Functions of VLAN-TRUNK

In a network topology with two switches, can the same VLAN communicate? The answer is yes !!

The Chinese meaning of trunk itself is: trunk, and trunk? In other words, it can carry something. It also plays a bearing role in the network, and it can carry multiple VLANs, enables communication between the same VLAN of different switching devices. As you can see, trunk also has its own structure. Since trunk is a relay link, we can't help but mention two types of links: 1. Access link: the access link is only a member of a VLAN. The devices connected to this port do not know that VLAN exists. The link connecting the host to the vswitch is the access link. A device is considered to be a part of a network or subnet Based on the layer 3rd information configured on the device. An access link is a port that belongs to only one VLAN. This port cannot receive or send information from another VLAN, unless the information is routed. 2. relay link: As I mentioned earlier, the relay link can carry multiple VLANs. The link between SW1 and sw2 is a relay link. Now let's look at the process of a data frame through a relay link. 1. When host a in vlan3 connected to SW1 sends a data frame to host B in vlan3 on sw2. The data sent by host a is a common data frame. 2. Switch SW1 receives this series of data frames based on the port information of the received data frame. After we know that the data frame is from vlan3 and the MAC address table is viewed, we know that we need to forward the data frame to sw2. Therefore, SW1 will mark the data frame with a VLAN, that is, insert a field in the data frame to write the vlan id information as this field. 3. After sw2 receives these data frames with vlan id, it forwards the data frames to host B in vlan3. The above is the process of data passing through the relay link. Next, let's talk about the VLAN labeling problem I just mentioned. VLAN tags play a significant role in the trunk. VLAN tags are frame tags that assign a unique user-defined ID to each frame transmitted on the relay link. This ID is the vlan id of a VLAN. If a frame is still sent to another relay link during transmission, the vlan id will remain in the frame header. Otherwise, if the frame is sent to a link, the switch deletes the vlan id in the frame header. There are two types of VLAN tags: Public tags 802.1Q encapsulate support from all vendors. Another type is isl encapsulation, which is only used by Cisco (Note: cisco2950 does not support ISL and only supports 802.1Q ). The formal name of IEEE 802.1Q is the virtual bridge LAN standard. It supports the ability to carry VLAN data streams on one layer through a relay link. It also defines the VLAN architecture. The services provided in the VLAN and the protocols and algorithms involved in providing these services adopt 802.1Q encapsulated frames. Four bytes are added to standard Ethernet frames. The original Ethernet frame cannot exceed 1518 bytes. If the maximum length of a frame is 8021, then. The frame is changed to 1522 bytes, which is called a small huge frame. However, the switch can still process such frames successfully. Tpid: indicates that the frame carries 802.1Q/802.1p. As for isl encapsulation, ISL adds a 26-byte frame header and a 4-byte frame tail to the frame. The end of the frame contains a cyclic redundancy Verification Code (CRC ). Now let's look back at the differences between 802.1Q and isl: 1. 802.1Q is a public VLAN-TRUNK label, and isl is a Cisco private label. 2. 802.1Q adds the mark to the middle of the Ethernet frame, while isl adds the mark to the beginning and end of the Ethernet frame. 3. the isl mark is 26 bytes longer than the 802.1Q mark. The 802.1Q mark is 4 bytes longer than the 802.1Q mark, and the isl is a total of ISL bytes longer. Now that we have understood the trunk principle, let's take a look at its configuration command: Switch (config) # interface fa Interface ID // enter the port mode switch (config-If) # switchport mode trunk // start trunkswitch # Show interface fa 0/0 switchport // view the trunk status