RIP routing protocol and working principle (1)

RIPRouting information Protocol (Routing information Protocol) is an earlier and widely used internal Gateway Protocol, Interior Gateway Protocol (IGP). It is applicable to an autonomous system AS of a small similar network) transmission of routing information in. The RIP Protocol is based on the Distance Vector algorithm Distance Vector Algorithms, DVA. It uses the "Number of hops", that is, metric, to measure the routing distance to the target address. For more information, see RFC1058 and RFC1723. It is a distance vector protocol used to exchange routing information between routers and hosts. The latest version is v4, or RIPv4.

As for the "Internal Gateway Protocol" mentioned above, we can understand it in this way. Due to historical reasons, the current INTERNET network is composed of a series of autonomous systems, each autonomous system connected to the backbone network through a core router. An autonomous system usually corresponds to a set of networks and routers within an organizational entity, such as a company or university. Each autonomous system has its own routing technology, which is different for different Autonomous Systems. The routing Protocol used for interfaces between autonomous systems is called the "external Gateway Protocol" (EGP Exterior Gateway Protocol). The routing Protocol used for internal autonomous systems is called the "Internal Gateway Protocol ", IGP for short. Different from the external gateway protocol, the internal gateway has only one external routing protocol, while the internal router protocol is a family. The difference between different internal router protocols is that the distance standard distance metric (distance Measurement Standard) is different from the routing refresh algorithm. RIP is one of the most widely used IGP protocols. The famous path refresh program Routed is implemented based on RIP. The RIP Protocol is designed for medium-sized networks that use the same technology, so it is applicable to most campus networks and regional networks with low usage rate changes that are not large consecutive lines. For more complex environments, the RIP Protocol is generally not used.

1. How RIP works
The RIP Protocol is based on the Bellham-Ford Distance Vector Algorithm. This algorithm was used for computer routing selection in 1969. The formal protocol was first developed by Xerox in 1970, it was part of the "Networking ServicesNXS)" protocol family of Xerox. Due to its simple implementation, RIP quickly becomes the most widely used routing protocol.

The key role of a vro is to Interconnect Networks. Each vro is connected to more than two actual networks and forwards data packets between these networks. When discussing IP address selection and packet forwarding, we always assume that the router contains the correct route, and the router can use the ICMP redirection mechanism to request the host connected to it to change the route. However, you must obtain the correct route table in some way before selecting the IP address. In a small, slowly changing Interconnect Network, managers can manually create and change route tables. In a large and rapidly changing environment, the manual update method is too slow to accept. In this case, the route table must be automatically updated, that is, the so-called dynamic routing protocol. The RIP Protocol is the simplest one.

When routing is implemented, RIP is used as a system long resident process daemon. It is in a vro and receives route information from other vrouters of the network system, in this way, the route table of the local IP layer is dynamically maintained to ensure that the correct route is selected when the IP layer sends a message. Broadcasts the route information of the vro and notifies the adjacent vro to make corresponding changes. The RIP Protocol is at the upper layer of the UDP protocol, and the route information received by the RIP is encapsulated in the UDP datagram. The RIP receives the route modification information from the remote router on the UDP port 520, modify the local route table and notify other routers. In this way, global routing is effective.

The RIP routing protocol uses the "Update UNPDATES)" and "request REQUESTS)" groups to transmit information. Each router with the RIP Protocol broadcasts update information to directly connected machines on the UDP520 port every 30 seconds. The update information reflects all route selection information databases of the vro. Each entry of the routing information database consists of "IP addresses accessible on the LAN" and "distance from the network. Request information is used to find other devices that can send RIP packets on the network.

RIP uses "Number of Route segments", that is, "Number of hops", as the network distance scale. Each router adds an internal distance to each route when it sends route information to the adjacent router. In 9-31, vro3 3 is directly connected to Network C. When it notifies vro2 2 of the path 142.10.0.0, it increases the number of hops by 1. Similarly, vro2 2 increases the number of hops to "2" and advertises the route to vro1 1, the distance between vro2 2 and vro1 1 and vro3 3 is 1 hop and 2 hop, respectively.

Figure 9-31 example of how RIP works