Advanced learning of RIP Routing Protocol

In the family of routing protocols, the most basic is the RIP routing protocol. Next, let's take a closer look at the specific principles and algorithms of this Protocol. Some of the basic principles are also used in subsequent protocols.

Measurement Method

The RIP measurement is based on the number of hops (hops count). Each time a router passes through, the number of hops in the path increases by one. As a result, the longer the number of hops, the longer the path, the RIP algorithm gives priority to the path with a few hops. the maximum number of hops supported by RIP is 15, and the number of hops is 16 ｡

Route update

Route updates in RIP are implemented through scheduled broadcast. By default, the router broadcasts its route table to the network connected to RIP every 30 seconds, the router that receives the broadcast adds the received information to its route table. Each router is broadcast so that all the routers on the network will know all the route information. Normally, the router receives a route message confirmation every 30 seconds. If a route entry is not confirmed after 180 seconds, that is, six update cycles, the router considers it to have expired. If the route entry remains unconfirmed after 240 seconds, that is, eight update cycles, it will be deleted from the routing table, the latency of 180 seconds and 240 seconds is controlled by the Timer. They are the Update Timer, Invalid Timer, and Flush Timer )｡

Routing cycle

Distance vector algorithms are prone to route loops. RIP is a distance vector algorithm, so the RIP routing protocol is no exception. If there is a route loop on the network, information will be transmitted cyclically, to avoid this problem, the RIP equi-distance vector algorithm implements the following four mechanisms ｡

Horizontal segmentation ensures that the router remembers the source of each route information, and do not send it again on the port that receives this message. This is the most basic measure to ensure that no route loop is generated ｡

Poison reverse. When a path becomes invalid, the router does not immediately delete it from the routing table, but uses 16, that is, an inaccessible measurement value broadcasts it out. Although this increases the size of the route table, it is helpful to eliminate the routing cycle and 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, when a router initiates RIP, it broadcasts the request message. The adjacent router that receives the broadcast immediately responds to an update message, instead of waiting for the next update cycle, changes in the network topology will spread over the network as quickly as possible, reducing the possibility of Route loops ｡

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, A vro learns that a path is invalid from one CIDR block, and then immediately learns that the route is valid on the other CIDR block. This valid information is often incorrect. suppressing timing avoids this problem, in addition, when a link starts and stops frequently, the suppression timing reduces the floating speed of the route, and the RIP route protocol increases the network stability ｡

Even if the above four methods are used, the routing loop problem cannot be completely solved, but the problem is minimized, when the metric value of a route entry is counted to an infinite number (Count to Infinity), this is because route information is transmitted cyclically. Each time a router is transmitted, the metric value is added to 1, when it reaches 16, the path becomes inaccessible. It is clever to select 16 as the inaccessible measurement value for the RIP routing protocol. It is large enough, this ensures that most networks can run normally and are small enough to minimize the time required to count to infinity ｡

Neighbors

Some networks are NBMA (Non-Broadcast MultiAccess, Non-Broadcast multi-channel access), that is, Broadcast is not allowed to transmit data on the network. For such networks, the RIP Routing Protocol cannot rely on the broadcast route table. There are many solutions. The simplest solution is to specify a neighbor (neighbor), that is, 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 VLSM (Variable Length Subnet Mask, Variable Length Subnet Mask) are not supported );

Slow convergence, often more than 5 minutes;

High bandwidth consumption ｡