QoS LLQ (Low Latency Queueing) on Cisco IOS

Hi Azm,

Congestion management is about dealing with congestion when it occurs and congestion avoidance is trying to avoid congestion. To understand congestion avoidance, you have to think about how the TCP window size and global synchronization works. I explained this in this lesson:


Congestion avoidance works by dropping certain TCP segments so that the window size reduces, slowing down the TCP transmission and by doing so, preventing congestion from happening.

The difference between priority and bandwidth is about the scheduler. Take a look at this picture:

There are four output queues here. Q1 is attached to the “LLQ” scheduler and Q2, Q3 and Q4 are attached to the “CBWFQ” scheduler.

Packets in Q2, Q3 and Q4 are emptied in a weighted round robin fashion. Round robin means that we move between the queues like this: Q2 > Q3 > Q4 and back to Q2. Weighted means that we can serve more packets from one queue if we want to. For example, when it’s Q2’s turn we send 2 packets…Q3 also gets to send 2 packets but Q4 only gets to send 1 packet.

Q1 is “special” since it’s attached to the LLQ scheduler. When something ends up in Q1, it is transmitted immediately…even if there are packets in Q2, Q3, and Q4. Traffic in Q1 is always prioritized over these queues.

Q1 is our priority queue, configured with the priority command. Q2, Q3, and Q4 have a bandwidth guarantee because of our scheduler, configured with the bandwidth command.

I hope this helps!

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