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Catalyst 2928 Switch Software Configuration Guide
OL-23389-01
Chapter 31 Configuring QoS
Understanding QoS
The classification is carried in the IP packet header, using 6 bits from the deprecated IP type of service
(ToS) field to carry the classification (
class
) information. Classification can also be carried in the
Layer 2 frame. These special bits in the Layer 2 frame are described here and shown in
.
Prioritization bits in Layer 2 frames:
•
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1p
CoS value in the three least-significant bits. On ports configured as Layer 2 ISL trunks, all traffic is
in ISL frames.
•
Layer 2 IEEE 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS
value in the three most-significant bits, which are called the User Priority bits. On ports configured
as Layer 2 IEEE 802.1Q trunks, all traffic is in IEEE 802.1Q frames except for traffic in the native
VLAN.
•
Other frame types cannot carry Layer 2 CoS values.
•
Layer 2 CoS values range from 0 for low priority to 7 for high priority.
Note
IPv6 QoS is not supported in this release.
Figure 31-1
QoS Classification Layers in Frames and Packets
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.
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 and 802.1p Frame
Preamble
Start frame
delimiter
DA
SA
Tag
PT
Data
FCS
3 bits used for CoS (user priority)
