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Voice VLANs and 802.1p

Hi folks,

I'm trying to get my head around the command...

switchport voice vlan dot1p

Here's what I think I understand...

The device and the IP phone use the same subnet.

The phone uses VLAN0

The Device uses Native VLAN of switchport

The phone and device can't see each other because of the VLAN Id Tag because it's not routed.

Dot1p is 3bit of a dot1Q.

Here's what I definetly don't understand...

If dot1p is 3bit of a dot1Q tag then does the port need to be a trunk port?

Can dot1p work with an Access port?

Does dot1p automatically configure the phone with a VLAN ID of 0?

What does all this mean if I have NEC phones compared to Cisco phones?

Why is dot1p recomended to use with non-cisco phones?

In our company we have NEC phones, the phones have to be manually configured with a VLAN ID, we then use a VOICE VLAN with the same VLAN ID, all ports are trunk. this is confusing me as I thought we should be using access ports, but then the dot1p rears it ugly head and I get all mixed up.

Any conformation or clarity will help me immensly

thanks

Dave

6 REPLIES

Re: Voice VLANs and 802.1p

cisco Ip phones bydefault mark the traffic with COS 5 and the COS will be carried over the voice vlan tag while the data vlan will be untagerd

the access port with voice vlan is called multi-access port so it is access port but technicaly act as trunk port that carry vlan tags of voice vlan and cos marking as wel but needs to be trusted on the switchport level

and the voice vlan negiciated by the help of CDP which is cisco protocol

with other vendors the marking methods migh be deffrent and there is no CDP

wish the info has gave u good background

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Community Member

Re: Voice VLANs and 802.1p

So your saying that although the port would be configured as access the IP Phone sends all it's traffic as 802.1Q? And in the 802.1p field of the phones 802.1Q traffic the cos bit will be set to 5?

If I configure "sw voice vlan dot1p" does it mean that I'm simply saying if a frame arrives at the switchport which has been encapsulated with 802.1Q then assign it to VLAN0? Anything else will be assigned to default VLAN which is if left unconfigured VLAN 1?

Is device and phone on same subnet?

Would Vlan 0 and 1 be able to communicate to each other within the switch if it's a layer 2 or would they need to use intervlan routing?

many thanks

Dave

Re: Voice VLANs and 802.1p

802.1Q/p Class of Service (CoS)-Ethernet frames can be marked at Layer 2 with their relative

importance by setting the 802.1p User Priority bits of the 802.1Q header. Only three bits are

available for 802.1p marking. Therefore, only 8 classes of service (0-7) can be marked on Layer 2

Ethernet frames

based on the details i gave you on the prevouse post

the switchport will be configured like

interface [port num]

switchport mod access

switchport access vlan [data vlan number]

voice vlan [voice vlan number]

mls qos trust cos

or

mls qos trust device cisco-phone

Commonly used markers include: CoS (ISL, 802.1p), DSCP,

and IP precedence.

cos L2

DSCP and IP Precedence L3

When you connect an IP phone to a switch using a trunk link, it can cause high CPU utilization in the switches. As all the VLANs for a particular interface are trunked to the phone, it increases the number of STP instances the switch has to manage. This increases the CPU utilization. Trunking also causes unnecessary broadcast / multicast / unknown unicast traffic to hit the phone link.

In order to avoid this, remove the trunk configuration and keep the voice and access VLAN configured along with Quality of Service (QoS). Technically, it is still a trunk, but it is called a Multi-VLAN Access Port (MVAP). Because voice and data traffic can travel through the same port, you should specify a different VLAN for each type of traffic. You can configure a switch port to forward voice and data traffic on different VLANs. Configure IP phone ports with a voice VLAN configuration. This configuration creates a pseudo trunk, but does not require you to manually prune the unnecessary VLANs

good luck

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Community Member

Re: Voice VLANs and 802.1p

Hi,

thanks for the great information, it's really helpful. Can you answer the questions on my first reply asking about 802.1p?

thanks

Dave

Re: Voice VLANs and 802.1p

as mentioned above

Commonly used markers include: CoS (ISL, 802.1p),

because ISL is cisco propriatory so not supported with non cisco devices!

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Community Member

Re: Voice VLANs and 802.1p

Sorry if I misunderstand but these are my questions...

If I configure "sw voice vlan dot1p" does it mean that I'm simply saying if a frame arrives at the switchport which has been encapsulated with 802.1Q then assign it to VLAN0? Anything else will be assigned to default VLAN which is if left unconfigured VLAN 1?

Is device and phone on same subnet?

Would Vlan 0 and 1 be able to communicate to each other within the switch if it's a layer 2 or would they need to use intervlan routing?

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