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Chapter 4: Web-Based Management
GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual 65
on another switch port, VLAN considerations come into play to decide if the packet is
dropped by the Switch or delivered.
• IEEE 802.1Q VLANs
IEEE 802.1Q (tagged) VLAN are implemented on the Switch. 802.1Q VLAN require
tagging, which enables them to span the entire network (assuming all switches on
the network are IEEE 802.1Q-compliant).
VLAN allow a network to be segmented in order to reduce the size of broadcast
domains. All packets entering a VLAN will only be forwarded to the stations (over IEEE
802.1Q enabled switches) that are members of that VLAN, and this includes
broadcast, multicast and unicast packets from unknown sources.
VLAN can also provide a level of security to your network. IEEE 802.1Q VLAN will only
deliver packets between stations that are members of the VLAN. Any port can be
configured as either tagging or untagging. The untagging feature of IEEE 802.1Q
VLAN allows VLAN to work with legacy switches that don't recognize VLAN tags in
packet headers. The tagging feature allows VLAN to span multiple 802.1Q-compliant
switches through a single physical connection and allows Spanning Tree to be
enabled on all ports and work normally.
Any port can be configured as either tagging or untagging. The untagging feature of
IEEE 802.1Q VLAN allows VLAN to work with legacy switches that don't recognize
VLAN tags in packet headers. The tagging feature allows VLAN to span multiple
802.1Q-compliant switches through a single physical connection and allows
Spanning Tree to be enabled on all ports and work normally.
Some relevant terms:
- Tagging - The act of putting 802.1Q VLAN information into the header of a packet.
- Untagging - The act of stripping 802.1Q VLAN information out of the packet header.
• 802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets
inserted after the source MAC address. Their presence is indicated by a value of
0x8100 in the Ether Type field. When a packet's Ether Type field is equal to 0x8100,
the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following
two octets and consists of 3 bits of user priority, 1 bit of Canonical Format Identifier
(CFI - used for encapsulating Token Ring packets so they can be carried across
Ethernet backbones), and 12 bits of VLAN ID (VID). The 3 bits of user priority are used
by 802.1p. The VID is the VLAN identifier and is used by the 802.1Q standard. Because
the VID is 12 bits long, 4094 unique VLAN can be identified.
The tag is inserted into the packet header making the entire packet longer by 4
octets. All of the information originally contained in the packet is retained.