802.1Q Tagging - Foundry Networks FESX Manual

Some network configurations may require that a port be able to reside in two or more Layer 2 broadcast domains
(port-based VLANs). In this case, you can enable a port to reside in multiple port-based VLANs by tagging the
port. See the following section.
If your network requires that you use VLAN ID 1 for a user-configured VLAN, you can reassign the default VLAN to
another valid VLAN ID. See "Assigning a Different VLAN ID to the Default VLAN" on page 11-15.

802.1Q Tagging

802.1Q tagging is an IEEE standard that allows a networking device to add information to a Layer 2 packet in
order to identify the VLAN membership of the packet. Foundry devices tag a packet by adding a four-byte tag to
the packet. The tag contains the tag value, which identifies the data as a tag, and also contains the VLAN ID of
the VLAN from which the packet is sent.
• The default tag value is 8100 (hexadecimal). This value comes from the 802.1Q specification. You can
change this tag value on a global basis on Foundry devices if needed to be compatible with other vendors'
equipment.
• The VLAN ID is determined by the VLAN on which the packet is being forwarded.
Figure 11.4 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 11.4 Packet containing Foundry's 802.1QVLAN tag
Untagged Packet Format
6 bytes 6 bytes
Destination Source
Address Address
6 bytes 6 bytes
Destination Source
Address Address
802.1q Tagged Packet Format
6 bytes 6 bytes
Destination Source
Address Address
6 bytes 6 bytes
Destination Source
Address Address
Octet 1
Tag Protocol Id (TPID)
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 Foundry
devices is compatible with other Foundry devices.
December 2005
2 bytes Up to 1500 bytes
Type Data
Field Field
2 bytes Up to 1496 bytes
Length Data
Field Field
2 bytes 4 bytes 2 bytes Up to 1500 bytes
Type 802.1q Type Data
Field Tag Field Field
2 bytes 4 bytes 2 bytes Up to 1496 bytes
Length 802.1q Length Data
Field Tag Field Field
Octet 2
1 2 3 4 5 6 7 8
802.1p
VLAN ID (12 bits)
(3 bits)
© Foundry Networks, Inc.
4 bytes
CRC
4 bytes
CRC
Up to 1500 bytes 4 bytes
Data CRC
Field
Up to 1496 bytes 4 bytes
Data CRC
Field
Octet 4
Configuring Virtual LANs (VLANs)
Ethernet II
IEEE 802.3
4 bytes
CRC
Ethernet II Ethernet II with 802.1q tag
4 bytes
CRC
IEEE 802.3 with 802.1q tag
IEEE 802.3
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