Cisco IOS Router Queues Serviced with CBWFQ Algorithm

CBWFQ Algorithm for Servicing Queues in Cisco IOS Routers

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How are queues serviced in Cisco IOS routers with the CBWFQ algorithm?



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A. B. C. D. E.


Class Based Weighted Fair queuing is an advanced form of WFQ that supports user defined traffic classes i.e.

one can define traffic classes based on match criteria like protocols, access control lists (ACLs), and input interfaces.

A flow satisfying the match criteria for a class contributes the traffic for that particular defined class.

A queue is allocated for each class, and the traffic belonging to that class is directed to the queue for that class.

The CBWFQ (Class-Based Weighted Fair Queuing) algorithm is a queuing algorithm used in Cisco IOS routers for Quality of Service (QoS) implementation. It provides better control over how traffic is handled in a network by allowing administrators to prioritize certain types of traffic over others.

CBWFQ uses the concept of classes, which are groups of packets that are assigned a specific treatment or priority level. Each class is assigned a bandwidth guarantee or a priority level, and packets are scheduled based on their class.

When a packet arrives, it is assigned to a class based on its characteristics (such as source/destination IP address, protocol, port number, etc.). The router then applies the appropriate queuing mechanism to the packet based on its assigned class.

In CBWFQ, each class has its own queue, and packets are scheduled based on the weight assigned to each queue. The weights are determined by the bandwidth or priority level assigned to each class.

The answer to the question is B, weighted round robin based on assigned bandwidth. This means that packets are served in a round-robin fashion, but each class is given a different weight based on the bandwidth guarantee assigned to it. Classes with higher bandwidth guarantees have a higher weight, which means they are serviced more frequently than classes with lower bandwidth guarantees.

For example, if there are two classes with bandwidth guarantees of 30% and 70%, respectively, the packets in the second class will be served more frequently than those in the first class, as the second class has a higher weight. However, both classes will still be serviced, and no class will be completely starved of bandwidth.

Strict priority queuing (option C) is another queuing mechanism used in QoS, but it is not part of CBWFQ. With strict priority queuing, packets in the highest-priority queue are served first, regardless of the traffic in other queues. This can lead to starvation of lower-priority queues if the high-priority queue is constantly full.

Option A, first-in, first-out (FIFO), and option D, last-in, first-out (LIFO), are simple queuing mechanisms that do not take class or priority into account. FIFO serves packets in the order they arrive, while LIFO serves the most recently arrived packet first.

Option E, weighted round robin based on assigned priority, is not a valid queuing mechanism in CBWFQ. Priority is used to determine which packets are served first in strict priority queuing, but it does not affect the queuing mechanism in CBWFQ.