buffer-size

Syntax

Hierarchy Level

Description

Specify buffer size in a scheduler configuration.

On all switches, you configure the proportion of port buffers allocated to a particular output queue using the following process:

1. Configure a scheduler and set the buffer-size option.

2. Use a scheduler map to map the scheduler to the forwarding class that is mapped to the queue to which you want to apply the buffer size.

   For example, suppose that you want to change the dedicated buffer allocation for FCoE traffic. FCoE traffic is mapped to the fcoe forwarding class, and the fcoe forwarding class is mapped to queue 3 (this is the default configuration). To use default FCoE traffic mapping, in the scheduler map configuration, map the scheduler to the fcoe forwarding class.

3. If you are using enhanced transmission selection (ETS) hierarchical scheduling, associate the scheduler map with the traffic control profile you want to use on the egress ports that carry FCoE traffic. If you are using direct port scheduling, skip this step.

4. If you are using ETS, associate the traffic control profile that includes the scheduler map with the desired egress ports. For this example, you associate the traffic control profile with the ports that carry FCoE traffic. If you are using port scheduling, associate the scheduler map with the desired egress ports.

   Queue 3, which is mapped to the fcoe forwarding class and therefore to the FCoE traffic, receives the dedicated buffer allocation specified in the buffer-size statement.

QFX10000 Switches

On QFX10000 switches, the buffer size is the amount of time in milliseconds of port bandwidth that a queue can use to continue to transmit packets during periods of congestion, before the buffer runs out and packets begin to drop.

The switch can use up to 100 ms total (combined) buffer space for all queues on a port. A buffer-size configured as one percent is equal to 1 ms of buffer usage. A buffer-size of 15 percent (the default value for the best effort and network control queues) is equal to 15 ms of buffer usage.

The total buffer size of the switch is 4 GB. A 40-Gigabit port can use up to 500 MB of buffer space, which is equivalent to 100 ms of port bandwidth on a 40-Gigabit port. A 10-Gigabit port can use up to 125 MB of buffer space, which is equivalent to 100 ms of port bandwidth on a 10-Gigabit port. The total buffer sizes of the eight output queues on a port cannot exceed 100 percent, which is equal to the full 100 ms total buffer available to a port. The maximum amount of buffer space any queue can use is also 100 ms (which equates to a 100 percent buffer-size configuration), but if one queue uses all of the buffer, then no other queue receives buffer space.

There is no minimum buffer allocation, so you can set the buffer-size to zero (0) for a queue. However, we recommend that on queues on which you enable PFC to support lossless transport, you allocate a minimum of 5 ms (a minimum buffer-size of 5 percent). The two default lossless queues, fcoe and no-loss, have buffer-size default values of 35 ms (35 percent).

Queue buffer allocation is dynamic, shared among ports as needed. However, a queue cannot use more than its configured amount of buffer space. For example, if you are using the default CoS configuration, the best-effort queue receives a maximum of 15 ms of buffer space because the default transmit rate for the best-effort queue is 15 percent.

If a switch experiences congestion, queues continue to receives their full buffer allocation until 90 percent of the 4 GB buffer space is consumed. When 90 percent of the buffer space is in use, the amount of buffer space per port, per queue, is reduced in proportion to the configured buffer size for each queue. As the percentage of consumed buffer space rises above 90 percent, the amount of buffer space per port, per queue, continues to be reduced.

On 40-Gigabit ports, because the total buffer is 4 GB and the maximum buffer a port can use is 500 MB, up to seven 40-Gigabit ports can consume their full 100 ms allocation of buffer space. However, if an eighth 40-Gigabit port requires the full 500 MB of buffer space, then the buffer allocations are proportionally reduced because the buffer consumption is above 90 percent.

On 10-Gigabit ports, because the total buffer is 4 GB and the maximum buffer a port can use is 125 MB, up to 28 10-Gigabit ports can consume their full 100 ms allocation of buffer space. However, if a 29th 10-Gigabit port requires the full 125 MB of buffer space, then the buffer allocations are proportionally reduced because the buffer consumption is above 90 percent.

QFX5100, EX4600, QFX3500, and QFX3600 Switches, and QFabric Systems

Set the dedicated buffer size of the egress queue that you bind the scheduler to in the scheduler map configuration. The switch allocates space from the global dedicated buffer pool to ports and queues in a hierarchical manner. The switch allocates an equal number of dedicated buffers to each egress port, so each egress port receives the same amount of dedicated buffer space. The amount of dedicated buffer space per port is not configurable.

However, the buffer-size statement allows you to control the way each port allocates its share of dedicated buffers to its queues. For example, if a port only uses two queues to forward traffic, you can configure the port to allocate all of its dedicated buffer space to those two queues and avoid wasting buffer space on queues that are not in use. We recommend that the buffer size should be the same size as the minimum guaranteed transmission rate (the transmit-rate).

Default

The default behavior differs on different switches.

QFX10000 Switches

If you do not configure buffer-size and you do not explicitly configure a queue scheduler, the default buffer-size is the default transmit rate of the queue. If you explicitly configure a queue scheduler, the default buffer allocations are not used. If you explicitly configure a queue scheduler, configure the buffer-size for each queue in the scheduler, keeping in mind that the total buffer-size of the queues cannot exceed 100 percent (100 ms).

Table 1 shows the default queue buffer sizes on QFX10000 switches. The default buffer size is the same as the default transmit rate for each default queue:

By default, only the queues mapped to the default forwarding classes receive buffer space from the port buffer pool. (Buffers are not wasted on queues that do not carry traffic.)

QFX5100, EX4600, QFX3500, and QFX3600 Switches, and QFabric Systems

The port allocates dedicated buffers to queues that have an explicitly configured scheduler buffer size. If you do not explicitly configure a scheduler buffer size for a queue, the port serves the explicitly configured queues first. Then the port divides the remaining dedicated buffers equally among the queues that have an explicitly attached scheduler without an explicitly configured buffer size configuration. (If you configure a scheduler, but you do not configure the buffer size parameter, the default is equivalent to configuring the buffer size with the remainder option.)

If you use the default scheduler and scheduler map on a port (no explicit scheduler configuration), then the port allocates its dedicated buffer pool to queues based on the default scheduling. Table 2 shows the default queue buffer sizes. The default buffer size is the same as the default transmit rate for each default queue:

By default, only the queues mapped to the default forwarding classes receive buffer space from the port buffer pool. (Buffers are not wasted on queues that do not carry traffic.)

On EX Series switches except EX4300 switches, the default scheduler transmission rate and buffer size percentages for queues 0 through 7 are 95, 0, 0, 0, 0, 0, 0, and 5 percent, respectively. On EX4300 switches, the default scheduler transmission rate and buffer size for queues 0 through 11 are 75, 0, 0, 5, 0, 0, 0, 0, 15, 0, 0 and 5 percent, respectively, of the total available buffer.

Options

Required Privilege Level

interface—To view this statement in the configuration.interface-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 9.0.