Product guide

Quality of Service (QoS): Managing Bandwidth More Effectively
Using QoS Classifiers To Configure Quality of Service for Outbound Traffic
Figure 8-23 shows an example of the ToS byte in the header for an IPv4 packet,
and illustrates the diffserv bits and precedence bits in the ToS byte. (Note that
the Precedence bits are a subset of the Differentiated Services bits.)
Field: Destination
MAC Address
Source MAC
Address
802.1Q Field Type &
Version
ToS Byte …
Packet: FF FF FF FF FF FF 08 00 09 00 00 16 08 00 45
E 0 ...
Differentiated Services Codepoint
Rsvd.
Precedence
Bits
1 1 1 0 0 0 0 0
E 0
Figure 8-23. The ToS Codepoint and Precedence Bits
Table 8-12. How the Switch Uses the ToS Configuration
Outbound Port ToS Option:
IP Precedence
(Value = 0 - 7)
Differentiated Services
IP Packet Sent Out Depending on the value of the IP
an Untagged Port Precedence bits in the packet’s
in a VLAN ToS field, the packet will go to one
of four outbound port queues in
the switch:
1 - 2 = low priority
0 - 3 = normal priority
4 - 5 = high priority
6 - 7 = high priority
For a given packet carrying a ToS codepoint that the switch has
been configured to detect:
• Change the codepoint according to the configured policy and
assign the 802.1p priority specified for the new codepoint in
the DSCP Policy Table (page 8-64).
• Do not change the codepoint, but assign the 802.1p priority
specified for the existing codepoint in the DSCP Policy Table
(page 8-64).
Depending on the 802.1p priority used, the packet will leave the
switch through one of the following queues:
1 - 2 = low priority
0 - 3 = normal priority
4 - 5 = high priority
6 - 7 = high priority
If No-override (the default) has been configured for a specified
codepoint, then the packet is not prioritized by ToS and, by
default, is sent to the “normal priority” queue.
8-48