Planet GSD-1002M User Manual download pdf (Page 99)

User’s Manual of GSD-1002M

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.

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

User Priority CFI VLAN ID (VID)

3 bits 1 bits 12 bits

TPID (Tag Protocol Identifier) TCI (Tag Control Information)

2 bytes 2 bytes

Preamble

Destination

Address

Source

Address

VLAN TAG

Ethernet

Type

Data FCS

6 bytes 6 bytes 4 bytes 2 bytes 46-1500 bytes 4 bytes

The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether Type/Length or Logical

Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be

recalculated.

1 2 ... 94 95 96 97 98 99 100 101 102 103 104 ... 312 313

No comments

Planet GSD-1002M User Manual Page 99