Trunk Ports and The Native VLAN
Clause 9 of the 1998 802.1Q standard defines the encapsulation protocol used to multiplex VLANs over a single link, by adding VLAN tags. However, it is possible to send frames either tagged or untagged, so to help explain which frames will be sent with or without tags, some vendors (most notably Cisco) use the concepts of a) trunk ports and b) the native VLAN for that trunk.
A trunk port is a port that sends and receives tagged frames on all VLANs, except the native VLAN, if one is configured.
Frames belonging to the native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the native VLAN configured on that port.
For example, if an 802.1Q port has VLANs 2, 3 and 4 assigned to it, with VLAN 2 being the native VLAN, frames on VLAN 2 that are sent from the aforementioned port are not given an 802.1Q header (i.e. they are plain Ethernet frames). Frames that are received on that port and have no 802.1Q header are assigned to VLAN 2. Tagging of frames sent to or received from VLANs 3 & 4 is the same as if no native VLAN had been configured - all frames on those VLANs must carry tags to identify their VLAN membership.
Note that unexpected results may occur if the native VLAN configuration is not the same on all sending and receiving ports on a link. Continuing the above example, if VLAN 2 is not configured as the native VLAN on some other 802.1Q port, that port will send tagged frames on VLAN 2. When the local port, on which VLAN 2 is configured as the native VLAN, receives these unexpectedly tagged frames, it will still assign them to VLAN 2, but it will send only untagged frames for VLAN 2. On receipt, the distant port will either associate the untagged frames with a different VLAN ID (the one locally configured as the native VLAN) or it will discard the untagged frames if it has no native VLAN configured. (Symmetrically, this remote port will send only untagged frames on its configured native VLAN, which will be associated with a different VLAN ID by the local port.)
Not all vendors use the concept of trunk ports and native VLANs. Annex D to the 1998 802.1Q standard uses the concept of trunk links, but the current (IEEE Std 802.1D- 2004) standard does not use the terms trunk or native. Some use the term "Qtrunk" to avoid confusion with 802.3ad "link aggregation" that is often named a trunk as well.
Read more about this topic: IEEE 802.1Q
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