Quality of Service (QoS): Managing Bandwidth More Effectively

Using QoS Types To Configure QoS for Outbound Traffic

Table 6-8. How the Switch Uses the ToS Configuration

Outbound Port

 

ToS Option:

 

IP Precedence

Differentiated Services

 

(Value = 0 - 7)

 

IP Packet Sent Out

Depending on the value of the IP

For a given packet carrying a ToS codepoint that the switch has

an Untagged Port

Precedence bits in the packet’s

been configured to detect:

in a VLAN

ToS field, the packet will go to one

• Change the codepoint according to the configured policy and

 

of four outbound port queues in

assign the 802.1p priority specified for the new codepoint in

 

the switch:

the DSCP Policy Table (page 6-32).

 

1 - 2 = low priority

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

 

0 - 3 = normal priority

specified for the existing codepoint in the DSCP Policy Table

 

4 - 5 = high priority

(page 6-32).

 

6 - 7 = high priority

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

Same as above, plus the IP Prece-

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

an Untagged Port

dence value (0 - 7) will be used to

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

in a VLAN

set a corresponding 802.1p priority

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

 

in the VLAN tag carried by the

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

 

packet to the next downstream

802.1p setting if not prioritized by other QoS types.

 

device. Refer to table 6-9,below.

 

 

 

 

Table 6-9. 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

 

 

 

6-28