29-3
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 29 Configuring QoS
Understanding QoS
Figure 29-1 QoS Classification Bits in Frames and Packets
Note
Layer 3 IPv6 packets are treated as non-IP packets and are bridged by the switch.
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:
•
•
•
Policing and Marking, page 29-8
•
•
Queueing and Scheduling, page 29-11
•
Packet Modification, page 29-17
46974
Encapsulated Packet
Layer 2
header
IP header
3 bits used for CoS
Data
Layer 2 ISL Frame
ISL header
(26 bytes)
Encapsulated frame 1...
(24.5 KB)
FCS
(4 bytes)
Layer 2 802.1Q/P Frame
Preamble
Start frame
delimiter
DA
Len
SA
Tag
PT
Data
FCS
Layer 3 IPv4 Packet
Version
length
ToS
(1 byte)
ID
Offset TTL
Proto FCS IP-SA IP-DA Data
3 bits used for CoS (user priority)
IP precedence or DSCP