Как работает VLAN IEEE 802.1Q ?

Протоколы, пинги, роутинг - ответы на вопросы как заставить сеть работать правильно
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27 июн 2003, 09:09
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Сообщение Alex 07 окт 2005, 10:23

Мне кажется что подобную тему надо было создавать в разделе "Стандарты и технологии", но все же:

Думаю что здесь http://www.citforum.ru/nets/protocols2/2_11_05.shtml

и здесь http://www.citforum.ru/nets/protocols2/index.shtml

Все доходчиво расписано.
Рожденный ползать, везде пролезет!

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16 май 2005, 19:56
Откуда: Рига

Сообщение Rain 07 окт 2005, 12:37

Спасибо, уже что-то... :)

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16 май 2005, 19:56
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Сообщение Rain 07 окт 2005, 20:11

К теме о том как надо составлять технические руководства, и как это может повлиять на решение покупателя при выборе поставщика техники :D !!! Вот как надо:

ftp://ftp.dlink.ru/pub/Switch/DES-3624/ ... 624_ru.doc

The Switch supports up to 96 IEEE 802.1Q (port-based) VLANs. Port-based VLANs limit traffic that flows into and out of switch ports. Thus, all devices connected to a port are members of the VLAN(s) the port belongs to, whether there is a single computer directly connected to a switch, or an entire department.
On port-based VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers. NICs 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 gets dropped by the switch or delivered.
There are two key components to understanding IEEE 802.1Q VLANs; Port VLAN ID numbers (PVID) and VLAN ID numbers (VID). Both variables are assigned to a switch port, but there are important differences between them. A user can only assign one PVID to each switch port. The PVID defines which VLAN a switch will forward packets from the connected segment on, when packets need to be forwarded to another switch port or somewhere else on the network. On the other hand, a user can define a port as a member of multiple VLANs (VIDs), allowing the segment connected to it to receive packets from many VLANs on the network. These two variables control a port’s ability to transmit and receive VLAN traffic, and the difference between them provides network segmentation, while still allowing resources to be shared across more than one VLAN.
VLAN Segmentation
Take for example a packet that is transmitted by a machine on Port 1 that is a member of VLAN 2 and has the Port VLAN ID number 2 (PVID=2). If the destination lies on another port (found through a normal forwarding table lookup), the switch then looks to see if the other port (Port 10) is a member of VLAN 2 (and can therefore receive VLAN 2 packets). If port 10 is not a member of VLAN 2, then the packet will be dropped by the switch and will not reach it’s destination. If Port 10 is a member of VLAN 2, the packet will go through. This selective forwarding feature based on VLAN criteria is how VLANs segment networks. The key point being that Port 1 will only transmit on VLAN 2, because it’s Port VLAN ID number is 2 (PVID=2).
Sharing Resources Across VLANs
Network resources such as printers and servers however, can be shared across VLANs. This is achieved by setting up overlapping VLANs as shown in the diagram below.

Figure 5-4. Example of typical VLAN configuration
In the above example, there are three different VLANs and each port can transmit packets on one of them according to their Port VLAN ID (PVID). However, a port can receive packets on all VLANs (VID) that it belongs to. The assignments are as follows:
Код: Выделить всё
Transmit on VLAN #		Member of VLAN #
Port	PVID		VID	Ports
Port 1	1		1	1,2,3,7
Port 2	1			
Port 3	1			
Port 7	3		3	1,2,3,7,11,12
Port 11	2		2	11,12,7
Port 12	2			

