Applying QoS to Input and Output Interfaces


Input interface

Classify Mark


(As close to the source as possible)

(Coming from a higher-speed link or aggregation)

Input interface

Classify Mark











(High-speed to


low-speed links or

aggregation points)


(Going to lower-speed


links or from points of





and Interleaving

WAN links)

In a QoS-enabled network, classification is performed on every input interface.

Marking should be performed as close to the network edge as possible—in the originating network device, if possible. Devices farther from the edge of the network, such as routers and switches, can be configured to trust or untrust the markings made by devices on the edge of the network. An IP Phone, for example, will not trust the markings of an attached PC, while a switch will generally be configured to trust the markings of an attached IP Phone.

It only makes sense to use congestion management, congestion avoidance, and traffic-shaping mechanisms on output interfaces, because these mechanisms help maintain smooth operation of links by controlling how much and which type of traffic is allowed on a link. On some router and switch platforms, congestion management mechanisms, such as weighted round robin (WRR) and modified deficit round robin (MDRR), can be applied on the input interface.

Congestion avoidance is typically employed on an output interface wherever there is a chance that a high-speed link or aggregation of links feeds into a slower link (such as a LAN feeding into a WAN).

Policing and shaping are typically employed on output interfaces to control the flow of traffic from a high-speed link to lower-speed links. Policing is also employed on input interfaces to control the flow into a network device from a high-speed link by dropping excess low-priority packets.

Both compression and LFI are typically used on slower-speed WAN links between sites to improve bandwidth efficiency.

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