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Configuring Virtual LANs (VLANs)
December 2005
© Foundry Networks, Inc.
11 - 7
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
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.
6 bytes
Destination
Address
2 bytes
Length
Field
Up to 1496 bytes
Data
Field
4 bytes
CRC
IEEE 802.3
6 bytes
Source
Address
6 bytes
Destination
Address
2 bytes
Type
Field
Up to 1500 bytes
Data
Field
4 bytes
CRC
Ethernet II
6 bytes
Source
Address
2 bytes
Length
Field
Up to 1496 bytes
Data
Field
4 bytes
CRC
IEEE 802.3 with 802.1q tag
2 bytes
Type
Field
Up to 1500 bytes
Data
Field
4 bytes
CRC
Ethernet II with 802.1q tag
4 bytes
802.1q
Tag
4 bytes
802.1q
Tag
802.1q Tagged Packet Format
Octet 1
Octet 2
Tag Protocol Id (TPID)
5 6 7 8
Octet 4
VLAN ID (12 bits)
1 2 3 4
802.1p
(3 bits)
6 bytes
Destination
Address
2 bytes
Length
Field
Up to 1496 bytes
Data
Field
4 bytes
CRC
IEEE 802.3
6 bytes
Source
Address
6 bytes
Destination
Address
2 bytes
Type
Field
Up to 1500 bytes
Data
Field
4 bytes
CRC
Ethernet II
6 bytes
Source
Address
Untagged Packet Format
Summary of Contents for FastIron Edge Switch X424
Page 36: ...Foundry Configuration Guide for the FESX FSX and FWSX 2 12 Foundry Networks Inc December 2005...
Page 56: ...Foundry Configuration Guide for the FESX FSX and FWSX 3 20 Foundry Networks Inc December 2005...
Page 70: ...Foundry Configuration Guide for the FESX FSX and FWSX 4 14 Foundry Networks Inc December 2005...
Page 198: ...Foundry Configuration Guide for the FESX FSX and FWSX 8 38 Foundry Networks Inc December 2005...
Page 316: ...Foundry Configuration Guide for the FESX FSX and FWSX 12 26 Foundry Networks Inc December 2005...
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Page 820: ...Foundry Configuration Guide for the FESX FSX and FWSX E 10 Foundry Networks Inc December 2005...