This window appears when you select one or more interfaces and click Configure in the QoS Queues window. Use it to configure the attributes of the four egress queues on device interfaces.
The tabs you see depend on which interface type, Fast Ethernet or Gigabit Ethernet, you selected. This list shows which tabs appear for which interface types:
When you are satisfied with the attribute configurations, click OK.
The CoS-Queue ID tab contains a table that maps CoS values to queue IDs; it directs outbound packets with certain CoS values to the mapped queues.
To change the default CoS-queue ID mappings:
If you later want all the entries to revert to the default mappings, click Set Defaults.
The Bandwidth tab shows, in relative terms, how much bandwidth you are allotting to each queue from the bandwidth available to all four queues.
To specify a relative allotment, enter a number from 0 to 65535 into the field beside each queue ID. The relative size difference in the numbers shows the relative differences in the allotments. For example, if the numbers top to bottom were 1, 2, 3, and 4, Queue 4 would have four times the bandwidth of Queue 1, twice the bandwidth of Queue 2 and, one-and-a third times the bandwidth of Queue 3.
If you give a queue a bandwidth of 0, it is allotted no bandwidth. The available bandwidth is shared among the remaining queues.
If you select Enabled from the Egress Priority Queue list on the Priority tab, Queue 4 becomes a priority queue. This means that its packets are sent before the packets in the other queues are sent.
A priority queue is exempt from an assigned allotment on the Bandwidth tab; the bandwidth that remains after the priority queue is served is distributed according to the allotment numbers for the other queues.
On this tab, you specify how much of a shared buffer you want to allocate to each queue. To do this, you enter an ID from 1 to 8 in the field beside each queue. The ID does not represent the percentage of the buffer you want to allocate. Rather, it maps to a value on the Min Reserves tab of the QoS Maps window. On that tab, the ratio of each value to the sum of the values determines the percentage of the buffer that is allocated to each queue.
If you change the IDs and want to return to the default settings, click Set Defaults.
The Limit tab shows, in relative terms, how much space you are allotting to each queue from the space available to all four queues.
To specify a relative allotment, enter a number from 0 to 255 into the field beside each queue ID. The relative size difference in the numbers shows the relative differences in the allotments. For example, if the numbers top to bottom were 1, 2, 3, and 4, Queue 4 would be four times larger than Queue 1, twice as large as Queue 2 and, one-and-a third times larger than Queue 3.
If you give a queue a limit of 0, it is allotted no space. The available space is shared among the remaining queues.
This tab maps every possible DSCP value to a threshold value: 1 for Threshold 1, 2 for Threshold 2. The degree of fullness that Threshold 1 and Threshold 2 represent is defined as percentages on the Threshold and WRED Threshold tabs. The percentages can differ from queue to queue. The way a queue behaves when a threshold is exceeded depends on whether the Threshold or WRED Threshold tab is controlling the queues.
To change the default DSCP-threshold mappings:
If you later want all the entries to revert to the default mappings, click Set Defaults.
To specify what happens to packets when queue thresholds are exceeded:
If the threshold type for a queue is Tail Drop:
If the threshold type for a queue is WRED:
If a queue is 100 percent full, additional packets are dropped regardless of the threshold type.
If packets are dropped from a WRED threshold and the source of the packets is using TCP, the source will decrease its transmission rate until all its packets reach their destination, an indication that traffic congestion has cleared. If the bulk of the traffic is not TCP traffic, there is no advantage to using a WRED threshold.