Interlogix NS3550-8T-2S User Manual - Manual (Page 99)

User’s Manual of NS3550-8T-2S
99
■ IEEE 802.1Q Standard
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.
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.
Adding an IEEE802.1Q Tag
Dest. Addr. Src. Addr. Length/E. type Data Old CRC
Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC
Priority CFI VLAN ID
■ Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant network
device to another with the VLAN information intact. This allows 802.1Q VLAN to span network devices (and indeed, the entire
network – if all network devices are 802.1Q compliant).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no VLAN are
defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged packets are assigned the
PVID of the port on which they were received. Forwarding decisions are based upon this PVID, in so far as VLAN are concerned.
Original Ethernet
New Tagged Packet