background image
1-13
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 1 Overview
What is the Quality of Service Toolset?
Class of Service
Packets can be marked as important by using Layer 2 Ethernet CoS settings in the User Priority bits of
the 802.1p portion of the 802.1Q header (as shown in
Figure 1-7
).
Figure 1-7
Layer 2 Ethernet (802.1Q) CoS Settings
Type of Service and Differentiated Services Code Points
As Layer 2 media often changes as a packet travels from source to destination, a more ubiquitous
classification would occur at Layer 3. The second byte in an IPv4 packet is the Type of Service (ToS)
byte. The first three bits (by themselves) of the ToS byte are referred to as the IP Precedence bits. These
same three bits, in conjunction with the next three bits, are known collectively as the DSCP bits. These
bits are illustrated in
Figure 1-8
.
Figure 1-8
Layer 3 ToS and DSCP Settings
PREAM. SFD
DA
PRI
CFI
VLAN ID
SA
Type
PT
DATA
FCS
TAG
4 Bytes
Three bits used for CoS
(User priority)
81034
Version
Length
ToS
1 Byte
LEN
7
6
5
4
3
2
1
0
ID
Offset
Proto
FCS IP-SA IP-DA Data
TTL
Unused bits;
Flow control for
DS CP
IP precedence
DS CP
Standard IPV4: Three MSB called IP precedence
(DiffServ may use six D.S. bits plus two for flow control)
81035
Layer 3 IPV4