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What is a router? A router is a network device that connects multiple networks or network segments. It can "translate" data information between different networks or network segments, so that they can "read" each other's data, thus forming one.

routers have two typical functions, namely, data channel function and control function. Data channel functions include forwarding decision, backplane forwarding and output link scheduling, which are generally completed by specific hardware; Control functions are generally realized by software, including information exchange with neighboring routers, system configuration, system management, etc.

over the years, the development of routers has been ups and downs. In the mid-199s, traditional routers became the bottleneck restricting the development of the Internet. Instead, ATM switch becomes the core of IP backbone network, and router becomes a supporting role. At the end of 199s, the scale of Internet was further expanded, the traffic doubled every six months, ATM network became a bottleneck again, and routers made a comeback. After the appearance of Gbps routing switches in 1997, people began to replace ATM switches with Gbps routing switches, and built a backbone network with routers as the core.

attachment: router principle and routing protocol

in recent ten years, with the continuous expansion of computer network scale and the rapid development of large-scale Internet (such as Internet), routing technology has gradually become a key part in network technology, and routers have also become the most important network equipment. The demand of users promotes the development of routing technology and the popularization of routers. People are not satisfied with only enjoying information on the local network, but want to make maximum use of various types of network resources in various regions of the world. At present, any computer network with a certain scale (such as enterprise network, campus network, intelligent building, etc.) can't operate and manage normally without the router, regardless of whether it uses fast Internet technology, FDDI technology or ATM technology.

1 network interconnection

The main driving force of network interconnection is to interconnect one's own network with other networks, get more information from the network and publish one's own news to the network. There are many ways to interconnect networks, among which bridge interconnection and router interconnection are the most commonly used.

1.1 network interconnected by bridges

Bridges work in the second layer of the OSI model, that is, the link layer. Complete data frame forwarding, the main purpose is to provide transparent communication between connected networks. Bridge forwarding is to judge whether a frame should be forwarded and to which port according to the source address and destination address in the data frame. The address in the frame is called "MAC" address or "hardware" address, which is generally the address with the network card.

The function of a bridge is to interconnect two or more networks and provide transparent communication. Devices on the network can't see the existence of the bridge, and the communication between devices is as convenient as on a network. Because the bridge forwards data frames, it can only connect the same or similar networks (data frames with the same or similar structure), such as the interconnection between Ethernet and token ring. For different types of networks (data frame structures are different), such as between Ethernet and X.25, the bridge is powerless.

The bridge has expanded the scale of the network, improved the performance of the network and brought convenience to the network application. In the previous network, the bridge was widely used. However, the interconnection of bridges has also brought many problems: one is the broadcast storm. Bridges do not block the broadcast messages in the network. When the network is large (several bridges and multiple Ethernet segments), it may cause a broadcasting storm, which will lead to the whole network being completely filled with broadcast information until it is completely paralyzed. The second problem is that when interconnected with the external network, the bridge will combine the internal and external networks into one network, and both sides will automatically and completely open their own network resources to each other. This interconnection method is obviously unacceptable when interconnecting with external networks. The main source of the problem is that the bridge only communicates with the network to the maximum extent, regardless of the information transmitted.

1.2 router interconnection network

router interconnection is related to the protocol of the network, and our discussion is limited to TCP/IP network.

the router works in the third layer of the OSI model, that is, the network layer. Routers use the network address (IP address) on the "logic" defined by the network layer to distinguish different networks, realize network interconnection and isolation, and maintain the independence of each network. Routers do not forward broadcast messages, but limit broadcast messages to their own networks. Data sent to other networks is first sent to the router, and then forwarded by the router.

IP routers only forward IP packets, and keep the rest in the network (including broadcasting), thus maintaining the relative independence of each network, which can form a large network with many networks (subnets) interconnected. By interconnection at the network layer, routers can easily connect different types of networks. As long as the network layer is running IP protocol, it can be interconnected through routers.

devices in the network communicate with each other with their network addresses (IP addresses in TCP/IP networks). An IP address is a "logical" address that has nothing to do with the hardware address. Routers only forward data based on IP addresses. The structure of IP address has two parts, one part defines the network number, and the other part defines the host number in the network. At present, subnet mask is used to determine the network address and host address in IP address in Internet network. The subnet mask, like the IP address, is 32bit, and they are in one-to-one correspondence, and it is stipulated that the part of the IP address corresponding to the number "1" in the subnet mask is the network number, and the part corresponding to the number "" is the main phone number. The network number and the host number are combined to form a complete IP address. The IP addresses of hosts in the same network must have the same network number. This network is called an IP subnet.

communication can only be conducted between IP addresses with the same network number. to communicate with hosts in other IP subnets, you must go out through a router or gateway on the same network. IP addresses of different network numbers cannot communicate directly, even if they are connected together.

the router has multiple ports for connecting multiple IP subnets. The network number of the IP address of each port is required to be the same as the network number of the connected IP subnet. Different ports have different network numbers and correspond to different IP subnets, so that the hosts in each subnet can send the required IP packets to the router through the IP address of their own subnet

2 Routing principle

When a host in an IP subnet sends an IP packet to another host in the same IP subnet, it will directly send the IP packet to the network, and the other party will receive it. When sending IP packets to hosts with different IP addresses on the network, it should choose a router that can reach the destination subnet and send IP packets to the router, which is responsible for sending IP packets to the destination. If no such router is found, the host sends the IP packet to a router called "default gateway". "Default Gateway" is a configuration parameter on each host, which is the IP address of a router port connected to the same network.

when a router forwards an IP packet, it only selects an appropriate port according to the network number of the destination IP address of the IP packet and sends the IP packet out. Like the host, the router should also determine whether the port is connected to the destination subnet. If so, it will directly send the packet to the network through the port. Otherwise, it should also choose the next router to transmit the packet. The router also has its default gateway, which is used to transmit IP packets without knowing where to send them. In this way, IP packets that know how to transmit are correctly forwarded through the router, and IP packets that don't know are sent to the "default gateway" router. In this way, IP packets will eventually be sent to the destination, and IP packets that can't reach the destination will be discarded by the network.

