Voice VLAN concepts

[sjacisin]

Traning material has question:

Suppose switchport is configured w/ access VLAN 10, voice VLAN 110, and native VLAN 99. The explanation does not make sense, as it seems the output they have, seperated on two different pages is some bad editing. as it shows

sh run int fa1/0/1

swtichport trunk native vlan 99

switchport access vlan 63

switchport voice vlan 306

then they do a sh spanning-tree int fa1/0/1 and it says there are two vlans vlan 10 and vlan 110 and they say vlan 10 is being used as data VLAN from IP Phone.

I would really like to know how it works if indeed native vlan99, access vlan 10, and voice vlan 110 were all configured on the same interface. My best guess is the following:

native vlan 99 means that there is an 802.1q trunk

access vlan 10 means that the switch will tag the data from the phone with vlan 10 (howevre all other documentation I have read says that the data traffic from the pc will be sent untagged using the native vlan. In that case the pc data would be sent untagged using vlan 99.

vlice vlan 110 means that the voice will be tagged w/ vlan 110, and also be using the 802.1p bits for priority forwarding.

So what is the vlan, 99 or 110, that the data from the pc is going to be sent as?

If it is sent as vlan 10, will it be tagged or untagged?

I am on pins and needles to get the solution.

Thank you,

Sally

[sureshsolomon]

Hi Sally,

I had a problem with this issue too. Let me give you what I understand on the VLAN.

1. switchport access vlan 63

The end device connecting does not need to understand dot1q headers. So any device will work without issue.

The packets with the vlan 63 will have the dot1q header stripped and the packets getting in to the switch will add on the dot1q header with the vlan 63 id .

2. switchport voice vlan 306

For a device to access this packet, it must be dot1q aware. If you connect a normal pc with only "switchport voice vlan 306" with out the "switchport access vlan 63 " command the PC will not get any information.

If you plug in an IP phone that understands dot1q ( has a vlan config option) then it will be able to read the packets by doing the decoding itself ( ip phone). In the "access vlan" cmd the switch port does the decoding.

3. switchport trunk native vlan 99

This basically is for the router that is connected to the switch. If the router has a command like this

=========================================

int fa 0/0 >>>>>>> Native vlan

ip add 10.22.100.1 255.255.255.0

int fa 0/0.306

encap dot1q 306 >>>>Vlan 210

ip add 10.11.222.3 255.255.255.0

==========================================

The packets entering the router will have the packets decoded of the vlan 99 dot1q ( stripped of the dot1q header) and the untagged packets entering the switch will have the packets encoded with a vlan 99 dot1q header.

By default the native vlan is vlan1.

The vlan 210 will be encoded and decoded by the subint to read the packets.

The vlan 63 and 302 seems out of place in the config you provided.

The concept is simple but seems a bit confusing to understand initially.

HTH

Suresh

[Giuseppe Larosa]

Hello Sally,

under a switch port configuration some commands can be effective and other can be just there to be used if something changes

One of these is the following:

swtichport trunk native vlan 99

this means if an 802.1Q trunk is manually configured or negotiated by DTP the native vlan is 99.

to check in real world you could use

sh int f1/0/1 switchport

to verify the port mode: access or trunk.

the pair of commands:

switchport access vlan 63

switchport voice vlan 306

create an implicit 802.1Q trunk with voice vlan 306 and data vlan 63.

so with this config PC frames are sent and received untagged in vlan 63.

Phone frames are sent tagged with voice vlan 306.

vlan 99 should be simply unused until the port becomes a trunk.

You say that later in the training material they do

sh spannning-tree f1/0/1

and this shows the port is in forwarding state for vlan 10 and vlan 110.

and they say that vlan 10 is used as data vlan by ip phone.

this should translate in

int f1/0/1

switchport access vlan 10

switchport voice vlan 110

vlan99 not being listed as in forwarding state on the port cannot send or receive frames so it can be ignored.

the pc frames should be sent and received untagged in vlan 10 if my intepretation is correct.

if for the phone vlan 10 is a data vlan is not a voice vlan that is the only one in which the phone is interested.

Hope to help

Giuseppe