Egress Quality of Service mechanisms are applied to frames and packets received by the switch in the outbound direction. The following Catalyst switch egress QoS mechanisms are described:
- Congestion Management and Avoidance
Congestion Management and Avoidance
Congestion management and avoidance in the egress direction is also comprised of the same three elements used in ingress congestion management and avoidance, which are queuing, dropping, and scheduling. Queuing is used to place packets into different software queues based on the received packet QoS labels. Catalyst switches typically have more egress queues than ingress queues. Depending on the platform and other variables, this number may range from two queues to four queues.
Once the packets have been placed into the appropriate queue based on their Quality of Service values, dropping is used to manage queues. Dropping is a congestion avoidance mechanism that uses drop priorities for different classes of traffic. Queues have drop thresholds which are used to indicate which packets can be dropped once the queue has filled beyond a certain threshold. The same congestion avoidance mechanism may be used for both ingress and egress queue congestion avoidance.
Scheduling refers to how the queues are serviced or emptied. As is the case with ingress QoS operation, if a priority queue is configured, it will be serviced before the normal queue. Strict Round Robin (SRR) is also used for egress scheduling. You are not required to implement any egress QoS configurations as they are beyond the scope of the SWITCH exam requirements.