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Understanding QoS
Figure 30-3
Layer 2 IEEE 802.1Q and IEEE 802.1P Frame
Preamble
Layer 3 IPv4 Packet
Version
These sections contain additional information about classification:
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Class Maps
As explained previously, you use an MQC class map to name a specific traffic flow (or class) and to
isolate it from all other traffic. A class map defines the criteria used to match against a specific traffic
flow to further classify it. If you have more than one type of traffic that you want to classify, you can
create another class map and use a different name. When you enter the class-map command with a
class-map name, the switch enters the class-map configuration mode. In this mode, you define the match
criterion for the traffic by using the match class-map configuration command. After a packet is matched
against the class-map criteria, it is acted on by the associated action specified in a policy map.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
30-6
QoS Classification Layers in Frames and Packets
Start frame
DA
delimiter
3 bits used for CoS
(IEEE 802.1D user priority)
ToS
Len
length
1 Byte
7
6
5
IP precedence
DSCP
Standard IPv4:
MSBs called IP precedence
"Class Maps" section on page 30-6
"The match Command" section on page 30-7
"Classification Based on Layer 2 CoS" section on page 30-7
"Classification Based on IP Precedence" section on page 30-8
"Classification Based on IP DSCP" section on page 30-8
"Classification Comparisons" section on page 30-9
"Classification Based on QoS ACLs" section on page 30-10
"Classification Based on QoS Groups" section on page 30-10
TAG
SA
Type
4 Bytes
PRI
CFI
ID
Offset
TTL
Proto
4
3
2
Chapter 30
PT
Data
VLAN ID
FCS
IP-SA
IP-DA
1
0
Flow control
for DSCP
Configuring QoS
FCS
Data
78-17058-01
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