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Chapter 4: Web-Based Management
destination lies on the same segment, communications take place using normal
Ethernet protocols. Even though this is always the case, when the destination for
a packet lies 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
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GE-DS-242-PoE Managed Ethernet Switch User Manual
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