ATM is a high-speed networking technology that handles data in fixed-size units called cells. It enables high-speed communication between edge routers and core routers in an ATM network.

ATM Interfaces

An ATM port can have a major interface and one or more subinterfaces. An ATM subinterface is a mechanism that enables a single physical ATM interface to support multiple logical interfaces. Several logical interfaces can be associated with a single physical interface.

ATM subinterfaces meet the specifications in RFC 2684—Multiprotocol Encapsulation over ATM Adaptation Layer 5 (September 1999), which replaces RFC 1483. All references to ATM subinterfaces in this chapter are still to ATM 1483 subinterfaces.

ATM 1483 subinterfaces are identified by user-defined numbers. To select a subinterface, you append a subinterface number to the port-level interface atm command.

When you create an ATM 1483 subinterface, you must configure a permanent virtual circuit (PVC). Protocols such as ATM require one or more virtual circuits over which data traffic is transmitted to higher layers in the protocol stack.

The ATM PVC on an ATM subinterface with an assigned IP address is reenabled after you reload the slot, or disable and reenable the slot, on which the ATM physical interface is configured.

Figure 1 shows a typical point-to-point ATM interface column.

Figure 1: ATM Interface Column

ATM Interface Column

ATM Physical Connections

ATM interfaces and subinterfaces support two types of connections—point-to-point and multipoint. The router defaults to point-to-point.

Depending on the type of connection you choose, you can specify one or more PVCs on each interface. For a standard point-to-point ATM interface, you configure only one PVC. For NBMA ATM connections, you configure multiple circuits.

ATM Virtual Connections

A virtual connection (VC) defines a logical networking path between two endpoints in an ATM network. ATM cells travel from one point to the other over a virtual connection. An ATM cell is a package of information that is always 53 bytes in length, unlike a frame or packet, which has a variable length. An ATM cell has a cell header and a payload. The payload contains the user data.

The cell header includes an 8-bit virtual path identifier (VPI) and a 16-bit virtual channel identifier (VCI).

An ATM network can have two types of VCs, depending on the addressing used to switch the traffic:

Virtual Channel Connection

A VCC uses all the addressing bits of the cell header to move traffic from one link to another. The VCC is formed by joining a series of virtual channels (VCs), which are logical circuits uniquely identified for each link of the network. On a VCC, switching is done based on the combined VPI and VCI values.

Virtual Path Connection

A VPC uses the higher-order addressing bits of the cell header to move traffic from one link to another. A VPC carries many VCCs within it. A VPC can be set up permanently between two points, and then switched.

VCCs can be assigned within the VPC easily and quickly. The VPC is formed by joining a series of virtual paths, which are the logical groups of circuits uniquely defined for each link of the network. On a VPC, switching is done based on the VPI value only.


JunosE Software does not support configuration and monitoring of ATM switched virtual circuits (SVCs) on the router.

ATM Adaptation Layer

The ATM Adaptation Layer (AAL) defines the conversion of user information into cells by segmenting upper-layer information into cells at the transmitter and reassembling them at the receiver. AAL1 and AAL2 handle intermittent traffic, such as voice and video, and are not relevant to the router. AAL3/4 and AAL5 support data communications by segmenting and reassembling packets.

E Series routers support the following AAL5 encapsulation types as specified in RFC 2684—Multiprotocol Encapsulation over ATM Adaptation Layer 5 (September 1999), which replaces RFC 1483:

Local ATM Passthrough

E Series routers support local ATM passthrough for ATM layer 2 services over Multiprotocol Label Switching (MPLS). Local ATM passthrough enables the router to emulate packet-based ATM switching. The ATM passthrough feature is useful for customers who run IP in most of their network but still have to carry a small amount of native ATM traffic.

Local ATM passthrough uses ATM Martini encapsulation to emulate ATM switch behavior. You can create pairs of cross-connected ATM VCs within the router. The router then passes AAL5 traffic between two VCs, regardless of the contents of the packets.

You can also use AAL0 encapsulation when you configure a local ATM passthrough connection. AAL0 encapsulation causes the router to receive raw ATM cells on this circuit and to forward the cells without performing AAL5 packet reassembly.

For more information, see Configuring Layer 2 Services over MPLS in JunosE BGP and MPLS Configuration Guide.

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 is useful for voice-over-ATM applications that use AAL2-encapsulated voice transmission.

VCC cell relay encapsulation enables the 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 passthrough connection between two ATM 1483 subinterfaces on the same router. The E Series implementation conforms to the required N-to-1 cell mode encapsulation method described in the Martini draft, Encapsulation Methods for Transport of ATM Over MPLS Networks—draft-ietf-pwe3-atm-encap-07.txt (April 2005 expiration), with the provision that only a single ATM virtual circuit (VC) can be mapped to an MPLS tunnel.

For more information, see Configuring Layer 2 Services over MPLS in JunosE BGP and MPLS Configuration Guide.

Note: The E120 and E320 routers do not support ATM over MPLS with VCC cell relay encapsulation in the current release.

Traffic Management

The scheduling priority for traffic classes depends on the type of router that you have. Table 3 describes the scheduling priorities for each type of router.

