Marking, policing, and shaping should be done at the edges of the network.
Queuing and dropping are done in the core based on packet marking done at the edge.
The figure illustrates the typical QoS configurations required at the PE and P routers within the service provider IP core network.
The service provider IP core is used to provide high-speed packet transport. Therefore, all the markings, policing, and shaping should be performed only at the PE router on the PE-to-CE link and not at the core.
Using the DiffServ model, only the edge requires a complex QoS policy. At the core, only queuing and WRED are required. The operation of queuing and WRED will be based on the markings done at the PE.
It is common to implement Gigabit Switch Routers (GSRs) within the service provider IP core. With the GSRs, the queuing mechanism is MDRR. If a router such as the Cisco 7500 is used in the core, LLQ will be the queuing mechanism.
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