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Where should i apply the QoS policy?
That depends on your overall end-to-end QoS policy. It also might depend whether you mean application of an actual policy configuration statement or other QoS configuration statements that support a logical QoS policy.
In general, edge port ingress QoS looks to analyze traffic. It might validate ingress markings and perhaps mark or remark some traffic. Once traffic is within your QoS domain, ingress traffic is usually just trusted.
Also in general, egress QoS deals with congestion management, both prioritization of transmission sequence and/or drop prioritization.
If you have a case where egress congestion isn't possible, for example egress to either of your PCs if they are the only PCs (as shown in your topology), you might not bother with egress QoS. However, keeping track of where you need egress QoS can be more troublesome than just doing it everywhere. Also doing it everywhere avoids issues with topology changes. For example, if the link between the two switches was upgraded to 100 Mbps, you could have congestion to PC1 (receiving traffic from PC2 and the Internet, concurrently).
In other words . . .
Where should i apply the QoS policy?
Where ever it's needed to accomplish your QoS goals.
