Platform Support for Priority Scheduling

Hardware platforms support queue priorities in different ways:

• On all platforms, you can configure one queue per interface to have strict-high priority. However, strict-high priority works differently on Juniper Networks J Series Services Routers than it does on M Series Multiservice Edge Routers and T Series Core Routers. For configuration instructions, see the J Series router documentation and Configuring Schedulers for Priority Scheduling.
• Strict-high priority works differently on AS PIC link services IQ (lsq-) interfaces. For link services IQ interfaces, a queue with strict-high priority might starve all the other queues. For more information, see the Junos Services Interfaces Configuration Guide.
• On Juniper Networks 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 percentQueue 2: priority high, transmit-rate 30 percent Traffic profile:Queue 0: 100 percent of the interface speedQueue 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 routers, you can include the transmit-rate statement at the [edit class-of-service schedulers scheduler-name] hierarchy level to assign the WRR weights within a given priority level and not between priorities.
• On J Series routers, adding the exact option with the transmit-rate statement 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 installed.

Table 27 shows the priority mappings by FPC type. Note, for example, that on Juniper Networks M320 Multiservice Edge Routers FPCs, T Series Core Routers 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.