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Since reallocating hardware resources in runtime is not an option, RouterOS cannot automatically free queue buffers reserved for inactive ports. Those buffers remain unused. However, if the user knows that the specific ports will never be used (e.g., stay physically disconnected), the respective queue resources can be manually freed by introducing using the built-in "offline" tx-manager with minimum resources:
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/interface/ethernet/switch/qos/tx-manager/ add name=offline comment="use this for always disconnected ports" /interface/ethernet/switch/qos/tx-manager/queue/ set [find where tx-manager=offline] queue-buffers=1 use-shared-buffers=no /interface/ethernet/switch/qos/port set [find where !(running or name~"cpu")] tx-manager=offline |
Traffic Prioritization
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- first 64 packets are always enqueued (96 - 32 = 64).
- WRED kicks in starting from 65th packet; the chance of 65th packet to be dropped is 1/32 or roughly 3%; the formula: (65 - 64) / 32;
- the drop chance of 72th packet is 25%: (72 - 64) / 32 = 8/32 = 1/4;
- half of the newly enquining packets are dropped when the queue fill level reaches 80 packets: (80 - 64) / 32 = 16/32 = 1/2;
- 75% of the packets are dropped at the fill level 88: (88 - 64) / 32 = 24/32 = 3/4.
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Choosing a WRED threshold value is a tradeoff between congestion anticipation and burst absorption. Setting a higher WRED threshold may lead to earlier traffic rate throttling and, therefore, resolve congestion. On the other hand, a high threshold leads to packet drops in limited traffic bursts that could be absorbed by the queue buffers and transformed losslessly if WRED didn't kick in. For instance, initiating a remote database connection usually starts with heavier traffic ("packet burst") at the initialization phase; then, the traffic rate drops down to a "reasonable" level. Any packet drop during the initialization phase leads to nothing but a slower database connection due to the need for retransmission. Hence, lowering the WRED threshold or entirely disabling WRED on such traffic is advised. The opposite case is video streaming. Early congestion detection helps select a comfortable streaming rate without losing too much bandwidth on retransmission or/and "overshooting" by sacrificing the quality level by too much.
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