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Applying Scheduler Maps to ATM Interfaces

To define a scheduler map, you associate it with a forwarding class. Each class is associated with a specific queue, as follows:

  • best-effort—Queue 0

  • expedited-forwarding—Queue 1

  • assured-forwarding—Queue 2

  • network-control—Queue 3


    For M320 and T Series routers only, you can configure more than four forwarding classes and queues. For more information, see Enabling Eight Queues on ATM Interfaces.

When you configure an ATM scheduler map, the Junos OS creates these CoS queues for a VC. The Junos OS prefixes each packet delivered to the VC with the next-hop rewrite data associated with each queue.

To configure an ATM scheduler map, include the scheduler-maps statement at the [edit interfaces at-fpc/pic/port atm-options] hierarchy level:

You can define the following options for each forwarding class:

  • epd-threshold—An EPD threshold provides a queue of cells that can be stored with tail drop. When a beginning-of-packet (BOP) cell is received, the VC’s queue depth is checked against the EPD threshold. If the VC’s queue depth exceeds the EPD threshold, the BOP cell and all subsequent cells in the packet are discarded.

  • linear-red-profile—A linear RED profile defines the number of cells using the queue-depth statement within the RED profile. (You configure the queue-depth statement at the [edit interfaces at-fpc/pic/port atm-options linear-red-profile profile-name] hierarchy level.)

By default, if you include the scheduler-maps statement at the [edit interfaces at-fpc/pic/port atm-options] hierarchy level, the interface uses an EPD threshold that is determined by the Junos OS based on the available bandwidth and other parameters. You can override the default EPD threshold by setting an EPD threshold or a linear RED profile.

If shaping is enabled, the default EPD threshold is proportional to the shaping rate according to the following formula:

The minimum value is 48 cells. If the formula results in an EPD threshold less than 48 cells, the result is ignored, and the minimum value of 48 cells is used.

  • priority—By default, queue 0 is high priority, and the remaining queues are low priority. You can configure high or low queuing priority for each queue.

  • transmit-weight—By default, the transmit weight is 95 percent for queue 0, and 5 percent for queue 3. You can configure the transmission weight in number of cells or percentage. Each CoS queue is serviced in WRR mode. When CoS queues have data to send, they send the number of cells equal to their weight before passing control to the next active CoS queue. This allows proportional bandwidth sharing between multiple CoS queues within a rate-shaped VC tunnel. A CoS queue can send from 1 through 32,000 cells or from 5 through 100 percent of queued traffic before passing control to the next active CoS queue within a VC tunnel.

The AAL5 protocol prohibits cells from being interleaved on a VC; therefore, a complete packet is always sent. If a CoS queue sends more cells than its assigned weight because of the packet boundary, the deficit is carried over to the next time the queue is scheduled to transmit. If the queue is empty after the cells are sent, the deficit is waived, and the queue’s assigned weight is reset.


If you include the scheduler-maps statement at the [edit interfaces at-fpc/pic/port atm-options] hierarchy level, the epd-threshold statement at the [edit interfaces interface-name unit logical-unit-number] or [edit interfaces interface-name unit logical-unit-number address address family family multipoint-destination address] hierarchy level has no effect because either the default EPD threshold, the EPD threshold setting in the forwarding class, or the linear RED profile takes effect instead.