Quality of Service (QoS): Managing Bandwidth More Effectively

Preparation for Configuring QoS

Preparation for Configuring QoS

You can use QoS regardless of whether your network has tagged VLANs:

Table 15-5. Summary of QoS Capabilities

Outbound Packet Options

Tagged VLAN

No Tagged

 

Environment

VLANs

 

 

 

Control Queue Priority for Packet Types

Yes

Yes

Carry the 802.1p Priority Assignment to Next Downstream Device

Yes

No

Configure a Service Policy and Carry It to Downstream Devices.

Yes 1

Yes 1

The policy includes:

 

 

Assigning a ToS Codepoint

 

 

Assigning an 802.1p Priority 2 to the Codepoint

 

 

1Except for packets processed using either the (Layer 3) Protocol or QoS IP-Precedence methods. Also, to use a service policy in this manner, the downstream devices must be configured to interpret and use the DSCP carried in the IP packets.

2This priority corresponds to the 802.1p priority scheme and is used to determine the packet’s port queue priority. When used in a tagged VLAN environment, this priority is also assigned as the 802.1p priority carried outbound in tagged VLAN packets.

Steps for Configuring QoS on the Switch

1.Determine the QoS policy you want to implement. This includes analyzing the types of traffic flowing through your network and identifying one or more traffic types to prioritize. In order of precedence, these are:

a.UDP/TCP applications

b.Device Priority—destination or source IP address (Note that destina­ tion has precedence over source. See Table 15-4.)

c.IP Type-of-Service Precedence Bits (Leftmost three bits in the ToS field of IP packets)

d.IP Type-of-Service Differentiated Service bits (Leftmost six bits in the ToS field of IP packets)

e.Protocol Priority

f.VLAN Priority (requires at least one tagged VLAN on the network)

g.Source-Port

h.Incoming 802.1p Priority (requires at least one tagged VLAN on the network)

For more on how QoS operates with the above traffic types, see Table 15- 4. "Precedence Criteria for QoS Classifiers" on page 15-9.)

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