background image
3-18
Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
956467
Chapter 3 QoS in an AVVID-Enabled Campus Network
Selecting an Access-Layer Switch
Step 5
Write the ACL to hardware.
cat6k-access> (enable) commit qos acl ACL_IP-PHONES
Step 6
Map the access list to the Auxiliary VLAN.
cat6k-access> (enable) set qos acl map ACL_IP-PHONES 110
Once QoS has been enabled on the Catalyst 6500 access-layer switch, you can use the following
command to place all traffic with a CoS of 3 (VoIP control) into the second transmit queue (with a low
drop threshold), to ensure successful call control during periods of heavy congestion. All traffic with a
CoS of 5 (VoIP RTP Bearer) is placed into the second queue by default.
cat6k-access> (enable) set qos map 2q2t tx 2 1 cos 3
Configuring the Uplink to the Distribution Switch
Once you have configured the access port queuing, you must also configure the uplink interfaces to the
distribution, or core, switch. This involves enabling trust for Ethernet frames coming into the trunk port
(1/1 in this example), manipulating the CoS-to-queue mapping entrance criteria, and mapping the CoS
and IP Precedence values to the appropriate DSCP value.
Configuring Transmit Queues
All VoIP (CoS of 5) traffic will be placed into the egress interface priority queue on 1p2q2t interfaces
and queue 2 on 2q2t interfaces as soon as you enable QoS. However, you must perform the additional
step of configuring the Catalyst 6500 CoS queue admission rules to ensure that traffic with a CoS of 3
(VoIP control) is placed into the second queue.
Step 1
Place CoS of 3 in queue 2 for 1p2q2t.
cat6k-access> (enable) set qos map 1p2q2t tx 2 1 cos 3
Step 2
Place CoS of 3 in queue 2 for 2q2t.
cat6k-access> (enable) set qos map 2q2t tx 2 1 cos 3
Modifying CoS-to DSCP and IP Precedence-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 CoS and
Layer 3 IP Precedence settings correctly to these DSCP values, you must modify the default CoS/IP
Precedence-to-DSCP mappings.
Step 1
Modify the CoS-to-DSCP mappings.
cat6k-access> (enable) set qos cos-dscp-map 0 8 16
26 34 46 48 56
Step 2
Modify the IP Precedence-to-DSCP mappings.
cat6k-access> (enable) set qos ipprec-dscp-map 0 8 16
26 34 46 48 56