3-49
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 3 QoS in an AVVID-Enabled Campus Network
Selecting a Distribution-Layer Switch
Configuring the Distribution Layer with a Layer 2 Switch
Once you have enabled QoS on the distribution-layer switch and have modified the default queue
admission, do the following to complete the integration with a Layer 2 access switch:
Step 1
Enable trust for CoS values from adjacent Layer 2 access switches. Use port-based QoS on the trunking
port and use mls qos trust cos. This configuration is used when the access-layer switch is only a Layer 2
device performing CoS classification.
Cat6k-distrib (config)#interface GigabitEthernet2/2
Cat6k-distrib (config-if)#no ip address
Cat6k-distrib (config-if)#wrr-queue cos-map 2 1 3 4
Cat6k-distrib (config-if)#mls qos vlan-based
Cat6k-distrib (config-if)#mls qos trust cos
Cat6k-distrib (config-if)#switchport
Cat6k-distrib (config-if)#switchport trunk encapsulation dot1q
Cat6k-distrib (config-if)#switchport mode trunk
Cat6k-distrib (config)#interface GigabitEthernet3/1
Cat6k-distrib (config-if)#no ip address
Cat6k-distrib (config-if)#wrr-queue cos-map 2 1 3 4
Cat6k-distrib (config-if)#mls qos vlan-based
Cat6k-distrib (config-if)#mls qos trust cos
Cat6k-distrib (config-if)#switchport
Cat6k-distrib (config-if)#switchport trunk encapsulation dot1q
Cat6k-distrib (config-if)#switchport mode trunk
Step 2
Configure CoS-to-DSCP mappings. Cisco follows the IETF recommendations for setting the DSCP
classification values for both the VoIP control plane traffic and VoIP bearer or media plane traffic. The
recommended settings are DSCP of AF31 for VoIP control plane and DSCP of EF for VoIP bearer plane.
To map the Layer 2 settings correctly to these DSCP values, you must modify the default CoS-to-DSCP
mappings as follows:
Cat6k-distrib (config-if)#mls qos map cos-dscp 0 8 16
26 34 46 48 56
Configuring QoS Policies and Layer 3 Access Lists for VoIP Control Traffic
In addition, you must configure QoS policies and Layer 3 access lists for VoIP control traffic.
The QoS configuration for the Catalyst 6500 with Native Cisco IOS is similar to the QoS configuration
for Cisco WAN routers, except how policing is used for marking traffic flows and how service policies
are applied to VLAN interfaces.
With Native Cisco IOS:
·
The physical gigabit Ethernet uplink ports are configured to use VLAN-based QoS with the MLS
QoS VLAN-based Native Cisco IOS interface commands.
·
The service-policy is applied to all VLAN traffic inbound on the uplink.
In the example below, three classes are defined: one for the VoIP media stream, one for the control
traffic, and one for all other traffic. Traffic is filtered for these classes based on the Layer 3 or 4 source
and destination IP addresses and ports. Each of these classes is referenced in the Voice-QoS policy map.
In the policy-map statements, a policing function is used to classify all traffic that meets the entrance
criteria matched with the class-map access lists.
The Catalyst 6500 Native Cisco IOS software does not support the set ip dscp commands. Although it
can be enabled, it should not be used because only software switched packets will be classified. Instead,
the policing algorithm should be used to classify traffic.