Also available as a PDF here
Introduction to Congestion Management and Queuing
- Congestion occurs when the rate of input to an interface exceeds the rate of output
- Speed mismatch problem – when traffic on a high speed interface enters the router but exits on a low speed interface
- Aggregation problem – when traffic from multiple interfaces aggregates into a single interface
- Tail drop – occurs when buffers fill up and new incoming traffic is dropped
- Default queuing method is FIFO (first in first out)
- Queuing mechanisms consist of hardware and software components
- Hardware queue is called the transmit queue (TxQ)
- If TxQ becomes full, packets are held in the software queue and release to the TxQ based on the queuing mechanism
- TxQ is always FIFO
- Software queuing has a number of queues – one for each class of traffic
- If the software queue is full, the packet is dropped
- If TxQ is full, packet is held in software queue based on it’s class
- Even if a packet is in one of the software queues, it can still be dropped if WRED is applied to that queue
- The IOS determines the hardware queue based on the configured bandwidth on the interface
- You can manually set the queue size with the tx-ring-limit command
