In-depth explanation of the RIP Routing Protocol

The use of the modern Internet is inseparable from the application of routing. In a vro, a standard is a prerequisite for the normal operation of the entire vro. Now let's take a look at the content of the RIP routing protocol. What is the RIP routing protocol? We will learn the specific functions from the following points. RIPRouting Information Protocols is the most widely used distance vector protocol, which was developed by Xerox in 1970s. At that time, RIP was part of the XNSXerox Network Service, Xerox Network Service) protocol cluster. The TCP/IP version of RIP is the license version of Xerox protocol. The biggest feature of RIP is that the implementation principle and configuration method are very simple.

Measurement Method

The RIP measurement is based on the hop count hops count). Each time a router passes through, the hop count of the path increases by one. As a result, the larger the number of hops, the longer the path, and the RIP algorithm will first select a path with fewer hops. The maximum number of hops supported by RIP is 15, and the number of hops is 16.

Route update

In the RIP routing protocol, route updates are implemented through scheduled broadcast. By default, the router broadcasts its route table to the network connected to it every 30 seconds. The router connected to the broadcast adds the received information to its route table. Every vro is broadcast so that all vrouters on the network will know all route information. Under normal circumstances, the router can receive a route message confirmation every 30 seconds. If the route entry is not confirmed after 180 seconds, that is, six update cycles, the router considers it invalid. If the route entry remains unconfirmed after 240 seconds, that is, eight update cycles, it will be deleted from the routing table. The above 30 s, 180 s, and 240 s are all controlled by the Timer, which are the Update Timer), the Invalid Timer), and the refresh Timer Flush Timer ).

Routing cycle

Distance vector algorithms are prone to route loops. The RIP routing protocol is a distance vector algorithm, so it is no exception. If there is a route loop on the network, information will be transmitted cyclically and never reach the destination. To avoid this problem, the RIP vector algorithm implements the following four mechanisms.

Horizontal split horizon ). Horizontal segmentation ensures that the router remembers the source of each route information and does not send it again on the port that receives this information. This is the most basic measure to prevent route loops.

Toxicity reversal poison reverse ). When a path becomes invalid, the router does not immediately delete it from the routing table, but broadcasts it with a 16, that is, an inaccessible measurement value. In this way, although the route table size is increased, it is very helpful to eliminate the routing cycle. It can immediately clear any loops between neighboring routers.

Trigger update ). When the route table changes, the update packet is immediately broadcast to all adjacent routers, instead of waiting for a 30-second update cycle. Similarly, when a router just starts the RIP routing protocol, it broadcasts request packets. The adjacent router that receives the broadcast immediately responds to an update message, instead of waiting for the next update cycle. In this way, the changes in the network topology will spread on the network as quickly as possible, reducing the possibility of loop routing.

Suppress the time holddown timer ). After a route entry is invalid, the route entry is in the suppression State for a period of time, that is, it no longer receives route updates for the same destination address within a certain period of time. If the router learns that a path is invalid from one CIDR block, it immediately learns that the route is valid on the other CIDR block. This effective information is often incorrect. Blocking timing avoids this problem. In addition, when a link starts and stops frequently, blocking timing reduces route fluctuations and increases Network stability.

Even if the above four methods are used, the routing loop problem cannot be completely solved, but the problem is minimized. Once a route loop occurs, the metric value of the route entry is counted to an infinite Count to Infinity. This is because route information is cyclically transmitted. Each time a vro is transmitted, the measurement value is added to 1 and the value is always added to 16, and the path becomes inaccessible. It is clever to select 16 as an inaccessible metric value for the RIP routing protocol. It is large enough to ensure that most networks can run normally and small enough, it takes the shortest time to count to infinity.

Neighbors

Some networks are NBMANon-Broadcast MultiAccess, and non-Broadcast multi-channel access), that is, Broadcast is not allowed to transmit data on the network. For such a network, RIP cannot rely on the broadcast route table. There are many solutions, the simplest is to specify the neighbor), that is, to specify to send the route table to a specific router.

RIP route protocol Defects

Although RIP is easy to use and has been tested for a long time, it also has some important defects, mainly including the following:

It is too simple. The measurement value is calculated based on the number of hops, and the unoptimal route is often obtained;

The measurement value is limited to 16, which is not suitable for large networks;

Poor security. Accept route updates from any device;

Classless IP addresses and VLSMVariable Length Subnet Mask are not supported, and the Subnet Mask is extended );
 
Slow convergence, often more than 5 minutes;

High bandwidth consumption.