Transmission Scheduling and Platform Differences

When you configure queue priorities, note the following hardware platform differences:

• On all platforms, you can configure one queue per interface to have strict-high priority. However, the strict-high priority works differently on J-series platforms than it does on M-series and T-series platforms. For more information, see Configuring Strict-High Priority on J-series Platforms and Configuring Strict-High Priority on M-series and T-series Platforms.
• The strict-high priority works differently on AS PIC link services IQ (lsq) interfaces. For link services IQ interfaces, a strict-high-priority queue might starve all the other queues. For more information, see the JUNOS Services Interfaces Configuration Guide.
• On J-series Services Routers, high priority queues might starve low priority queues. For example:

  Queue priority and transmission rate:

  Queue 0: priority low, transmit-rate 50 percent

  Queue 2: priority high, transmit-rate 30 percent

   

  Traffic profile:

  Queue 0: 100 percent of the interface speed

  Queue 2: 100 percent of the interface speed

   

  Results:

  Queue 0: 0 percent of traffic is delivered.

  Queue 2: 100 percent of traffic is delivered.

• On J-series Services Routers, you can use the transmit-rate statement to assign the WRR weights within a given priority level and not between priorities.
• On J-series Services Routers, the transmit-rate exact option is useful within a given priority and not between the priorities.
• The priority levels you configure map to hardware priority levels. These priority mappings depend on the FPC type in which the PIC is mounted.

Table 34 shows the priority mappings by FPC type. Note, for example, that on M320 FPCs and T-series enhanced FPCs, the software priorities medium-low and medium-high behave similarly because they map to the same hardware priority level.

Table 34: Scheduling Priority Mappings by FPC Type