Cisco NM-8B-U - HW ROUTERS L-M User Manual page 30

Feature Overview
Understanding Quality of Service (QoS)
Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an
equal chance of being dropped.
With the QoS feature configured on your switch, you can select specific network traffic, prioritize it
according to its relative importance, and use congestion-management and congestion-avoidance
techniques to provide preferential treatment. Implementing QoS in your network makes network
performance more predictable and bandwidth utilization more effective.
The QoS implementation for this release is based on the DiffServ architecture, an emerging standard
from the Internet Engineering Task Force (IETF). This architecture specifies that each packet is
classified upon entry into the network. 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 or
a Layer 3 packet are described here and shown in
Prioritization values in Layer 2 frames:
•
Layer 2 802.1Q 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 interfaces configured
as Layer 2 802.1Q trunks, all traffic is in 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.
Prioritization bits in Layer 3 packets:
•
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.
Figure 14
Encapsulated Packet
Layer 2
header
Layer 2 802.1Q/P Frame
Preamble
Layer 3 IPv4 Packet
Version
length
Note Layer 2 ISL Frame is not supported in this release.
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
30
16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
QoS Classification Layers in Frames and Packets
IP header Data
Start frame
DA SA
delimiter
ToS
Len ID
(1 byte)
DSCP
Figure 14:
Tag PT Data FCS
3 bits used for CoS (user priority)
Offset TTL Proto FCS IP-SA IP-DA Data