Switchport Connections to Voice Vlans and Qos/Cos

*Because the sound quality of an IP phone call can deteriorate if the data is unevenly transmitted, the switch supports quality of service (QoS) based on IEEE 802.1P class of service (CoS)
*The Port Fast feature is automatically enabled when voice VLAN is configured
-Expansion on this, as soon as I enter in
Switchport access voice vlan 100 on fa0/1
if I
Sh run int Fa0/1
it will have spanning-tree portfast enabled.
*802.1p = CoS
*Voice Vlan is only supported on access ports and not on trunk ports..
but, you can do it.  I think the issue with this is that you used to be able to do this since you could set a trunk with another Cisco switch if it uses dot1q and start VLAN-hopping if all VLANS are allowed… this is why we use the switchport access vlan #
*Interesting thing about voice is spanning-tree, there are actually multiple instances of it ran if you have a Voice Vlan with the switchport Voice Vlan ID VVID, it will show up as a second instance.
*CDP has to be enabled on the interface connected to the voice port.  Reason why is if I put a computer or something on that interface, it will not work!!!  Phones will Tag their VLANs for Voice vlan 110 lets say and use CDP to enable it Security!!!!  If you are not using Cisco phones in this case then oh noeeessss!!!
*Most Cisco IP phones contain a 3 port switch internal
*Skinny – > When a IP phone picksup the phone to the CCM

Commands to setup voice
Switchport access voice vlan 110
Switchport access voice vlan dot1p
Switchport access voice vlan untagged
Switchport access voice ?

Configure how the Cisco IP Phone carries voice traffic:

•vlan-id—Configure the Cisco IP Phone to forward all voice traffic through the specified VLAN. By default, the Cisco IP Phone forwards the voice traffic with an 802.1Q priority of 5. Valid VLAN IDs are from 1 to 4094.

•dot1p—Configure the Cisco IP Phone to use 802.1p priority tagging for voice traffic and to use the default native VLAN (VLAN 0) to carry all traffic. By default, the Cisco IP Phone forwards the voice traffic with an 802.1p priority of 5.

•none—Allow the IP phone to use its own configuration to send untagged voice traffic.

•untagged—Configure the phone to send untagged voice traffic.

Commands to verify

Sh Vlan x
Sh int fa0/1 switchport
Sh Spanning-tree x

Wireless stuff

-QoS/Cos When it comes to Voice

-One thing to keep in mind with QoS and layer 2 switching is that since its layer 2 every frame is going to look the same to a switch, so adding any QoS in a enviornment is added a switch that switches that layer 2 frame will not put any kind of priority on it.  At Layer 2 at least.

That is why we use a pile of Junk Called CoS(Class of service)

CoS – Is used so there is no need for a QoS At layer 2, this works so that there are no opening of packets since it cannot… and reduces over head since there is no type of stateful inspection.

ToS – Is a Layer 3 type of Inspection where also reduces overhead since there does not have to be a statefull connection to open up the packet and look inside to view what exactly is it so it is tagged at a ToS Field.

Marking is Marking CoS or ToS as a certain #… Voice by default is a CoS #5. you can go 0-7.

*When CoS is applied within a Dot1q tag on a frame.  CoS Is normally added within that frame within Switch Boundries to make “like” QoS Decisions.

Delay – > Time Required to send a packet from A to B

Jitter – > The best way for me to remember this is if I was streaming internet radio, if half way through the packets drop, no mussicczzzz then start again.  Voice is very sensitive to jitter since a conversation cannot drop packets.

Loss – > Packets being congested and dropped without being Delivered.

QoS Can be delivered amoung three different Ways…

Best Effort – > This is not exactly QoS… when a network forwards packets amoung what order it came in on, this is more for as quick as possible rather than quality. So voice packets are the same quality as data packets which ever ones are first get switched first.

Integrated Services Model – > I might have to come back to this but what it sounds like is QoS Will check every different path it will take if that is the following and check to see if it is okay congested or the fastest way to route / switch.

Differentiated services Model(DSCP) – > What this will do is put different services on different QoS Levels, allowing it to mark a certain number for the priority… everything else that is not QoS is simply at a best effort.

Classification – > How important a packet is to QoS Can be mapped back to a ACL,Protocol(TCP/UDP) Port number or in some cases stateful inspection)

Trust Boundry – > Something I might have to come back on, but usually where a network would connect its edge to end users on a Acces level switch.  This is sort of where the ToS and DSCP come into play. *KEEP IN MIND* a IP phone since it has a internal switch it is always considered part of the trusted network.

How to Enable QoS on a Interface for a Phone?

Switch(Config)#mls qos
When this is entered in all Switchports are untrusted untill further commands are entered on each individual interface.  This is what I view as turning on QoS on a Switch.

Switch(config-if)# mls qos trust {cos | ip-precedence | dscp}

This is clearly entered in on a Per interface line level.  for Incomming packets.

Switch(config-if)# mls qos trust device cisco-phone

This is just like using the VVID this will only trust the packets if a Phone is plugged in and acitvely sending CDP packets.

Switch(Config-if)# mls qos trust cos

Should in most cases be used on trunk ports.


Uses the best possible QoS Features, I would say this is the best bargain here….

Switch(config-if) auto qos voip {cisco-phone | cisco-softphone | trust}

In this case voip ciscophone/cisco softphone are pretty obvious, trust would be a router – > switch switch – > switch

Switch(config-if) auto qos voip

Is a Marco that will configure all the commands for you.

How to Check is QoS is functioning?

Sh run int Fa0/1
sh mls qos int Fa0/1
Sh int fa0/1 switchport – there is a trust field.
sh auto qos int fa0/1

Post a comment or leave a trackback: Trackback URL.


  • seal a meal vacuum food sealer vs230  On May 16, 2013 at 3:34 am

    Wow, wonderful blog layout! How long have you ever been running
    a blog for? you made blogging look easy. The entire look of your site
    is wonderful, as well as the content!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: