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

Outbound Port

 

ToS Option:

 

802.1p (Value = 0 - 7)

Differentiated Services

 

 

 

IP Packet Sent Out

Depending on the value of the IP

For a given packet carrying a ToS codepoint that the switch

an Untagged Port

Precedence bits in the packet’s ToS

has been configured to detect:

in a VLAN

field, the packet will go to one of four

• Change the codepoint according to the configured policy

 

outbound port queues in the switch:

and assign the 802.1p priority specified for the new

 

1-2 = low priority (queue 1)

codepoint in the DSCP Policy Table (page 5-33).

 

• Do not change the codepoint, but assign the 802.1p priority

 

 

 

0-3 = normal priority (queue 2)

specified for the existing codepoint in the DSCP Policy

 

Table (page 5-33).

 

 

 

4-5 = medium priority (queue 3)

Depending on the 802.1p priority used, the packet will leave

 

the switch through one of the following queues:

 

 

 

6-7 = high priority (queue 4)

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

Same as above, plus the IP Prece-

Same as above, plus the Priority value (0 - 7) will be used to

an Untagged Port

dence value (0 - 7) will be used to set

set a corresponding 802.1p priority in the VLAN tag carried by

in a VLAN

a corresponding 802.1p priority in the

the packet to the next downstream device. Where No-over-

 

VLAN tag carried by the packet to the

ride is the assigned priority, the VLAN tag carries a “0” (normal

 

next downstream device. Refer to

priority) 802.1p setting if not prioritized by other QoS classi-

 

table 5-8,below.

fiers.

 

 

 

Table 5-8. ToS IP-Precedence Bit Mappings to 802.1p Priorities

ToS Byte IP

Corresponding

Service Priority Level

Precedence Bits

802.1p Priority

 

 

 

 

000

1

Lowest

001

2

Low

002

0

Normal

003

3

 

004

4

 

005

5

 

006

6

 

007

7

Highest

 

 

 

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