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 117.

Figure 117: 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 118. The two routers pass traffic between two interfaces through an MPLS tunnel using Martini encapsulation, regardless of the contents of the packets.

Figure 118: 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:

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 119, 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 119: 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:

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.


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

Control Word Support

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

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:

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:

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).

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