Untagged traffic is used for switch-to-switch communication, with CDP as an example protocol. When trunking switches, if the native VLANs do not match it does not cause problems, as tagged traffic will pass and untagged traffic will be tagged with the receiving switch's native VLAN. Connecting switches with access ports instead of trunk ports allows traffic to flow but only for one VLAN at a time between switches.

3. Carrying untagged traffic has its uses. This happens when one switch wants to send information to another switch.
An example of switch-to-switch communication is CDP. CDP is a Cisco protocol used to share information about connected devices.
In this case, if there is a trunk link between two switches, how does the sending switch decide which VLAN to use? In short, it sends untagged traffic, which is on the native VLAN.
Other VLAN Types
Be aware that there are other VLAN types and uses, which fall outside the scope of this article. So far data VLANs have been covered, but there are also voice VLANs.
There are also different ways of using data VLANs. This includes reserving VLANs for management, or creating ‘remote VLANs’ for use in ERSPAN ports.
Additionally there are methods of manipulating VLANs for security, such as private VLANs. This is a method of subdividing VLANs to segregate traffic within a VLAN.
It’s also possible to use double-tagging, which is adding two tags to a frame. This is sometimes used by service providers to keep customer traffic separate. It may also be used to extend the number o
Special Scenarios
Native VLAN Mismatch
When two switches are connected via trunk ports, and the native VLAN between the two does not match, the switch logs an error like this:
%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/2 (2), with Switch FastEthernet0/2 (1)
The question is, does this cause a problem? The short answer is; no. There are two types of frames that could pass through this link; tagged and untagged. Any tagged traffic will be passed without b
Have a look at this diagram for an example:
In this example, the two switches are connected with a trunk link. However, the native VLANs (15 and 20) do not match, resulting the error above. So what happens if an untagged packet crosses th
As discussed earlier, when an untagged frame enters a switch port, the native VLAN is tagged on the frame. So if switch-A were to send a frame to switch-B, it would be sent untagged, and switch-B
What would happen if a host on Switch-A were on VLAN 15, and it sent a frame to a host on Switch-B? Firstly, the frame would be tagged as VLAN 15 when entering Switch-A. However, the interest
So, if traffic will still flow between the switches, even if there is a native VLAN mismatch, what’s the syslog error all about? This is CDP informing you that the current configuration is not best-pract
Linking Switch Access Ports
What would happen if two access ports were used to link two switches, instead of trunk ports? This is very similar to the Native VLAN Mismatch scenario above. Incoming frames will be added to t
The difference is that although traffic will flow, essentially only one VLAN is being allowed from one switch to the other. Although, keep in mind that each switch, in this case, would see a different
In general, use trunking to connect switches.