Planet WGSW-2840 Инструкция по эксплуатации онлайн [45/104] 65303

an entire department.

On port-based VLAN.NIC do not need to be able to identify 802.1Q tags in packet headers. NIC send and receive

normal Ethernet packets. If the packet's 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 allow

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.

802.1Q Tag