weird scenario! switch, ip phone

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Jan 3rd, 2009
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HI every body!

Please consider the following configuration

sw-----------------ip phone----pc


switchport voice vlan 3

switchport mode access vlan 4

switchport trunk native vlan 5.

The result is:

voice frame will use vlan 3 and pc data frame will use vlan 4


Now consider the following Scenario:

sw1------------sw2

sw1

switchport trunk encapsulation negotiate

switchport trunk native vlan 2

switchport mode access vlan 3

what would be the result?

1) would vlan 3 frames on the trunk will be untagged or native vlan 2?

2)In first example, we see that a port is in vlan 4 and still be configured for trunk with ip phone.

Is it only possible if switch port is connected to ip phone?

thanks a lot!


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Giuseppe Larosa Sat, 01/03/2009 - 12:49
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Hello Sarah,

some switch commands are effective only when the port is in trunk mode


this is the case of

switchport trunk native vlan X

switchport trunk enc negotiate


if the port has config


switchport

switchpor mode access

switchport access vlan 4

switchport voice vlan 3


the commands that specify what the port will do if it is a trunk have no effect.


The port is an implicit trunk carrying only two vlans but it is still seen as in access mode

the command

sh int type x/y switchport


provides all this information includind voice vlan


To be noted that if you need to pass three vlans on the port you need to configure the port as trunk

In this way you will have one voice vlan and two data vlans


Hope to help

Giuseppe



sarahr202 Sat, 01/03/2009 - 19:33
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Thanks a lot Giuseppe!

A fellow net pro forwarded a very good link about it. .javascript:newWin('http://cciepursuit.wordpress.com/2009/01/01/group-study-good-explanation-of-the-voice-vlan/


As noted in above link, Cisco calls it multi-vlans acces port.

I just want to confirm one thing if the link has to carry more than two vlans say 3, it could not be configuured as muli-vlan access port so it must be configured as trunk. Am i correct?

to put simply an muli-vlans access port cannot carry more than two vlans right or wrong?




how about if we have following configuration

switch 'port f0/1---------ip phone---pc

switchport voice vlan untagged

Will this command cause ip phone and pc to use vlan 1 ? why? by default all ports are in vlan 1. Since f0/1 is not in any other vlan, it is therefore in vlan 1. IP phone will and pc will use vlan 1, ip phone will send untagged frame to switch.AM i correct?



thanks a lot!





lejoe.thomas Sun, 01/04/2009 - 16:24
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Hi Sarah,


With configuration of multi vlan, particularly because the switchport is configured as access with voice vlan statements.


to put simply an muli-vlans access port cannot carry more than two vlans right or wrong?

Yes


switch 'port f0/1---------ip phone---pc

Will this command cause ip phone and pc to use vlan 1 ? why? by default all ports are in vlan 1. Since f0/1 is not in any other vlan, it is therefore in vlan 1. IP phone will and pc will use vlan 1, ip phone will send untagged frame to switch.AM i correct?


When using voice vlan untagged, even the voice frames from the phone fall into the access-vlan, because they are untagged (if the port is in access-mode) or the native vlan (if the port is int trunk mode).


HTH


Lejoe


Giuseppe Larosa Mon, 01/05/2009 - 02:14
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Hello Sarah,


>> I just want to confirm one thing if the link has to carry more than two vlans say 3, it could not be configuured as muli-vlan access port so it must be configured as trunk. Am i correct?

to put simply an muli-vlans access port cannot carry more than two vlans right or wrong?


Correct the implicit mini trunk can carry only two vlans.

We actually use the switchport voice vlan command also on these trunk ports (switchport mode trunk)with 3 vlans.


Hope to help

Giuseppe



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