Table 3: Scheduling Priorities for Traffic Classes

Scheduling Priority (from Highest to Lowest)

ERX7xx Models, ERX14xx Models, or the ERX310 Broadband Services Router

E120 and E320 routers


The following traffic classes are prioritized equally:

  • CBR
  • VBR-RT



The following traffic classes are prioritized equally:

  • UBR with a peak cell rate (PCR)



UBR without PCR



UBR with or without PCR

The level of support for traffic management depends on the specific I/O module or IOA. See Supported Features.

Connection Admission Control

ATM networks use connection admission control (CAC) to determine whether to accept a connection request, based on whether allocating the connection’s requested bandwidth causes the network to violate the traffic contracts of existing connections. CAC is a set of actions that the network takes during connection setup or renegotiation.

The router supports CAC on PVCs on major ATM interfaces. This implementation of CAC determines available bandwidth based on port subscription bandwidth. The router maintains available bandwidth for each major ATM port. Bandwidth for VP tunnels is included in CAC computations.

Table 4 lists the traffic parameter that the router uses for each service category to compute the bandwidth that the connection requires. For example, the peak cell rate is used to calculate how much bandwidth is required for CBR connections.

Table 4: Traffic Parameters Used to Compute Bandwidth

Service Category

Traffic Parameter Used to Calculate Required Bandwidth








UBR bandwidth configured on the ATM major interface

UBR with PCR

UBR bandwidth configured on the ATM major interface

How CAC Works

With no connections, the available bandwidth is equal to the subscription port bandwidth. While connections are requested, the required bandwidth, which is based on the service category and traffic parameters of the connection, is compared against the available port bandwidth. If sufficient bandwidth is available, the router accepts the connection and updates the available port bandwidth accordingly.

Similarly, when a connection is deleted, the available port bandwidth is updated accordingly.

Configuring CAC

You enable and configure CAC on an ATM major interface using atm cac. When you enable CAC on an ATM interface, you can optionally specify a subscription bandwidth and a UBR weight:

CAC and ATM Bulk Configuration

You cannot configure CAC on an ATM interface on which you have created a bulk-configured virtual circuit (VC) range for use by a dynamic ATM 1483 subinterface. Conversely, you cannot create a bulk-configured VC range on an ATM interface on which you have configured CAC. The router rejects these configurations, which causes them to fail.

If you are upgrading to the current JunosE Software release from a lower-numbered release, configurations that use CAC and bulk configuration on the same ATM interface continue to work. However, we recommend that you disable CAC on these ATM interfaces to ensure continued compatibility with future JunosE releases.

For information about how to use the atm cac command to configure CAC, see Setting Optional Parameters. For information about how to use the atm bulk-config command to create a bulk-configured VC range, see Bulk Configuration of VC Ranges Overview in Configuring Dynamic Interfaces Using Bulk Configuration.


ATM interfaces support the ATM Forum integrated local management interface (ILMI), versions 3.0, 3.1, and 4.0. An important feature of ILMI is the ability to poll or send keepalive messages across the UNI. ATM interfaces always respond to such messages, which are sent by an ATM peer device. Optionally, you can configure ATM major interfaces to generate keepalive messages, a process that enables a continuous ATM-layer connectivity verification; if the ATM peer stops responding to keepalive messages, the router disables the ATM interface.

The ATM interface is not reenabled until the keepalive message’s responses are received (or until the keepalive feature is disabled on the ATM port). To enable ILMI and control the generation of keepalive messages, use the atm ilmi-enable and atm ilmi-keepalive commands.

VPI/VCI Address Ranges

The VPI/VCI address ranges allowed on ATM interfaces are module dependent. Certain modules on ERX14xx models, ERX7xx models, or the Juniper Networks ERX310 Broadband Services Router have a fixed allocation scheme, whereas others have a configurable allocation scheme. In the configurable allocation scheme, a bit range is shared across the VPI and VCI fields.

For example, if an ATM interface has a bit range of 18, and 4 bits are allocated to the VPI space, then 14 bits are left for the VCI space. The resulting numeric range is 0 to 2n-1, where n is the number of bits for each space. Completing the example, if 4 bits were allocated for the VPI space and 14 for the VCI space, the configurable range would be 0 to 15 for VPI and 0 to 16,383 for the VCI space. To configure the bit range, use atm vc-per-vp.

See Supported Features for details on how various line module and I/O modules support configurable VPI/VCI address ranges.

Note: The E120 and E320 routers support the full VPI/VCI address range; therefore, it has a fixed allocation scheme.

VP Tunneling

Virtual path (VP) tunneling enables traffic shaping to be applied to the aggregation of all VCs within a single VP. Thus, VP tunnels can be used to ensure that the total traffic transmitted on a VP does not exceed the specified PCR. VP tunneling uses a round-robin algorithm to guarantee fairness among all of the VCs within the tunnel.

You can change the PCR associated with a tunnel even when VCs have already been configured on the tunnel. The individual VCs within a tunnel must be specified as UBR VCs. In other words, they may not have their own traffic-shaping parameters.

The level of support for VP tunneling is dependent on the specific I/O module. See Supported Features for details.