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5-26
Quality of Service (QoS): Managing Bandwidth More Effectively
Using QoS Classifiers to Configure Quality of Service for Outbound Traffic
Table 5-7. How the Switch Uses the ToS Configuration
Table 5-8. ToS IP-Precedence Bit Mappings to 802.1p Priorities
Outbound Port ToS Option:
802.1p (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 (queue 1)
0-3 = normal priority (queue 2)
4-5 = medium priority (queue 3)
6-7 = high priority (queue 4)
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 5-33).
• Do not change the codepoint, but assign the 802.1p priority
specified for the existing codepoint in the DSCP Policy
Table (page 5-33).
Depending on the 802.1p priority used, the packet will leave
the switch through one of the following queues:
1 - 2 = low priority (queue 1)
0 - 3 = normal priority (queue 2)
4 - 5 = medium priority (queue 3)
6 - 7 = high priority (queue 4)
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 5-8, 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-over-
ride is the assigned priority, the VLAN tag carries a “0” (normal
priority) 802.1p setting if not prioritized by other QoS classi-
fiers.
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