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Configuring Layer 2 Circuit Trunk Mode Scheduling Overview


For ATM2 IQ interfaces configured to use Layer 2 circuit trunk mode, you can share a scheduler among 32 trunks on an ATM port. A weighted round robin scheduling algorithm ensures each trunk receives a proportional share of the bandwidth when all trunks are active, and redistributes bandwidth that would have otherwise been reserved by an inactive trunk, thus minimizing the latency on each trunk. For general information about Layer 2 circuit trunk mode, see Configuring Layer 2 Circuit Transport Mode. For general information about ATM CoS scheduling, see ATM2 IQ VC Tunnel CoS Components Overview.

Each trunk is associated with a trunk bandwidth. The trunk bandwidth is the maximum bandwidth used each time a trunk is serviced. We recommend configuring trunk bandwidths so that the ratio between the minimum and maximum bandwidths does not exceed 1:500.

To minimize latency, the Junos OS does not shape the trunks. As cells are received, they are immediately transmitted.

To configure trunk bandwidth, include the trunk-bandwidth statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

The trunk bandwidth can be from 1,000,000 through 542,526,792 bps. You can specify the rate in bits per second or cells per second (cps). You can specify a bits-per-second value either as a complete decimal number or as a decimal number followed by the abbreviation k (1000), m (1,000,000), or g (1,000,000,000). You can specify a cells-per-second value by entering a decimal number followed by the abbreviation c; values expressed in cells per second are converted to bits per second by means of the formula 1 cps = 384 bps.

The Junos OS rounds off the configured value. Therefore, we recommend that you configure a minimum trunk bandwidth of 1m. From 1m, configure values in increments of 500k.