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Understanding CoS Tail-Drop Profiles

When the number of packets queued is greater than the ability of the switch to empty an output queue, the queue requires a method for determining which packets to drop to relieve the congestion. Weighted random early detection (WRED) drop profiles define the drop probability of packets as the output queue fills. During periods of congestion, as the output queue fills, the switch drops incoming packets as determined by a drop profile until the output queue becomes less congested.

Depending on the drop probabilities, a drop profile can drop many packets long before the buffer becomes full, or it can drop only a few packets even if the buffer is almost full.

You configure drop profiles in the drop profile section of the class-of-service (CoS) configuration hierarchy. You apply drop profiles using a drop profile map in each scheduler configuration. For each scheduler, you can configure separate drop profiles for each combination of loss priority (low, medium-high, and high) and protocol.

Drop profiles define the meaning of each of the loss priorities by setting the values for when to drop packets and the probability that packets will drop.

You can configure a maximum of 32 drop profiles.

Note: You cannot apply drop profiles to multidestination (multicast) queues. Do not apply drop profiles to lossless flows such as FCoE traffic, because the corresponding queues require lossless behavior. Use priority-based flow control (PFC) to prevent packet drop.

Drop Profile Parameters

• Fill level—The queue fullness value, which represents a percentage of the memory used to store packets in relation to the total amount of memory allocated to the queue.
• Drop probability—The percentage value that corresponds to the likelihood that an individual packet is dropped.

You set two queue fill levels and two drop probabilities in each drop profile. The two fill levels and the two drop probabilities create two pairs of values. The first fill level and the first drop probability create one value pair and the second fill level and the second drop probability create the second value pair.

The first fill level value specifies the percentage of queue fullness at which packets begin to drop, known as the drop start point. Until the queue reaches this level of fullness, no packets are dropped. The second fill level value specifies the percentage of queue fullness at which all packets are dropped, known as the drop end point.

The first drop probability value is always 0 (zero). This pairs with the drop start point and specifies that until the queue fullness level reaches the first fill level, no packets drop. When the queue fullness exceeds the drop start point, packets begin to drop until the queue exceeds the second fill level, when all packets drop. The second drop probability value, known as the maximum drop rate, specifies the likelihood of dropping packets when the queue fullness reaches the drop end point. As the queue fills from the drop start point to the drop end point, packets drop in a smooth, linear pattern (called an interpolated graph) as shown in Figure 1. After the drop end point, all packets drop.

Figure 1: Tail-Drop Profile Packet Drop

The thick line in Figure 1 shows the packet drop characteristics for a sample tail-drop profile. At the drop start point, the queue reaches a fill level of 30 percent. At the drop end point, the queue fill level reaches 50 percent, and the maximum drop rate is 80 percent.

No packets drop until the queue fill level reaches the drop start point of 30 percent. When the queue reaches the 30 percent fill level, packets begin to drop. As the queue fills, the percentage of packets dropped increases in a linear fashion. When the queue fills to the drop end point of 50 percent, the rate of packet drop has increased to the maximum drop rate of 80 percent. When the queue fill level exceeds the drop end point of 50 percent, all of the packets drop until the queue fill level drops below 50 percent.

Default Drop Profile

If you do not configure default profiles and apply them to queue schedulers, the switch uses the default drop profile for lossy traffic classes. In the default drop profile, when the fill level is 0 percent, the drop probability is 0 percent. When the fill level is 100 percent, the drop probability is 100 percent. As soon as packets arrive on a queue, the default profile might begin to drop packets.

Packet Drop Method

When a packet reaches the head of the queue, the switch generates a random number between 0 and 100. The switch plots the random number against the drop profile using the current fullness of the queue. When the random number falls above the graph line, the packet is transmitted. When the number falls below the graph line, the packet is dropped.

To create the linear drop pattern from the drop start point to the drop end point, the drop probabilities are derived using a linear approximation with eight sections, or steps, from the minimum queue fill level to the maximum queue fill level. The fill levels are divided into the eight sections equally, starting at the minimum fill level and ending at the maximum fill level. As the queue fills, the percentage of dropped packets increases. The percentage of packets dropped is based on the maximum drop rate.

For example, the default drop profile (which specifies a maximum drop rate of 100 percent) has the following drop probabilities at each section, or step, in the eight-section linear drop pattern:

• First section—The minimum drop probability is 6.25 percent of the maximum drop rate. The maximum drop probability is 12.5 percent of the maximum drop rate.
• Second section—The minimum drop probability is 18.75 percent of the maximum drop rate. The maximum drop probability is 25 percent of the maximum drop rate.
• Third section—The minimum drop probability is 30.25 percent of the maximum drop rate. The maximum drop probability is 37.5 percent of the maximum drop rate.
• Fourth section—The minimum drop probability is 43.75 percent of the maximum drop rate. The maximum drop probability is 50 percent of the maximum drop rate.
• Fifth section—The minimum drop probability is 56.25 percent of the maximum drop rate. The maximum drop probability is 62 percent of the maximum drop rate.
• Sixth section—The minimum drop probability is 68.75 percent of the maximum drop rate. The maximum drop probability is 75.5 percent of the maximum drop rate.
• Seventh section—The minimum drop probability is 81.25 percent of the maximum drop rate. The maximum drop probability is 87.5 percent of the maximum drop rate.
• Eighth section—The minimum drop probability is 92.75 percent of the maximum drop rate. The maximum drop probability is 100 percent of the maximum drop rate.

Packets drop even when there is no congestion, because packet drops begin at the drop start point regardless of whether congestion exists on the port. The default drop profile example represents the worst-case scenario, because the drop start point fill level is 0 percent, so packet drop begins when the queue starts to receive packets.

You can specify when packets begin to drop by configuring a drop start point at a fill level greater than 0 percent. For example, if you configure a drop profile that has a drop start point of 30 percent, packets do not drop until the queue is 30 percent full.

The smaller the gap between the minimum drop rate (which is always 0) and the maximum drop rate, the smaller the gap between the minimum drop probability and the maximum drop probability at each section (step) of the linear drop pattern. The default drop profile, which has the maximum gap between the minimum drop rate (0 percent) and the maximum drop rate (100 percent), has the highest gap between the minimum drop probability and the maximum drop probability at each step. Configuring a lower maximum drop rate for a drop profile reduces the gap between the minimum drop probability and the maximum drop probability.

Drop Profile Maps

Drop profile maps are part of scheduler configuration. A drop profile map maps a drop profile to a loss priority and a protocol. Specifying the drop profile map in a scheduler associates the drop profile with the queues (forwarding classes) that you map to the scheduler in a scheduler map.

You configure loss priority for a queue in the classifier section of the CoS configuration hierarchy, and the loss priority is applied to the queue at the ingress interface.

Each scheduler can have multiple drop profile maps, one for each combination of loss priority and protocol.

Congestion Prevention

Configuring drop profiles on output queues prevents them from impacting other queues on the egress ports. If you do not configure drop profiles and map them to output queues, output queues without drop profiles can impact the bandwidth available to other output queues.

Note: Do not configure drop profiles for lossless traffic such as the default FCoE and no-loss forwarding classes because queues mapped to lossless forwarding classes require lossless behavior. Use priority-based flow control (PFC) to prevent packet drop.

For example, queues might not receive the egress bandwidth configured in the queue scheduler when a port’s egress queues are congested because the ingress port buffer fills above a certain threshold. If this happens, configuring a drop profile for the queue on the port that is receiving its configured bandwidth alleviates the problem by preventing the queue from impacting the other queues on the port.