11.4: Voice QoS

11.4.1: QoS Trust

It is extremely essential to apply and practice the correct QoS level when moving voice traffic over a switched campus network. A trust boundary is defined for QoS in a network. Since each network mechanism has comparable QoS policies configured, QoS information within this boundary is trusted. QoS information arising from outside the boundary can either be overwritten for a particular circumstance or completely overwritten. When an IP Phone is set up as a component of the network, the QoS information transmitted by the phone can be trusted.

The IP Phone has the following two data sources:

• User PC data switch port: Packets from the user PC data switch port are produced somewhere else. Therefore, QoS information contained in these packets should not just be trusted to be accurate or truthful.

• VoIP packets native to the phone: The IP Phone produces these packets. Therefore, it can accurately manage QoS information that is integrated in these voice packets

A switch instructs a connected IP Phone on the manner in which it has to offer QoS trust to its user data switch port. This is done by using CDP messages. QoS Trust Configuration and Verification

Use the following interface configuration command to configure the trust extension:

Switch(config-if)# switchport priority extend {cos value | trust}

QoS contained in the packets from the user PC data switch port are not usually trusted because the PC's applications can attempt to send-up CoS or DSCP settings, in order to try and access premium network service. By using the cos keyword, the CoS bits are overwritten to value by the IP Phone, when packets are sent to the switch. The CoS values from the PC are overwritten to 0 when they cannot be trusted. However, the PC can be operating certain trusted applications that are authorized to request particular QoS levels. Use the trust keyword to enable the IP Phone to pass total QoS trust to the PC. The CoS values remain unchanged and are forwarded via the phone.

Use the following EXEC command to verify the manner in which QoS trust was extended to the IP Phone:

Switch# show mls qos interface type mod/num

Use the following EXEC command to verify the manner in which the IP Phone has been set up to deal with incoming QoS information from a connected PC or mechanism:

Switch# show interface type mod/num switchport

Use the following EXEC command to verify the switch port queuing methods:

Switch# show interface type mod/num capabilities

Use one of the following EXEC commands to verify the manner in which the CoS values places packets into egress port queues:



mls qos



mod/num Queuing








The first EXEC command is utilized by the Catalyst 3550 switches. The second EXEC command is utilized by the Catalyst 6500 switches.

11.4.2: Voice Packet Classification

The Cisco IP Phones makes use of the subsequent voice call control Skinny Protocols:

• Skinny Client Control Protocol (SCCP): TCP port 2000

• Skinny Station Protocol (SSP): TCP port 2001

• Skinny Gateway Protocol (SGP): TCP port 2002

Real-time Transport Protocol (RTP) conveys all voice-bearer traffic by utilizing UDP ports negotiated by call control protocols.

Switches that have to classify voice call control traffic that are utilized by IP Phones ought to match against the static TCP ports 2000 - 2002. Matching can be achieved by utilizing an IP Access List and the following command: match accessgroup.

A switch needs to single out RTP packets that are on the negotiated UDP port numbers in order to classify the voice-bearer traffic. This can be done by using the following NBAR command: match protocol rtp

A Cisco IP Phone sets its QoS information according to the rules listed below:

• PC data packets can be left in its original state or can be marked with a configurable CoS value

• SCCP voice control packets get CoS 3, IP Precedence, and DSCP 26 - AF31

• RTP voice-bearer packets get CoS 5, IP Precedence 5 and DSCP 46 - EF

The default switch behaviour supplies the suitable QoS to voice traffic. Packets with CoS 3 are normally located in the higher threshold lower-priority standard queue. Packets with CoS 5 are located in the strict-priority queue. Voice call control packets are marked with CoS 3.They are placed into the egress queues for better service than normal data. Voice bearer packets are marked with CoS 5. They placed into the strict-priority queue.