Table 5-2. Example of possible VLAN assignments
The server attached to Port 7 is shared by VLAN 1 and VLAN 2 because Port 7 is a member of both VLANs (it is listed as a member of VID 1 and 2). Since it can receive packets from both VLANs, all ports can successfully send packets to it to be printed. Ports 1, 2 and 3 send these packets on VLAN 1 (their PVID=1), and Ports 11 and 11 send these packets on VLAN 2 (PVID=2). The third VLAN (PVID=3) is used by the server to transmit files that had been requested on VLAN 1 or 2 back to the computers. All computers that use the server will receive transmissions from it since they are all located on ports which are members of VLAN 3 (VID=3).
VLANs Spanning Multiple Switches
VLANs can span multiple switches as well as your entire network. Two considerations to keep in mind while building VLANs of this sort are whether the switches are IEEE 802.1Q-compliant and whether VLAN packets should be tagged or untagged.
Definitions of relevant terms are as follows:
Ё Tagging The act of putting 802.1Q VLAN information into the header of a packet. Ports with tagging enabled will put the VID number, priority, and other VLAN information into all packets that flow into and out it. If a packet has previously been tagged, the port will not alter the packet, thus keeping the VLAN information intact. Tagging is used to send packets from one 802.1Q-compliant device to another.
Ё Untagging The act of stripping 802.1Q VLAN information out of the packet header. Ports with untagging enabled will take all VLAN information out of all packets that flow into and out of a port. If the packet doesn’t have a VLAN tag, the port will not alter the packet, thus keeping the packet free of VLAN information. Untagging is used to send packets from an 802.1Q-compliant switch to a non-compliant device.
Ё Ingress port A port on a switch where packets are flowing into the switch and VLAN decisions must be made. Basically, the switch examines VLAN information in the packet header (if present) and decides whether to forward the packet. If the packet is tagged with VLAN information, the ingress port will first determine if the ingress port itself is a member of the tagged VLAN and can thus receive the packet (if the Ingress Filter is enabled), and then it decides if the destination port is a member of the VLAN. Assuming both ports are members of the tagged VLAN, the packet will be forwarded. If the packet doesn’t have VLAN information in its header (is untagged), the ingress port first determines if the ingress port itself can receive the packet (if the Ingress Filter is enabled), will tag it with its own PVID (if it defined as a tagging port), and check to see if the destination port is on the same VLAN as its own PVID and can thus receive the packet. If Ingress filtering is disabled and the destination port is a member of the VLAN used by the ingress port, the packet will be forwarded. If the ingress port is an untagging port, it will only check the filter condition--if the filter condition is enabled-- before forwarding the packet.
Ё Egress port A port on a switch where packets are flowing out of the switch, either to another switch or to an end station, and tagging decisions must be made. If an egress port is connected to an 802.1Q-compliant switch, tagging should be enabled so the other switch can take VLAN data into account when making forwarding decisions. If an egress connection is to a non-compliant switch or end-station, tags should be stripped so the (now normal Ethernet) packet can be read by the receiving device.
VLANs Over 802.1Q-compliant Switches
When switches maintaining the same VLANs are 802.1Q-compliant, it is possible to use tagging. Tagging puts 802.1Q VLAN information into each packet header, enabling other 802.1Q-compliant switches that receive the packet to know how to treat it. Upon receiving a tagged packet, an 802.1Q-compliant switch can use the information in the packet header to maintain the integrity of VLANs, carry out priority forwarding, etc.
Data transmissions between 802.1Q-compliant switches take place as shown below.

Figure 5-5. Data transmissions between 802.1Q-compliant Switches
In the above example, step 4 is the key element. Because the packet has 802.1Q VLAN data encoded in its header, the ingress port can make VLAN-based decisions about its delivery: whether server #2 is attached to a port that is a member of VLAN 2 and, thus, should the packet be delivered; the queuing priority to give to the packet, etc. It can also perform these functions for VLAN 1 packets as well, and, in fact, for any tagged packet it receives regardless of the VLAN number.
If the ingress port in step 4 were connected to a non-802.1Q-compliant device and was thus receiving untagged packets, it would tag its own PVID onto the packet and use this information to make forwarding decisions. As a result, the packets coming from the non-compliant device would automatically be placed on the ingress ports VLAN and could only communicate with other ports that are members of this VLAN.

Как говориться - вопросов больше не имею... :D

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09 июн 2016, 13:25

Re: Как работает VLAN IEEE 802.1Q ?

Сообщение ruslan33390 09 июн 2016, 13:33

А в чём недостатки стандарта 802.1Q?

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