Advanced Configuration and Management Guide

Untagged Packet Format

6 bytes

6 bytes

2 bytes

Up to 1500 bytes

4 bytes

 

Destination

Source

Type

Data

CRC

Ethernet II

Address

Address

Field

Field

 

 

 

 

 

 

 

 

6 bytes

6 bytes

2 bytes

Up to 1496 bytes

4 bytes

 

Destination

Source

Length

Data

CRC

IEEE 802.3

Address

Address

Field

Field

 

 

802.1q Tagged Packet Format

6 bytes

6 bytes

4 bytes

2 bytes

Up to 1500 bytes

4 bytes

 

Destination

Source

802.1q

Type

Data

CRC

Ethernet II with 802.1q tag

Address

Address

Tag

Field

Field

 

 

 

 

 

 

 

 

 

6 bytes

6 bytes

4 bytes

2 bytes

Up to 1496 bytes

4 bytes

 

Destination

Source

802.1q

Length

Data

CRC

IEEE 802.3 with 802.1q tag

Address

Address

Tag

Field

Field

 

 

Octet 1

Octet 2

1

2

3

4

5

6

7

8

Octet 4

Tag Protocol

Id (TPID)

802.1p

 

 

 

VLAN

ID (12 bits)

 

 

(3 bits)

 

 

 

 

 

 

Figure 16.4 Packet containing the 802.1Q VLAN tag

NOTE: You cannot configure a port to be a member of the default port-based VLAN and another port-based VLAN at the same time. Once you add a port to a port-based VLAN, the port is no longer a member of the default VLAN. The port returns to the default VLAN only if you delete the other VLAN(s) that contains the port.

If you configure a VLAN that spans multiple devices, you need to use tagging only if a port connecting one of the devices to the other is a member of more than one port-based VLAN. If a port connecting one device to the other is member of only a single port-based VLAN, tagging is not required.

If you use tagging on multiple devices, each device must be configured for tagging and must use the same tag value. In addition, the implementation of tagging must be compatible on the devices.

Figure 16.5 shows an example of two devices that have the same Layer 2 port-based VLANs configured across them. Notice that only one of the VLANs requires tagging.

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