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Figure 24
Bandwidth Versus Quality of Service
Low bandwidth
needs QoS
High bandwidth
might not need QoS
IP Precedence
Use the ip precedence command to give voice packets a higher priority than other IP data traffic. Every IP packet is given a
precedence level: the numbers 1 through 5 identify classes for IP flows; the numbers 6 through 7 are used for network and
backbone routing and updates. You can configure voice packets for higher priority by setting the IP precedence value to 5.
Internal routers using weighted fair queuing give these packets priority. This command applies only to VoIP dial peers. The
following example sets the IP precedence to 5:
Router> enable
Password:
Router# configure terminal
Router(config)# dial-peer voice 221 voip
Router(config-dial-peer)# ip precedence 5
Use the ip precedence command if RSVP is not enabled and if you would like to give voice packets a higher priority than other
IP data traffic.
Low Latency Queuing
Low latency queuing (LLQ) provides a low-latency, strict-priority transmit queue for real-time traffic, such as VoIP traffic.
Strict-priority queuing allows delay-sensitive data such as voice to be dequeued and sent first (before packets in other queues
are dequeued), giving delay-sensitive data preferential treatment over other traffic. This reduces jitter in voice conversations.
Configuring LLQ
Ensure that the voice and data packets have different IP precedence values. This will allow the router to differentiate between
them. Normally, data packets should have an IP precedence of 0, while voice packets should have an IP precedence of 5. If the
VoIP packets are generated from within the router, you may set IP precedence to 5 for these packets by using the procedure in
the "IP Precedence" section on page 32.
Create an access list and a class map for the voice packets.
Router(config)# access-list 101 permit ip any any precedence 5
Router(config)# class-map match-all voice
Link the class map to the access list.
Router(config-cmap)# match access-group 101
Configure LLQ for voice traffic by creating a policy map and defining its class.
Router(config)# policymap mypolicy
Router(config-pmap)# class voice
Assign priority bandwidth to the voice traffic. The priority bandwidth assigned (in kilobits per second) depends on the codec
used and the number of simultaneous calls allowed.
Router(config-pmap-c)# priority 200
Attach LLQ to the dialer interface, and create a service policy.
Router(config)# interface dialer 1
Router(config-if)# service-policy out mypolicy
32
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