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PC connected to a trunk port

Posted on 2014-12-22
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I am just curious why when I plug my PC to a trunk port and it is working. In my network, vlan1 is for voice and vlan2 is for data. But the port that the PC connected to is a trunk. So if I guess correctly, the PC will be in the default vlan1 and that is why it is working. Correct?
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Question by:leblanc
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Predrag Jovic earned 250 total points
ID: 40513256
The reason that your PC is working is - native VLAN. Native VLAN has no VLAN tag.
So, all untagged traffic that acomes into switch port is considered as part of native VLAN (in your case VLAN1).
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by:rharland2009
rharland2009 earned 250 total points
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Not necessarily. Many networks who share infrastructure for voice and data employ a strategy that allows the phones to be identified as such by DHCP options and given an appropriate address, after which they boot up into the appropriate VLAN.
For example, in one of our facilities we have VLAN 14 for computers and VLAN 8 for phones. Each port that a phone and computer connect to are untagged (native in Cisco-speak, I believe) in the computers VLAN and tagged in the voice VLAN.
Our VoIP phones have a 2-port switch for passthrough to the computer.
So what happens is this. The first time a phone is connected to the port, it powers up and asks for a DHCP address. We've configured the voice DHCP scope to pass some options (instructions) to the phones - in this case, reboot and come back up in VLAN 8. The computers ignore these options and will get a DHCP address as normal since their VLAN is the 'default' for this port.
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by:leblanc
ID: 40513277
I assume that your phones are connected to ports configured with voice and data vlan. With my phones (NEC), it is a bit wired because my ports have to be configured as a trunk.
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Expert Comment

by:Predrag Jovic
ID: 40513290
As I told you.
Trunk is created, but only voice traffic is tagged.
PC VLAN traffic is not tagged, so all untagged traffic is considered as part of native VLAN.
If you change native VLAN to VLAN 10, all untagged traffic will be part of VLAN 10 in that case.
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