Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Telephony: Congiguring Voice VLANS

Hello Learned Friends. Some help required here!! :-)

1. Whats Layer 3 Precedence mean/do &

what does Layer 2 COS mean(I know

what it does)?

2. When configuring Voice VLAN:-

Whats the trusted mode/ untrusted

mode by CDP request from the switch

access port & how is it achieved?

Does it have any link to the cmd "mls

qos trust cos"? What actually happens

at these steps?

3. Has anyone any clearer steps on this

whole configuration process than what

Mr. Lammle(SYBEX) shows? Maybe with

schematics,flowcharts & diagrams? I'm



BS =)

Hall of Fame Super Silver

Re: Telephony: Congiguring Voice VLANS

Hello Baljit,


with the introduction of Quality of Service in the IP networks and the differentiated Services models packets (OSI L3) or ethernet Frames (OSI L2) can be treated in a different manner according to their marking:

TOS or DSCP is a form to signal that an IP packet has to treated according to the value: for example in the case of VoIP packets that are part of the voice conversation are marked with TOS 5 or DSCF EF = expedit forwarding that means that these packets are placed in a prioritizated queue

802.1Q frames are ethernet frames with Vlan tagging information (that tells to which Vlan the frame belong) in the 4 bytes 802.1Q header there are 3 bits called CoS = Class of Service specified in 802.1p that allows to implement differentiated services in a LAN switched environment.

So an IP packet traveling inside an ethernet frame out a 802.1Q trunk port has:

a CoS 3 bits field in the L2 802.1Q header

a TOS or DSCP settings inside the IP header (L3).

Usually CoS is a copy of TOS 3leftmost bits of DSCP.


Multilayer switches implement QoS using the trust boundary concept:

mls qos trust instructs the switch to accept the CoS settings on received frames (accept=don't modify)

the switch associate to the frame an internal DSCP that reflects the CoS settings seen.

If the port is not trusted all incoming frames will have their CoS and ToS /DSCP set to a single value (0 by default or the CoS value set for the port with mls qos cos x)

CDP version 2 is used to communicate with a Cisco IP Phone: what is the voice vlan (the voice vlan tag to be used), the power to be provided by the switch if the port supports PoE and is configured to provide it, the QoS behaviour that the IP phone has to use on downstream connected frames.

So the switch can even extend the trust boundary to the IP phone port to the PC.

The phone will send VoIP packets with CoS 5 and DSCP = EF.

Again frames coming from PC can be:

untrusted with CoS reset to 0

trusted = accepted


on modern IOS switches

int fas0/1


switchport access vlan

switchport mode access

switchport voice vlan

from a L2 point of view this is an implicit 802.1Q trunk with untagged frames in data-vlan and frames tagged with voice-vlan

Older switches may require

int f0/1


switchport trunk enc dot1q

switchport mode trunk

switchport native vlan

switchport voice vlan

switchport trunk allowed vlan ,

Hope to help