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4.0 CCNA - IP Services

4.7 Forwarding per-hop behavior (PHB) for QoS such as classification, marking, queuing, congestion, policing, shaping

Congestion Control

In Cisco IOS devices, congestion can be controlled using the following techniques:

1. Queuing: Queuing is the process of buffering packets in a queue before they are transmitted. Cisco IOS devices support multiple queuing mechanisms, including priority queuing, custom queuing, and Weighted Fair Queuing (WFQ). These mechanisms allow network administrators to prioritize different types of traffic and prevent congestion.

2. Shaping: Shaping is the process of regulating the rate at which packets are transmitted, so that the outgoing traffic does not exceed a specified rate. Cisco IOS devices support several shaping mechanisms, including Traffic Shaping (TS), Frame Relay Traffic Shaping (FRTS), and Class-Based Shaping.

3. Policing: Policing is the process of regulating incoming traffic by discarding or marking packets that exceed a specified rate. Cisco IOS devices support several policing mechanisms, including Committed Access Rate (CAR), Class-Based Policing (CB policing), and Hierarchical Policing.

4. Congestion Avoidance: Congestion avoidance mechanisms are designed to dynamically adjust the rate of transmission in response to changes in network congestion. Cisco IOS devices support several congestion avoidance mechanisms, including Random Early Detection (RED), Weighted Random Early Detection (WRED), and Adaptive Shaping (AD-SHAPING).

The specific configuration of each of these mechanisms may vary depending on the specific network requirements. It is important to understand the specific requirements and the impact of each congestion control mechanism before implementing it in a live network.

WFQ Example Configuration

Here is an example of configuring weighted fair queuing (WFQ) on a Cisco IOS device:

Router(config)#interface [interface_name]
Router(config-if)#bandwidth [bandwidth_value]
Router(config-if)#fair-queue [queue_value]
Router(config-if)#end
Router#show interfaces [interface_name] | include Fairqueue

In this example, interface_name is the name of the interface on which WFQ is being configured, bandwidth_value is the bandwidth of the interface in kilobits per second (Kbps), and queue_value is the number of queues for the WFQ process. The show interfaces command is used to verify the WFQ configuration.

Note that this is just a simple example of configuring WFQ on a Cisco IOS device, and specific configurations may vary based on the network requirements.

Traffic Policing

An example of configuration on a Cisco IOS device

Router(config)#interface [interface_name]
Router(config-if)#service-policy output [policy_name]
Router(config-if)#end
Router(config)#policy-map [policy_name]
Router(config-pmap)#class [class_name]
Router(config-pmap-c)#police [cir_value] [cbs_value] [be_value]
Router(config-pmap-c)#end
Router#show policy-map interface [interface_name]

In this example, interface_name is the name of the interface on which traffic policing is being configured, policy_name is the name of the policy-map applied on the interface, class_name is the name of the class in the policy-map, cir_value is the committed information rate in bits per second (bps), cbs_value is the committed burst size in bytes, and be_value is the maximum burst size in bytes. The show policy-map interface command is used to verify the traffic policing configuration.

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