Understanding Native VLANs

Question

As I understand it (and that is questionable):

This pertains to Cisco equipment.

1-Switch traffic on a port, configured in access mode with a static VLAN, is tagged with a VLAN field before being sent out the trunk port?
2-Control traffic is untagged. Untagged frames are "tagged" with the PVID of the Native VLAN?
3-The purpose of trunking is to allow multiple VLAN traffic across the same link?
4-The native VLAN is for legacy or untagged LAN traffic. The purpose is to allow any untagged traffic or traffic not associated with a VLAN to use the trunk link and still communicate with the network?

Is that right?

score 3 · Accepted Answer · answered Mar 04 '12 at 18:30

3

Indeed Trunks only carry tagged frames, the purpose of a trunk is to be able to transfer data from different VLAN's. When your frame reaches the other side, the switch will look at the tag and forward it to the correct VLAN.

A native VLAN is indeed a VLAN that carries untagged frames. However if you mean by control traffic the management VLAN, it's by default on VLAN1 just like the native VLAN, however you can configure a seperate VLAN for management traffic (which is best practice). But it's indeed also untagged.

Also if a port get's a VLAN tag it doesn't know, it's automatically placed on the native VLAN.

answered Mar 04 '12 at 18:30

Lucas Kauffman

16,910

I did mean the management VLAN when I wrote 'control', thank you. In setting the management VLAN on something other than the default VLAN, would I then also change the native VLAN to the same VID or should the native VLAN and management VLAN be separate? – Kernel Panic Mar 04 '12 at 18:39
You seperate them, there are some attacks possible when you leave them on one VLAN, but I can't recall them immediately. – Lucas Kauffman Mar 04 '12 at 18:41
I thought you could have one untagged VLAN on a trunk port? – Robin Gill Mar 04 '12 at 19:29
Yes, that's the Native VLAN – Lucas Kauffman Mar 04 '12 at 19:43

Understanding Native VLANs

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