Life of a High Priority VoIP Packet

As it begins its life in the IP Phone, the VoIP packet is immediately marked with both of the following:

■ Layer 2: 802.1Q CoS = 5 (highest priority in an Ethernet LAN)

■ Layer 3: DSCP = EF (highest priority in an IP network)

Note The IEEE 802.1Q standard is an IEEE specification for implementing VLANs in Layer 2

switched networks. IEEE 802.1Q and its use in QoS will be discussed further in the "Classification and Marking" module in this course.

With the frame marked at class of service (CoS) = 5 and differentiated services code point (DSCP) = Expedited Forwarding (EF), this frame should receive priority treatment every time it encounters any QoS mechanism in the network.


A User Datagram Protocol (UDP) header is used for voice packets rather than TCP.

Notice that a Real-Time Transport Protocol (RTP) header has been added because this is a voice packet. RTP helps synchronize real-time transmissions such as voice by time-stamping packets so that they can be resynchronized at the receiving end. This helps minimize jitter.

In the Cisco Catalyst 2950 switch, CoS = 5 means to treat the frame with priority queuing (PQ). This means that you should move the frame before any other frames with a lower CoS.

The default CoS-to-DSCP mapping is set to recognize the CoS = 5 as DSCP = 40. The EF value is 46 on input to the switch as set by the IP Phone. Because the default CoS-to-DSCP marking is CoS 5 = DSCP 40 in the Cisco 2950 switch (not 46), DSCP is set to 40 on output.

When the frame arrives at the Cisco Catalyst 2950 switch, it is instantly recognized as a high-priority frame because of the CoS = 5 and is immediately placed in the high-priority, no-delay queue. Because the switch recognizes the frame as a CoS = 5, it re-marks the DSCP field to 40.

Default CoS-to-DSCP Mapping in Cisco Catalyst 2950 Switch

COS Value

DSCP Value

















When the packet hits the edge router, the router recognizes the packet as a voice packet due to the DSCP = 40 setting (as was set by the Cisco Catalyst 2950 switch).

The packet is immediately dispatched ahead of any nonvoice packets using low-latency queuing (LLQ). LLQ is designed to provide instant dispatch of voice packets ahead of data while carefully managing the dispatch of data.

If the link to the service provider is a relatively slow link, both header compression (in this case, class-based RTP header compression) and link fragmentation and interleaving (LFI) would be employed to improve the bandwidth efficiency of the link.

If the WAN link is a Frame Relay link, the packet would use Frame Relay traffic shaping (FRTS) and Frame Relay Forum 12 (FRF.12).

Note Both of these technologies are explained further in the "Traffic Shaping and Policing" module in this course.

When the packet arrives at the service provider, the service provider reclassifies the packet to fit within the service provider QoS classification policy.

In this case, the service provider has defined these four traffic classes:

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