The second and third layers are relative, generally referring to the second and third layers in the seven layers of OSI. For example, when a QQ message is clicked and sent, it will definitely go through seven layers of protocols, and finally it will go to the physical layer, and then it will be transmitted through electrical signals. OSPF is only a routing protocol, which has nothing to do with Layer 2 and Layer 3. This just means that if the OSPF protocol is to be deployed, it must be in a three-layer network, because the related messages of OSPF are transmitted through IP.

PPP, HDLC, FR and Ethernet are all Layer 2 technologies, but each has its own advantages. Take Ethernet as an example, which is the MAC address we often talk about, or take QQ messages as an example. QQ messages will be abbreviated to 7-5 layers after entering the computer. After arriving at the fourth layer, the TCP source port and destination port number will be affixed, and then sent to the third layer for processing, plus the source IP address and destination IP address, and then to the second layer, plus Ethernet. . . . . .

In short, it is necessary to distinguish levels, and the use of the network cannot be separated from any of the two or three layers. They are continuous, not parallel.