IEEE 802.1Q
IEEE 802.1Q is a standardized by the IEEE prioritization and VLAN technology, implemented in contrast to the older, only port-based VLANs, packet-based tagged VLANs. The term "Tagged " is derived from the English. Expression of material from the day, which are goods label with which goods are marked. So it is with tagged VLANs to networks using the network packets that carry a specific VLAN tag. [ Footnote 1]
History and normalization
Before the standardization by the IEEE consortium there were various proprietary tagged VLAN solutions like Cisco's Inter-Switch Link Protocol (ISL ) or 3Com's VLT (Virtual LAN Trunk ) tagging, which were now grouped together in the IEEE 802.1Q standard to a multi-vendor standard. The standard describes a mechanism that allows multiple virtual networks share a common physical or logical interface, without compromising safety aspects or to enable seamless data exchange between the VLANs. By the normalization, the advantage is that all LAN switches is where implemented 802.1Q, can be integrated into a common virtual network structure and administration, the exchange VLAN information in question with each other.
Tags in the Ethernet packet
In 802.1Q standard data fields for VLAN tagging are defined, which are inserted into the data field of an Ethernet packet. This has the advantage that usually older switches can forward such packets. The inserted tag consists of four fields, with a total length of 32 bits. Two bytes for the Priority field, three bits for the indicator of Canonical format a bit and for the twelve -bit VLAN ID to be used for the protocol ID.
The Protocol ID field is always set for 802.1Q VLANs on the value 8100hex. This value is reserved. The ensuing priority field governs the priority of the Ethernet frame (see Traffic Shaping ). The Canonical field indicates the direction from which the VLAN ID must be read ( ie a maximum of 4096 4096 VLANs).
Operation according to IEEE 802.1Q
Each VLAN is assigned a unique number. This number is called the VLAN ID. A device that belongs to the VLAN with the ID = 1 can communicate with any other device on the same VLAN, but not with a device in another VLAN, such as ID = 2, 3, ...
To distinguish between the VLANs according to IEEE 802.1Q Ethernet frame ( 32 bits =) is extended by four bytes. Of these 12 bits containing the VLAN ID are provided so that (without use of the Canonical Format bit) 4096 total - 2 = 4094 possible VLANs are ( VLAN IDs " 0" and " 4095 " are reserved and not allowed).
- TPID - Tag Protocol Identifier: Fixed value 8100hex. Frame carries the 802.1Q/802.1p-Tag-Information.
- TCI - Tag Control Information: PCP - Priority Code Point: User - priority information.
- DEI - Drop Eligible Indicator: Can be used separately or in conjunction with PCP to indicate that frames can be dropped in the presence of congestion ( formerly CFI [note 1] ). .
- VID - VLAN Identifier: identification of the VLAN to which the frame belongs.
In addition, a prioritization VLAN. It may, for each frame of an 8 (3 bits) priority be given (IEEE 802.1p). This makes it possible, for example, voice data forwarded preferably as HTTP data is thwarted. This functionality is winning in terms of the increasing use of VoIP ( IP telephony ) is becoming increasingly important. This interference can be avoided when making calls with a ' limited ' bandwidth. ( See also Quality of Service)
Switch types
- Older switches: you do not read the VLAN tag, can usually - as in all 802.1Q tagged packets according to all OSI Layer 2 information is set as normal and the day of Layer 2 view is in the data area - but still Packages forward to set VLAN tags.
- Simple switches without management interface: Current models understand the importance of VLAN tags and process tags correctly; they work in the so-called automatic learning mode, but can not itself define tagging features (no day -Insert or Remove).
- Managbare professional devices: Such switches understand the importance of VLAN tags and process tags correctly; they may additionally but also self- tagging features define ( insert day, day - Remove). Course work Optionally, this switch in the automatic learning mode (see above). Within a VLAN infrastructure, elderly and others, not even 802.1Q VLAN - capable terminals only in such switches are operated.
Types of LAN cards
- Simple and older cards: These can be either due to limitations in the hardware or due to lack of software that does not deal with VLAN tags; they will drop packets with VLAN tags set. Thus, these cards must be connected to 802.1Q - capable switches, which can analyze and remove tags as needed and paste.
- Higher Cards: These were once used mainly in server area and were from manufacturers such as 3Com, Adaptec, AMD, Compaq, DEC, HP, Intel, packet engines or SysKonnect. Current 100-Mbps and Gigabit cards are VLAN - capable typically. A matching, VLAN - capable operating system (current Unix-like operating systems such as AIX, Solaris or Linux, current Windows server editions etc.. ) And the appropriate driver provided, these cards can handle (day -detect, insert, remove) with VLAN tags and process them correctly.