Talking about The Concept of Routing

The five-layer Internet protocol stack of a computer network is from top to bottom: application layer, transport layer, network layer, link layer, and physical layer. The routing concept appears in the network layer because the three important functions of the network layer are: Forwarding, routing, and connection establishment. This article mainly analyzes the concept of routing.

    First distinguish the difference between forwarding and routing:

   Forwarding refers to the local action of the router that transfers packets from an input link interface to an appropriate output link interface;

   Routing refers to a network-wide process to determine the end-to-end route taken by a packet from the source to the destination

    The choice of routing requires an algorithm to determine, so the task of the routing algorithm is to determine a good path from the sender to the receiver, and this path can be understood as a route.

    For example, suppose a driver is driving from Shanghai to Beijing. As we all know, there are too many roads to travel from Shanghai to Beijing, passing through many cities, so the driver needs to determine his own best route before setting off. The best basis may be the shortest distance, the least cost, the shortest time, etc. This is equivalent to a routing algorithm, and ultimately the driver will determine a route, which is equivalent to routing. This route will pass through multiple cities, then forwarding is equivalent to passing through the city, that is, the driver enters the city from one road and decides which way to leave the city, which can be understood as the difference between macro and micro.

    Each router has a forwarding table. The router forwards the packet by checking the value of the header field of the arriving packet, and then uses this value to index and query the router's forwarding table. The value of the header stored in the forwarding entry indicates the output link interface of the router to which the packet will be forwarded. The value in the packet header may be the destination address of the packet or a hint of the connection to which the packet belongs, depending on the network layer protocol.

    How is the forwarding table configured in the router? This is a key issue because it reveals the important interaction between routing protocols and forwarding. The routing algorithm determines the value of the forwarding table inserted into the router. The routing algorithm may be centralized or distributed, but in either case, the router accepts routing protocol packets, and this information is used to configure its forwarding table. The specific implementation process will not be explained in detail, please refer to computer network books (mainly because I am not very clear myself =.=|)

     The forwarding function of the network layer actually transfers the packet from the inbound link of a router to the appropriate outbound link

The four components of a router are: input port, switching structure, output port, and routing processor.


1. Input port: perform the physical layer function of connecting an input physical link to the router; perform the data link layer function that needs to interact with the data link layer at the far end of the inbound link; perform the search function on the input port .

2. Switch structure: Connect the input port and output port of the router. This switch structure is completely contained in the router, that is, it is a network in a network router.

3. Output port: Store the packets received from the switching junction and transmit these packets on the input link by performing necessary link layer and physical layer functions.

4. Routing processor: execute routing protocol, maintain routing table and link status information of the connection.