An individual router interface has multiple queues assigned to store packets temporarily before transmission. The router uses a scheduling method, often based on packet type, to determine the order in which the queues are serviced. JUNOS schedulers allow you to define the priority, bandwidth, delay buffer size, rate control status, and RED drop profiles to be applied to a particular queue for packet transmission. For more information, see Scheduler Settings.
On J-series Services Routers, you can configure per-unit scheduling (also called logical interface scheduling). Per-unit scheduling allows you to enable multiple output queues on a logical interface and associate an output scheduler with each queue.
The transmission rate determines the traffic transmission bandwidth for each forwarding class you configure. The rate is specified in bits per second (bps). Each queue is allocated some portion of the bandwidth of the outgoing interface.
This bandwidth amount can be a fixed value, such as 1 megabit per second (Mbps), a percentage of the total available bandwidth, or the rest of the available bandwidth. You can limit the transmission bandwidth to the exact value you configure, or allow it to exceed the configured rate if additional bandwidth is available from other queues. This property helps ensure that each queue receives the amount of bandwidth appropriate to its level of service.
On J-series Services Routers, the minimum transmit rate supported on an interface is one-thousandth of the speed of that interface. For example, on a Gigabit Ethernet interface with a speed of 1000 Mbps, the minimum transmit rate is 1 Mbps (1000 Mbps x 1/1000). You can configure transmit rate in the range 3200 through 160,000,000,000 bps. When the configured rate is less than the minimum transmit rate, the minimum transmit rate is used instead.
On J-series Services Routers, transmit rate assigns the weighted round-robin (WRR) priority values within a given priority level and not between priorities. For more information, see Transmission Scheduling on J-series Services Routers.
You can configure the delay buffer size to control congestion at the output stage. A delay buffer provides packet buffer space to absorb burst traffic up to a specified duration of delay. When the buffer becomes full, packets with 100 percent drop probability are dropped from the head of the buffer.
The system calculates the buffer size for a queue based on the buffer allocation method you specify for it in the scheduler. See Delay Buffer Size Allocation Methods for different buffer allocation methods and Specifying Delay Buffer Sizes for Queues for buffer size calculations.
By default, all J-series Services Router interfaces other than channelized T1/E1 interfaces support a delay buffer time of 100,000 microseconds. On channelized T1/E1 interfaces, the default delay buffer time is 500,000 microseconds for clear-channel interfaces, and 1,200,000 microseconds for NxDS0 interfaces.
On J-series Services Routers, you can configure larger delay buffers on channelized T1/E1 interfaces. Larger delay buffers help these slower interfaces to avoid congestion and packet dropping when they receive large bursts of traffic. For more information, see Configuring Large Delay Buffers with a Configuration Editor.
Scheduling priority determines the order in which an output interface transmits traffic from the queues, thus ensuring that queues containing important traffic are provided better access to the outgoing interface.
The queues for an interface are divided into sets based on their priority. Each set contains queues of the same priority. The router examines the sets in descending order of priority. If at least one queue in a set has a packet to transmit, the router selects that set. If multiple queues in the set have packets to transmit, the router selects a queue from the set according to the weighted round-robin (WRR) algorithm that operates within the set.
The packets in a queue are transmitted based on the configured scheduling priority, the transmit rate, and the available bandwidth. For more information, see Transmission Scheduling on J-series Services Routers.
Shaping rates control the maximum rate of traffic transmitted on an interface. You can configure the shaping rate so that the interface transmits less traffic than it is physically capable of carrying.
On J-series Services Routers, you can configure shaping rates on logical interfaces. By default, output scheduling is not enabled on logical interfaces. Logical interface scheduling (also called per-unit scheduling) allows you to enable multiple output queues on a logical interface and associate an output scheduler and shaping rate with the queues.
By default, the logical interface bandwidth is the average of unused bandwidth for the number of logical interfaces that require default bandwidth treatment. You can specify a peak bandwidth rate in bits per second (bps), 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). The range is from 1000 through 32,000,000,000 bps.
A drop profile is a feature of the random early detection (RED) process that allows packets to be dropped before queues are full. Drop profiles are composed of two main values—the queue fullness and the drop probability. The queue fullness represents percentage of memory used to store packets in relation to the total amount that has been allocated for that queue. The drop probability is a percentage value that correlates to the likelihood that an individual packet is dropped from the network. These two variables are combined in a graph-like format.
When a packet reaches the head of the queue, a random number between 0 and 100 is calculated by the router. This random number is plotted against the drop profile having the current queue fullness of that particular queue. When the random number falls above the graph line, the packet is transmitted onto the physical media. When the number falls below the graph line, the packet is dropped from the network.
When you configure the RED drop profile on an interface, the queue no longer drops packets from the tail of the queue (the default). Rather, packets are dropped after they reach the head of the queue.