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JUNOS 10.1 Network Interfaces Configuration Guide
and the physical interface MTU. You can use the following formula to calculate the
maximum lifetime that packets can sustain while transiting a PVC:
For example, if you configure a PVC on an ATM1 interface with the default MTU, a
CBR shaping rate of 3,840,000 bps (10,000 cps), and a queue length of 25 packets.
The maximum lifetime is:
This is the worst-case lifetime assuming all packets in the queue are MTU sized, and
the traffic using the PVC is oversubscribing its configured shaping contract.
In general, we recommend that you use a maximum lifetime under 500 ms.
If you add or change the queue-length setting on the VC, the logical interface
associated with the VC is deleted and re-added.
Configuring the ATM2 IQ EPD Threshold
The EPD threshold is a limit on the number of transmit cells that can be queued.
Cells that exceed the limit are discarded. 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. This prevents a single queue from draining all the buffers on
the PIC.
By default, for UBR the EPD threshold is approximately 1 percent of the available
cell buffers. If shaping is enabled, the default EPD threshold is proportional to the
shaping rate according to the following formula:
By default, the software estimates how much buffer space is needed for each PVC.
However, you can configure the per-VC buffer space. In general, ATM PVCs need
larger buffers for data traffic and smaller buffers for time-sensitive applications.
Unnecessarily deep buffers might cause excessive delays on congested PVCs. Overly
shallow buffers might cause premature random early detection (RED) or tail packet
drops in bursty conditions.
The minimum EPD threshold value is 48 cells. If the default EPD threshold formula
results in an EPD threshold of less than 48 cells, the result will be ignored, and the
minimum value of 48 cells will be used.
To set the EPD threshold of a PVC, include the
For a list of hierarchy levels at which you can include this statement, see
epd-threshold.
The allowable range for EPD threshold varies by interface type, as shown in Table
29 on page 335.
334
Configuring the ATM2 IQ EPD Threshold
( PVC queue-length in packets x MTU ) / ( PVC shaping in bps / 8 )
( 25 x 4482 ) / ( 3,840,000 / 8 ) = 233 ms
default epd-threshold = number of buffers * shaping rate / line rate
epd-threshold cells;
statement:
epd-threshold
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