatenate ATM cells and transmit the cells in an MPLS packet on the pseudowire.
    • Numeric identifier (1, 2, or 3) that indicates which of the three ATM Martini cell packing timers you want to use to detect timeout of the cell collection threshold.

    Based on this configuration, the router attempts to concatenate the specified maximum number of ATM cells into an MPLS packet within the time interval allowed by the ATM Martini cell packing timer you selected. When the timer detects that the allotted time interval has expired, the router forwards the MPLS packet even if it contains fewer than the specified maximum number of aggregated cells per packet.

    Cell Concatenation and Latency

    Cell concatenation increases network latency, which is undesirable for voice-over-ATM applications. To minimize these effects, use care in choosing values for the ATM Martini cell packing timers.

    We recommend that for voice-over-ATM configurations, you select timeout values between 6 microseconds and 3 x 6 microseconds. Values within this range are generally low enough to maintain a reasonable cell delay and high enough to take advantage of the cell concatenation mechanism.

    If traffic shaping is enabled on the egress router, the JunosE implementation of VCC cell relay encapsulation preserves the spacing between cells.

    Control Word Support

    VCC cell relay encapsulation requires use of a control word. The control word contains the T, E, C, and U bits, as well as an optional sequence number field.

    The JunosE implementation of VCC cell relay encapsulation supports the T bit, which is always set to indicate raw ATM cells, and the optional sequence number. The E, C, and U bits have no meaning for VCC cell relay configurations because the router forwards a complete ATM cell with as much header information as possible.

    Unsupported Features

    VCC cell relay encapsulation on JunosE routers does not support the following features:

    • Mapping multiple ATM VCs to a single MPLS pseudowire
    • Optional Martini one-to-one cell encapsulation method with cell headers removed

    For information about AAL5 SDU encapsulation, see Encapsulation Methods for Transport of ATM Over MPLS Networks—draft-ietf-pwe3-atm-encap-07.txt (April 2005 expiration). For information about, VCC cell relay encapsulation, see Encapsulation Methods for Transport of ATM Over MPLS Networks—draft-ietf-pwe3-atm-encap-07.txt (April 2005 expiration).

    Published: 2014-08-18