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Figure 21, Figure 22, Overview of virtual local area networks (vlans) – Brocade BigIron RX Series Configuration Guide User Manual

Page 366: Vlans configured across multiple devices

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BigIron RX Series Configuration Guide

53-1002484-04

Overview of Virtual Local Area Networks (VLANs)

11

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

shows the format of packets with and without the 802.1q tag. The tag format is

vendor-specific. To use the tag for VLANs configured across multiple devices, make sure all the
devices support the same tag format.

FIGURE 21

Packet containing Brocade’s 802.1QVLAN tag

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 devices is compatible with other Brocade devices.

Figure 22

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.

FIGURE 22

VLANs configured across multiple devices

Untagged Packet Format

6 bytes

Destination

Address

6 bytes

Source

Address

2 bytes

Type

Field

Up to 1500 bytes

Data Field

4 bytes

CRC

Ethernet II

IEEE 802.3

802.1q Tagged Packet Format

4 bytes

802.1q

Tag

Ethernet II with 802.1q tag

IEEE 802.3 with 802.1q tag

Tag Protocol Id (TPID)

Octet 1

Octet 2

802.1p
(3 bits)

VLAN ID (12 bits)

Octet 4

1 2 3 4 5 6 7 8

6 bytes

Destination

Address

6 bytes

Source

Address

2 bytes

Length

Field

Up to 1496 bytes

Data Field

4 bytes

CRC

6 bytes

Destination

Address

6 bytes

Destination

Address

6 bytes

Source

Address

6 bytes

Source

Address

4 bytes

802.1q

Tag

2 bytes

Type

Field

2 bytes

Length

Field

Up to 1500 bytes

Data Field

Up to 1496 bytes

Data Field

4 bytes

CRC

4 bytes

CRC