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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
Environment
No Tagged
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
1
Except 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.
2
This 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.)
15-11