Shulou(Shulou.com)06/01 Report--

The application of switch in Ethernet solves the problem of collision domain that hub can not solve, but traditional switching technology can not effectively suppress broadcast frames, that is, when a device accessing the switch sends a broadcast frame to the switch, the switch will forward the received broadcast frame to all devices connected to other ports of the switch, causing a sharp increase in communication traffic on the network, and even causing network collapse.

In addition, in traditional networks, since users can access all devices on the network, the security of the network is not guaranteed.

For this reason, switch VLAN technology appeared in the mid-1990s to solve the above two problems.

Question: Why not use a router to solve these two problems?

In fact, we can use routers to solve these two problems, but their performance is low. The router's main function is to realize heterogeneous network interconnection and network routing, and can not provide fast and efficient packet forwarding.

VLAN Technology in Switches

VLAN is the abbreviation of Virtual Local Area Network(Virtual local area network), which is a logical network divided on the physical network. VLAN technology is a technology that logically divides the devices in a local area network into network segments to realize virtual workgroups.

VLAN technology has the following characteristics:

VLAN is fundamentally different from traditional LAN determined by physical location. It is not limited by the physical location of the network and can span multiple physical networks and switches. Devices in a network can be functionally divided into logical workgroups, each of which is a VLAN

Broadcasts in the same VLAN can only be heard by members of the VLAN and will not be transmitted to other VLANs. Therefore, VLANs can isolate broadcast information. Each VLAN is a broadcast domain. Users can restrict broadcast domains by dividing VLANs to prevent broadcast storms.

VLAN division can effectively improve bandwidth, we can divide the network equipment into multiple logical working groups according to business functions, each group is a VLAN. In this way, most of the daily communication information is limited to a VLAN, so that bandwidth is effectively used.

VLAN is defined and divided by software, which makes it very flexible to establish and reorganize VLAN. When users are added, removed, or modified from a VLAN, there is no need to physically adjust the network.

VLAN classification:

port-based VLANs

VLAN based on MAC address

VLAN based on IP address

VLAN Based on Network Layer Protocol

VLAN Based on IP Multicast

VLAN data frame identification

Traditional Ethernet data frames cannot identify VLANs or subnets. VLAN frame identification is to put a unique identifier in each data frame. Every switch checks the VLAN identifier of this data frame to determine the VLAN to which the data frame belongs, and the switch can make a corresponding judgment and send the data frame to the destination port in this VLAN.

In addition, the switch is responsible for deleting VLAN information before the data frame is sent to the receiving device. Because data frames containing VLAN information are not standard Ethernet data frames.

VLAN data frame identification methods are:ISL(Cisco dedicated), IEEE802.1q(international standard), LANE and so on. IEEE802.1q and ISL.

The general data frame format is as follows:

preamble frame delimiter destination MAC address source MAC address type data part frame detection sequence 7 b1 b6 b2B46B-1500B4B

Data frame formats identified using IEEE 802.1q methods are as follows:

preamble frame delimiter destination MAC address source MAC address PIDCI type data part frame detection sequence 7 b1 b6 b2 b2B46B-1500B4B

PID identifies the protocol used and its value is 0x8100

CI contains a 12-bit VLAN identifier. The number of VLANs available is 2^12 -2 = 4094, of which 0 and 4095 are not used.

The frame detection sequence is recalculated

The format of the data frame identified by ISL is as follows:

Preamble Frame Separator ISL Header Destination MAC Address Source MAC Address Type Data Partial Frame Detection Sequence VLAN Data Frame Detection Sequence 7B1B26B6 B2B46B-1500B4B4B

The ISL header contains VLAN identification information

VLAN data frame detection sequence pairs from ISL header to original detection sequence

Ports in VLAN technology

There are two types of VLAN ports:

Access port

It can belong to only one VLAN and is assigned by manual settings. This port cannot receive information directly from another VLAN, nor can it send information to other VLANs.

Trunk port

By default, all VLANs belonging to this switch. Can forward frames for all VLANs. It can also be restricted by setting allowed-vlans.

The ports in the switch work by default in Access mode in Layer 2 mode.

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