Importance of Native VLANs

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802.1Q trunks define a native VLAN for frames that are not tagged by default. An 802.1Q native VLAN is defined as one of the following:

The VLAN that a port is in when not in trunking operational mode

The VLAN from which Layer 2 frames will be transmitted untagged on an 802.1Q trunk port

■ The VLAN to which Layer 2 frames will be forwarded if received untagged on an 802.1Q trunk port

With an 802.1Q native VLAN, a switch can forward any Layer 2 frame received on a trunk port, whether tagged or not, to an intended VLAN. Any Layer 2 frames from a native VLAN are transmitted from the trunk port untagged. Compare 802.1Q to ISL, where any unencapsulated frames received on a trunk port are immediately dropped and all frames transmitted from a trunk port are encapsulated.

Note A frame with the frame tag is called a "baby giant." For ISL, a baby giant is 1548 bytes; while for 802.1 Q, a baby giant is 1522 bytes.

By default on Catalyst switches, all switch ports and native VLANs are initially assigned to VLAN1. The 802.1Q trunk ports connected to each other via physical or logical segments must all have the same native VLAN configured. A trunk port will support only one native VLAN. If you do not configure the same native VLAN on all switches and you use CDP, CDP will issue a "VLAN mismatch" error message to any active consoles. There are some specific cases where either CDP is turned off or cannot be transmitted through an intermediate Layer 2 device in the same manner that 802.1Q frames can. If the native VLAN is misconfigured for trunk ports on the same trunk link, Layer 2 loops can occur.

Each physical port has a parameter called a Port VLAN identifier (PVID). Every 802.1Q port is assigned a PVID value based on its native VID (default is VLAN1). All untagged frames are assigned to the PVID parameter. When a port receives a tagged frame, the tag is respected. If the frame is untagged, the value contained in the PVID is considered a tag. This allows the coexistence on the same Ethernet segment of VLAN-aware bridges or stations and VLAN-unaware bridges or stations.

When configuring 802.1Q on a Cisco switch, it is possible to define a different native VLAN or access VLAN to an access port from that defined for an operational trunk port.

Continue reading here: VTP Pruning

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