> # Information About QoS

> QoS implementation in the Cisco MDS 9000 Family follows the differentiated services (DiffServ) model. The DiffServ standard is defined in RFCs 2474 and 2475. The Cisco MDS 9000 Family supports QoS for internally and externally generated control traffic. Within a switch, control traffic is sourced to the supervisor module and is treated as a high priority frame. By default, the QoS feature for certain critical control traffic is enabled. These critical control frames are assigned the highest (absolute) priority. A high priority status provides absolute priority over all other traffic and is assigned in the following cases:

> - Internally generated time-critical control traffic (mostly Class F frames).
> - Externally generated time-critical control traffic entering a switch in the Cisco MDS 9000 Family from a another vendor’s switch. High priority frames originating from other vendor switches are marked as high priority as they enter a switch in the Cisco MDS 9000 Family.

> Quality of service (QoS) offers the following advantages:

> - Provides relative bandwidth guarantee to application traffic.
> - Controls latency experienced by application traffic.

> Prioritizes one application over another (for example, prioritizing transactional traffic over bulk traffic) through bandwidth and latency differentiation.

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