Dell PowerConnect B-RX Configuration Manual page 352

11
Overview of Virtual Local Area Networks (VLANs)
Tagged ports allow the device to add a four-byte 802.1q tag to the packet. 802.1q tagging is an
IEEE standard that allows a networking device to add information to Layer 2 packets. This
information identifies the VLAN membership of the packet, as well as the VLAN ID of the VLAN from
which the packet is sent. Furthermore, the default tag value of the 802.1q tag is 8100
(hexadecimal). This value comes from the 802.1q specification. You can change this tag value on a
global basis on device if needed to be compatible with other vendors' equipment.
Figure 21
vendor-specific. To use the tag for VLANs configured across multiple devices, make sure all the
devices support the same tag format.
FIGURE 21
Destination
802.1q Tagged Packet Format
Destination
Destination
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 a 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.
The tagging on all device switches is compatible with other Brocade devices.
280
shows the format of packets with and without the 802.1q tag. The tag format is
Brocade
Packet containing
Untagged Packet Format
6 bytes 2 bytes
6 bytes
Source Type
Address Field
Address
6 bytes 6 bytes
2 bytes
Destination Source
Length
Address Address
Field
4 bytes
6 bytes
6 bytes
802.1q
Source
Address Tag
Address
4 bytes
6 bytes
6 bytes
802.1q
Source
Tag
Address
Address
Octet 1 Octet 2 1 2 3 4 5 6 7 8
802.1p
Tag Protocol Id (TPID)
(3 bits)
's 802.1QVLAN tag
Up to 1500 bytes
Data Field
Up to 1496 bytes
Data Field
2 bytes
Up to 1500 bytes
Type
Data Field
Field
2 bytes
Up to 1496 bytes
Length
Data Field
Field
Octet 4
VLAN ID (12 bits)
4 bytes
Ethernet II
CRC
4 bytes
IEEE 802.3
CRC
4 bytes
Ethernet II with 802.1q tag
CRC
4 bytes
IEEE 802.3 with 802.1q tag
CRC
BigIron RX Series Configuration Guide
53-1001810-01