Cisco Catalyst 3550 series Software Configuration Manual page 589

Chapter 29 Configuring QoS
Figure 29-1 QoS Classification Bits in Frames and Packets
Encapsulated Packet
Layer 2
header
Layer 2 ISL Frame
ISL header
(26 bytes)
Layer 2 802.1Q/P 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
To give 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 have 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.
These sections describe the QoS stages and how they work:
Basic QoS Model, page 29-4
•
Classification, page 29-5
•
Policing and Marking, page 29-8
•
Mapping Tables, page 29-10
•
• Queueing and Scheduling, page 29-11
• Packet Modification, page 29-17
78-11194-09
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 3550 Multilayer Switch Software Configuration Guide
Understanding QoS
29-3