Quality of Service (QoS): Managing Bandwidth More Effectively

Using QoS Classifiers to Configure Quality of Service for Outbound Traffic

a.Configure a specific DSCP with a specific priority in an edge switch.

b.Configure the switch to mark a specific type of inbound traffic with that DSCP (and thus create a policy for that traffic type).

c.Configure the internal switches in your LAN to honor the policy.

(For example, you could configure an edge switch to assign a codepoint of 000001 to all packets received from a specific VLAN, and then handle all traffic with that codepoint at high priority.)

For a codepoint listing and the commands for displaying and changing the

DSCP Policy table, refer to “Differentiated Services Codepoint (DSCP)

Mapping” on page 5-33.

■Precedence Bits: This element is a subset of the DSCP and is comprised of the upper three bits of the ToS byte. When configured to do so, the switch uses the precedence bits to determine a priority for handling the associated packet. (The switch does not change the setting of the prece- dence bits.) Using the ToS Precedence bits to prioritize IPv4 packets relies on priorities set in upstream devices and applications.

Figure 5-10shows 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

Source MAC

802.1Q Field

Type &

ToS Byte

…

 

 

MAC Address

Address

 

Version

 

 

 

 

 

 

 

 

 

 

 

Packet:

FF FF FF FF FF FF

08 00 09 00 00 16

08 00

45

E 0

...

 

 

 

 

 

 

 

 

 

Differentiated Services Codepoint

Precedence

 

 

 

Rsvd.

 

 

Bits

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

1

1

0

0

0

0

0

 

 

 

 

 

 

 

 

 

 

E

 

 

 

0

 

 

 

 

 

 

 

 

 

Figure 5-10. The ToS Codepoint and Precedence Bits

5-25