Quality of Service (QoS): Managing Bandwidth More Effectively
Using QoS Classifiers To Configure QoS for Outbound Traffic
Table 6-9. How the Switch Uses the ToS Configuration
Outbound Port ToS Option:
IP Precedence
(Value = 0 - 7)
Differentiated Services
IP Packet Sent Out
an Untagged Port
in a VLAN
Depending on the value of the IP
Precedence bits in the packet’s
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 6-58).
• Do not change the codepoint, but assign the 802.1p priority
specified for the existing codepoint in the DSCP Policy Table
(page 6-58).
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.
IP Packet Sent Out
an Untagged Port
in a VLAN
Same as above, plus the IP Prece-
dence value (0 - 7) will be used to
set a corresponding 802.1p priority
in the VLAN tag carried by the
packet to the next downstream
device. Refer to table 6-10, below.
Same as above, plus the Priority value (0 - 7) will be used to set
a corresponding 802.1p priority in the VLAN tag carried by the
packet to the next downstream device. Where No-override is the
assigned priority, the VLAN tag carries a “0” (normal priority)
802.1p setting if not prioritized by other QoS classifiers.
Table 6-10. ToS IP-Precedence Bit Mappings to 802.1p Priorities
ToS Byte IP
Precedence Bits
Corresponding
802.1p Priority
Service Priority Level
000 1 Lowest
001 2 Low
002 0 Normal
003 3
004 4
005 5
006 6
007 7 Highest
6-45
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