Cisco Systems 3750E manual Basic QoS Model, 36-3

Chapter 36 Configuring QoS

Understanding QoS

Figure 36-1 QoS Classification Layers in Frames and Packets

Encapsulated Packet

Layer 2 header

IP header

Data

Layer 2 ISL Frame

ISL header	Encapsulated frame 1...							FCS
(26 bytes)		(24.5 KB)					(4 bytes)
	3 bits used for CoS
Layer 2 802.1Q and 802.1p Frame
Preamble	Start frame	DA	SA	Tag		PT		Data	FCS
	delimiter
					3 bits used for CoS (user priority)

Layer 3 IPv4 Packet

Version	ToS	Len	ID	Offset	TTL	Proto	FCS	IP-SA	IP-DA	Data
length	(1 byte)
		IP precedence or DSCP

46974

All switches and routers that access the Internet rely on the class information to provide the same forwarding treatment to packets with the same class information and different treatment to packets with different class information. The 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 edge of the network so that the core switches and routers are not overloaded with this task.

Switches and routers along the path can use the 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.

Basic QoS Model

To implement QoS, the switch must distinguish packets or flow from one another (classify), assign a label to indicate the given quality of service as the packets move through the switch, make the packets comply with the configured resource usage limits (police and mark), and provide different treatment (queue and schedule) in all situations where resource contention exists. The switch also needs to ensure that traffic sent from it meets a specific traffic profile (shape).

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 36-3