Router is the necessary equipment for interconnection between different networks, and also the main node equipment on the Internet.
1. Router working level and function
Routers work in the network layer of the OSI reference model. Routers of different models, brands and applications have different numbers of WAN ports and ports used to connect to the LAN. As shown in Figure 3-12, a router with two WAN ports and three LAN ports has different numbers.
The WAN port of the router is used to connect external networks, which can be a WAN or a WAN communication subnet.
The LAN port is used to connect to the local network. One LAN port is connected to one network segment. The network segments connected by each LAN port must have the same network architecture. Each port of the router contains a MAC entity for data transmission between adjacent nodes.
There are two main functions:
(1) Path selection function: find the best path for each packet forwarded through the router Website production Forwarding path.
(2) Forwarding/filtering function: responsible for forwarding data packets and filtering network broadcasts to ensure the independence of each network.
2. Working principle of router
The router distinguishes different networks according to the network identification of the network connected to each port (see Section 4.3.2 for basic knowledge of network identification). Therefore, there is an internal routing table in the router for path selection.
As shown in Figure 3-13, two LAN ports of the router connect two internal networks. The network architecture of both ends of the network is the same, and they are connected to router ports E0 and El respectively. The router has a built-in routing table based on the path length as the measurement standard, which gives information such as which port must be forwarded to access which network and the distance from the destination node.
If host A sends a packet to host Z, router port El receives the packet sent by A and looks up the corresponding port E2 of the network ID of destination host Z in the routing table, with the shortest distance of., It can be forwarded from port E2; If no forwarding port is found in the routing table, the router will discard the packet. It can be seen from the above that routers do not forward unknown data packets, which plays the role of isolating the network and limiting the broadcast messages in their own networks to the inside of the network.
3. Creating a Routing Table
The routing table is a very important database in the router. It contains specific information about how to choose a path (routing algorithm) and which path to choose (path selection) for forwarding packets in the network layer. There are static and dynamic methods to establish the routing table, namely manual setting and automatic updating. The content of the routing table varies according to the metric based on which the routing algorithm is based. It is also complex to build, and will not be introduced here.
