Each interface has two levels of queuing available. One is a hardware queue (called tx-ring) which is serviced by FIFO (First In First Out) method. Second is a software queue which is configurable (default serviced by FIFO as well). As Voice and business critical application’s traffic is more important than other corporate traffic (like Web traffic) it is recommended to make use from software queue and prioritize some traffic over the other. Prioritize in software queue will allow important traffic to go sooner to the hardware queue than non-important traffic. This is most useful for latency-dependant traffic like Voice or Video. Voice traffic is usually marked by EF (Expedited Forwarding) bit in the Layer 3 header. We can use this information to match the traffic and prioritize it. We can also use an ACL to mark the traffic.
It is important to enable priority queuing on the respective interface before configuring action for class map. Finally, our policy map must be attached globally or on the interface. Attaching it globally has effect on every interface where priority queuing is enabled.
Also note that priority queuing is an outbound only solution. We cannot prioritize inbound traffic.
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