Cisco Catalyst 3750 Software Configuration Manual page 483

Chapter 26 Configuring QoS
Figure 26-1 QoS Classification Layers in Frames and Packets
Encapsulated Packet
Layer 2
header
Layer 2 ISL Frame
ISL header
(26 bytes)
Layer 2 802.1Q and 802.1p Frame
Preamble
Layer 3 IPv4 Packet
Version
length
Layer 3 IPv6 packets are treated as non-IP packets and are bridged by the switch.
Note
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 inbound and outbound traffic.
78-15870-01
IP header Data
Encapsulated frame 1...
(24.5 KB)
3 bits used for CoS
Start frame
DA SA
delimiter
ToS
Len ID
(1 byte)
IP precedence or DSCP
FCS
(4 bytes)
Tag PT Data FCS
3 bits used for CoS (user priority)
Offset TTL Proto FCS IP-SA IP-DA Data
Catalyst 3750 Metro Switch Software Configuration Guide
Understanding QoS
26-3