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Quality of Service: Managing Bandwidth More Effectively
Using Quality of Service Policies
Edge Switch
Honor Priority
Downstream
Switch
Classify inbound traffic
on these Class-of-
Honor New Priority
Downstream
Downstream
Tagged VLANs on some
Service (CoS) types:
Switch
Switch
or all inbound and
• IP-device (address)
outbound ports.
Tagged VLANs on
Tagged VLANs on at
• Protocol (LAN)
inbound and outbound
least some inbound
ports.
Classify inbound traffic
ports.
on CoS types.
• VLAN-ID (VID).
Traffic arrives with
Traffic arrives with the
priority set by edge
Change priority on
• Source-Port
priority set in the VLAN
Apply 802.1p priority to
selected CoS type(s).
switch
tag. Carry priority
Forward with 802.1p
selected outbound
downstream on tagged
priority.
Forward with 802.1p
traffic on tagged VLANs.
VLANs.
Set Priority
priority.
Change Priority
Figure 6-1. Example of 802.1p Priority Based on CoS (Class-of-Service) Types and
Use of VLAN Tags
Honor Policy
Downstream
Switch
Edge Switch
Classify inbound traffic
on IP-device (address)
Honor New Policy
Downstream
Downstream
Classify on ToS DiffServ
and VLAN-ID (VID).
Switch
Switch
and Other CoS
Apply DSCP markers to
Traffic arrives with DSCP
Classify on ToS Diffserv
selected traffic.
Apply new DSCP markers
markers set by edge
to selected traffic.
switch
Set Policy
Classify on ToS DiffServ.
Change Policy
Figure 6-2. Example Application of Differentiated Services Codepoint (DSCP)
Policies
Apply QoS to inbound traffic at the network edge. At the edge switch,
QoS classifies certain traffic types and in some cases applies a DSCP policy.
At the next hop (downstream switch) QoS honors the policies established at
the edge switch. Further downstream, another switch may reclassify some
traffic by applying new policies, and yet other downstream switches can be
configured to honor the new policies.
Preserve QoS in outbound traffic in a VLAN. QoS is implemented in the
form of rules or policies that are configured on the switch. Although you can
use QoS to prioritize traffic only while it moves through the switch, you derive
the maximum benefit by using QoS in an 802.1Q VLAN environment (with
802.1p priority tags) or in an untagged VLAN environment (with DSCP policies
in which QoS sets priorities that downstream devices can support without re-
classifying the traffic).
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