To provide the same forwarding treatment to packets with the same class information and different treatment to packets with different class information, all switches and routers that access the Internet rely on class information. Class information in the packet can be assigned by end hosts or by switches or routers along the way, based on a configured policy, detailed examination of the packet, or both. Detailed examination of the packet is expected to happen closer to the network edge so that core switches and routers are not overloaded.
Switches and routers along the path can use class information to limit the amount of resources allocated per traffic class. The behavior of an individual device when handling traffic in the DiffServ architecture is called per-hop behavior. If all devices along a path provide a consistent per-hop behavior, you can construct an end-to-end QoS solution.
Implementing QoS in your network can be a simple or complex task and depends on the QoS features offered by your internetworking devices, the traffic types and patterns in your network, and the granularity of control that you need over incoming and outgoing traffic.
By default the 3550 will have these values for the CoS to DSCP mapping:
CoS value 0 1 2 3 4 5 6 7 DSCP value 0 8 16 24 32 40 48 56
You can modify these values with the command 搈ls qos map cos-dscp?as shown in the following example:
This command modifies dscp1卍scp8, with a range up to 63.
Auto QoS
You can configure the 3550 switch to take the QoS values of a packet that are set by the originator of the flow, and 搕rust?them. To trust the DSCP value configured by another device you would use the following command under the ingress interface:
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