At present, TCP/IP networks are all interconnected by routers, and the Internet is an international network in which thousands of IP subnets are interconnected by routers. This kind of network is called router-based network, which forms an "internetwork" with routers as nodes. In the "internetwork", the router is not only responsible for forwarding IP packets, but also for communicating with other routers. * * * It is the same as determining the routing of the "internetwork" and maintaining the routing table.

routing action includes two basic contents: routing and forwarding. Path-finding is to determine the best path to the destination, which is realized by routing algorithm. Because it involves different routing protocols and routing algorithms, it is relatively complicated. In order to determine the best path, the routing algorithm must start and maintain a routing table containing routing information, which is different depending on the routing algorithm used. The routing algorithm fills the collected different information into the routing table, and according to the routing table, the relationship between the destination network and the nexthop can be told to the router. Routers exchange information to update the route, update and maintain the routing table to correctly reflect the topology change of the network, and the router determines the best path according to the metric. This is the routing protocol, such as Routing Information Protocol (RIP), Open Shortest Path First Protocol (OSPF) and Border Gateway Protocol (BGP).

forwarding means transmitting information packets along the best path with good routing. The router first looks up in the routing table to determine whether it knows how to send the packet to the next site (router or host). The router does not know how to send the packet and usually discards the packet. Otherwise, the packet will be sent to the next site according to the corresponding entry in the routing table. The destination network is directly connected to the router, and the router will send the packet directly to the corresponding port. This is the routed protocol.

Routing and forwarding protocol and routing protocol are complementary and independent concepts. The former uses the routing table maintained by the latter, while the latter uses the functions provided by the former to publish routing protocol data packets. Unless otherwise specified, the routing protocols mentioned below refer to routing protocols, which is also a common habit.

3 routing protocol

There are two typical routing methods: static routing and dynamic routing.

a static route is a fixed routing table set in the router. Static routes will not change unless the network administrator intervenes. Because static routing can't reflect the change of the network, it is generally used in networks with small network scale and fixed topology. The advantages of static routing are simple, efficient and reliable. Of all the routes, the static route has the highest priority. When the dynamic route conflicts with the static route, the static route shall prevail.

dynamic routing is a process in which routers in the network communicate with each other, transmit routing information, and update the router table with the received routing information. It can adapt to the change of network structure in real time. When the routing update information indicates that the network has changed, the routing software will recalculate the route and send out new routing update information. These messages pass through various networks, causing routers to restart their routing algorithms and update their routing tables to dynamically reflect the network topology changes. Dynamic routing is suitable for networks with large network scale and complex network topology. Of course, various dynamic routing protocols will occupy network bandwidth and CPU resources to varying degrees.

static routing and dynamic routing have their own characteristics and application scope, so dynamic routing is usually used as a supplement to static routing in the network. When a packet is pathfinding in a router, the router first looks for a static route, and forwards the packet according to the corresponding static route. Otherwise, look for dynamic routes.

according to whether it is used within an autonomous domain, dynamic routing protocols are divided into internal gateway protocol (IGP) and external gateway protocol (EGP). The autonomous domain here refers to a network with unified management organization and unified routing policy. The routing protocols used in the autonomous domain are called internal gateway protocols, and commonly used are RIP and OSPF；. The external gateway protocol is mainly used for routing between multiple autonomous domains, and BGP and BGP-4 are commonly used. The following are briefly introduced respectively.

3.1 RIP routing protocol

RIP protocol was originally designed for Xerox parc general protocol of xerox network system, and it is a common routing protocol in the Internet. RIP uses distance vector algorithm, that is, routers choose routes according to distance, so it is also called distance vector protocol. The router collects all the different paths that can reach the destination, and keeps the path information about the minimum number of stops to reach each destination, and discards any other information except the best path to reach the destination. At the same time, the router also informs other neighboring routers of the collected routing information by RIP protocol. In this way, the correct routing information gradually spread to the whole network.

RIP is widely used, which is simple, reliable and easy to configure. But RIP is only suitable for small homogeneous networks, because the maximum number of allowed sites is 15, and any destination with more than 15 sites is marked as unreachable. RIP's routing information broadcast every 3s is also one of the important reasons for the network broadcast storm.

3.2 OSPF routing protocol

In the mid-198s, RIP could not adapt to the interconnection of large-scale heterogeneous networks, and SPF came into being. It is a routing protocol developed for IP networks by the Internal Gateway Protocol Working Group of the Internet Engineering Task Force (1ETF).

SPF is a routing protocol based on link state, which requires each router to send link state broadcast information to all other routers in the same management domain. The link state broadcast of OSPF includes all interface information, all metrics and other variables. Routers using SPF must first collect relevant link state information and calculate the shortest path to each node according to a certain algorithm. However, the routing protocol based on distance vector only sends routing update information to its neighboring routers.

unlike RIP, OSPF subdivides an autonomous domain into zones, and accordingly there are two types of routing methods: when the source and destination are in the same zone, intra-zone routing is adopted; When the source and destination are in different areas, interval routing is adopted. This greatly reduces the network overhead and increases the stability of the network. When the router in one area breaks down, it will not affect the normal work of routers in other areas in the autonomous domain, which will also bring convenience to the management and maintenance of the network.

3.3 BGP and BGP-4 routing protocol

BGP is an external gateway protocol designed for TCP/IP Internet, which is used between multiple autonomous domains. It is neither based on pure link state algorithm nor based on.