Queing - FIFO,PQ,WFO,CBWFQ

PQ - High priorty queue is always serviced first irrrespective traffic coming fron other queue.
WFQ - Flow based ,each flow consist of source port ,destination port ,source and destination WFO always give prefernce smaller flows and lower packet size
CBWFQ - Each traffic is classifed and placed in class ,each class is allcated some amount of bandwidth ,queues are always serviced on basis amount of allocated bandwidth to queue .

Random Early Detection (RED) is a congestion avoidance mechanism that takes advantage of the congestion control mechanism of TCP. By randomly dropping packets prior to periods of high congestion, RED tells the packet source to decrease its transmission rate. WRED drops packets selectively based on IP precedence. Edge routers assign IP precedences to packets as they enter the network. (WRED is useful on any output interface where you expect to have congestion. However, WRED is usually used in the core routers of a network, rather than at the edge.) WRED uses these precedences to determine how it treats different types of traffic.

When a packet arrives, the following events occur:

1. The average queue size is calculated. 
2. If the average is less than the minimum queue threshold, the arriving packet is queued. 
3. If the average is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic. 
4. If the average queue size is greater than the maximum threshold, the packet is dropped.