Basic Qos Model - Cisco Catalyst 2950 Software Manual

Chapter 13 Configuring QoS
Figure 13-1 QoS Classification Layers in Frames and Packets
Encapsulated Packet
Layer 2
Layer 2 802.1Q/P Frame
Preamble
Layer 3 IPv4 Packet
Version
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.
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

Figure 13-2
policing, and marking:
If you have the standard software image installed on your switch, only the queueing and scheduling
Note
features are available.
78-11380-03
Layer 3 IP packets can carry a Differentiated Services Code Point (DSCP) value. The supported
DSCP values are 0, 8, 10, 16, 18, 24, 26, 32, 34, 40, 46, 48, and 56.
IP header
header
Start frame
DA
delimiter
ToS
Len
length (1 byte)
DSCP
shows the basic QoS model. Actions at the ingress interface include classifying traffic,
Data
SA Tag PT Data
3 bits used for CoS (user priority)
ID Offset TTL Proto FCS IP-SA IP-DA Data
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding QoS
FCS
13-3