Cisco ME 3400 - Page 632

33-6
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-07
Chapter 33 Configuring QoS
Understanding QoS
• Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports the use
of either value because DSCP values are backward-compatible with IP precedence values.
IP precedence values range from 0 to 7. DSCP values range from 0 to 63.
• Output remarking is based on the Layer 2 or Layer 3 marking type, marking value and packet type.
Figure 33-3 QoS Classification Layers in Frames and Packets
These sections contain additional information about classification:
• “Class Maps” section on page 33-7
• “The match Command” section on page 33-7
• “Classification Based on Layer 2 CoS” section on page 33-8
• “Classification Based on IP Precedence” section on page 33-8
• “Classification Based on IP DSCP” section on page 33-8
• “Classification Comparisons” section on page 33-9
• “Classification Based on QoS ACLs” section on page 33-10
• “Classification Based on QoS Groups” section on page 33-10
• “Classification Based on VLAN IDs” section on page 33-12
141151
Layer 2 IEEE 802.1Q and IEEE 802.1p Frame
Preamble
Start frame
delimiter
DA
Len
SA Type
TAG
2 Bytes
PT Data FCS
Layer 3 IPv4 Packet
Version
length
To S
1 Byte
ID Offset TTL Proto FCS IP-SA IP-DA
PRI
7
6 5 4 3 2 1 0
CFI VLAN ID
Data
3 bits used for CoS
(IEEE 802.1p user priority)
IP precedence
DSCP
Standard IPv4:
MSBs called IP precedence
Flow control
for DSCP