Principle: Before sending data, first monitor whether someone is sending on the channel. If there is, it means that the line is busy, and then decide according to the predetermined strategy.

algorithm

Principle: CSMA (monitoring) can only reduce the probability of conflict, but cannot completely avoid it, so the collision detection method of listening and transmitting is adopted

method

Principle: After detecting a conflict, immediately stop sending, send interference signals, wait for a random time before sending data

Random time formula: waiting time = t*Random[0,1,...2k-1 (k is the square)], so the backoff time is not necessarily longer than the previous one. The greater the number of retransmissions, the larger the backoff window, thereby reducing the probability of collision.

1. The preceding pilot field (7 bytes) and start identifier (1 byte) are not included in the frame length; 2. The data length is 46-1500 bytes. If it is not enough, it will be filled to 46 bytes; 3. Check bit 4 bytes, CRC cyclic redundancy check 32 bits 4. Destination address 6 bytes, source address 6 bytes, length: 2 bytes 5. Minimum frame: 64 bytes; (46 6 6 2 4) 6. The longest frame: 1518 bytes (1500 6 6 2 4)

1. Ethernet header: destination source address length = 6 6 2 2. IP header: 20 bytes 3.TCP header: 20 bytes 4. Application data: minimum 46 - IP - TCP =6 bytes 5. Application data: Maximum 1500 - IP - TCP = 1460 bytes 6. Checksum: 4 bytes

Fast Ethernet (100M)

Gigabit Ethernet (1000M)

10 Gigabit Ethernet (10G)

A logical network formed by segmenting a switched LAN based on management function, organization, or application type.

Different VLAN communications must pass through three-layer devices: routers, three-layer switches, and firewalls.

A hub or repeater is a collision domain, a VLAN is a broadcast domain, and an interface on a switch is a collision domain.

Static VLAN: based on switch port

Dynamic VLAN: based on MAC address, based on policy, based on network layer protocol, based on network layer address

Controlling network traffic helps control broadcast storms, reduce conflicts, and improve network bandwidth utilization;

Improve network security, different VLANs cannot communicate;

Flexible network configuration, VLAN configuration can break through geographical location restrictions and use management functions to divide;

PRI (3 bits): Priority indicates priority, providing a total of 8 priorities from 0 to 7. When multiple frames are waiting, data packets are sent in order of priority.

VID (12 bits): VALN identifier, up to 2^12=4096 VLANs, of which VID0 is used to identify priority, VID4095 is reserved for use, up to 4094 VLANs can be configured, the default VLAN1, cannot be deleted

Note: The Vlan labeling process of the switch is implemented by dedicated hardware and will not cause too much delay in data.

VLAN tags are transparent to users. When the PC receives data, the VLAN tags will be torn off;

Access interface: can only transmit single Vlan data, generally used for PCs, cameras, etc.;

Trunk interface: capable of transmitting multiple VLAN tags, generally used between switches;

Hybrid interface: hybrid interface, including Access interface and Trunk interface

QINQ (802.a1d): double-layer label, generally used in carrier metropolitan area networks;

MAC-IN-MAC: also called PBB, 802.1ah

MAN technology

Once broadcast data frames appear, these data will be continuously broadcast and flooded by interconnected switches, causing broadcast storms;

After a broadcast storm occurs, it will seriously occupy CPU resources and network bandwidth;

Phenomenon: The network is slow, all indicators flash at high speed, CPU usage is high, and CLI freezes

Since all switches are connected together, the ports will automatically learn the MAC address, so when a PC sends out a message, each switch will continuously forward the information, causing the switch to continuously change the device MAC address.

8 bytes, consisting of 2 priority bytes and 6 bytes of MAC address (default: 32768)

The smaller the value, the higher the priority. When manually modifying it, it must be a multiple of 4096.

Determine the root bridge: choose the bridge with the smallest priority and MAC address

Determine the root ports of other bridges: the port closest to the root bridge from the non-root bridge port

Select a designated port for each segment: the designated bridge is given priority, and the designated port is on the designated bridge.

Select non-specified ports

Transmit BPDU messages

Spanning Tree Protocol: 802.1d STP (slow, convergence takes 30S-50S)

Rapid Spanning Tree Protocol: 802.1W RSTP (fast, 6S)

Multiple Spanning Tree Protocol: 802.1S MSTP (to achieve multiple Vlan load balancing)