6-13
Quality of Service (QoS): Managing Bandwidth More Effectively
Preparation for Configuring QoS
Preparation for Configuring QoS
QoS operates in VLAN-tagged and VLAN-untagged environments. If your
network does not use multiple VLANs, you can still implement the 802.1Q
VLAN capability for packets to carry their 802.1p priority to the next
downstream device. To do so, configure ports as VLAN-tagged members on
the links between switches and routers in your network infrastructure.
Table 6-6. Summary of QoS Capabilities
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 QoS 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 6-5.)
c. IP Type-of-Service Precedence Bits (Leftmost three bits in the ToS
field of IP packets)
d. VLAN
e. Interface
For more on how QoS operates with the preceding traffic types, see
‘‘Precedence Criteria for QoS types’’, on page 6-11.)
Outbound Packet Options Port Membership in VLANs
Tagged Untagged
Control Port Queue Priority for Packet Types Yes Yes
Carry 802.1p Priority Assignment to Next Downstream Device Yes No
Carry DSCP Policy to Downstream Devices. The policy includes: Yes
1
Yes
1
Assigning a ToS Codepoint
Assigning an 802.1p Priority
2
to the Codepoint
1
Except for non-IPv4 packets or packets processed using the QoS IP-Precedence method,
which does not include the DSCP policy option. 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 VLAN-tagged environment, this priority is also assigned
as the 802.1p priority carried outbound in packets having an 802.1Q field in the header.
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