Definition of the RIP route protocol and update of the route table

In terms of routers, protocols are a key point. Protocol learning can help us better configure and manage vrouters. As the basis, we should understand the RIP Protocol. The RIP routing protocol is mainly a routed-based program. At the same time, routed runs on Unix systems. When the RIP routing protocol is enabled on the network, the network has a powerful function to automatically update the route table. However, some major internal gateway protocols, such as RIP/RIP2, OSPF, or IGRP/OSPF, have certain protocols.

The Internal Gateway Protocol is applicable to scenarios where only one Administrator is responsible for network operations and operations. Otherwise, a configuration error may cause network performance degradation or unstable network operation. For networks shared by many administrators, such as the Internet, consider using EGP InteriorGatewayProtocol, external gateway protocol), such as BGP4.

If there is only one vro in the network, there is no need to use the RIP routing protocol. It is necessary to share information only when there are multiple vrouters in the network. However, if you only have a small network, you can use static routes to manually update the route table.

Routing Information Protocol

RIPRoutingInformationProtocol) is based on a program called "routed". The program runs on a BSDI Unix system and is standardized in RFC1058 in 1988. In version 2 described in RFC1388, support for VLSMVariableLengthSubnetMasks and variable-length subnet shielding are added, but this does not make up for the main defects of the RIP routing protocol. For example, in a network with multiple paths to the same destination, it takes a lot of time for RIP to determine a selectable path. In a network without multiple paths, the RIP routing protocol has been widely used.

The RIP routing protocol is listed as the distance vector protocol, which means that distance is used to determine the optimal path, as measured by the number of Route hops. The router sends broadcast information to each other every 30 seconds. Each vro that receives the broadcast information adds a hop count. If the broadcast information is received by multiple routers, the path with the lowest hop count on the router is the selected path. If the preferred path does not work properly, the path with a higher hop count will be used as a backup.

For the RIP and other routing protocols), the router on the network must go through the process of deciding the alternative path when a path is unavailable. This process is called Convergence ). It is a major problem that the RIP routing protocol spends a lot of time on convergence. Before the RIP routing protocol realizes that the path cannot be reached, it is set to wait until it has missed 6 updates for a total of 180 seconds. Then, before using the new path to update the route table, it waits for the next information of another feasible path to arrive. This means that it takes at least three minutes before the backup path is used, which is a long time for most applications to time out.

The fact that RIP records only one route for each destination requires that RIP actively maintain the integrity of the route table. This is done by requiring all active RIP routers to broadcast their route table content to the adjacent RIP Router at a fixed interval. All received updates automatically replace the information already stored in the route table.
RIP relies on three timers to maintain the route table:

· Update Timer

· Route timeout Timer

· Route refresh Timer

The update timer is used to initialize route table updates at the node level. Each RIP node uses only one update timer. On the contrary, the route timeout timer and route refresh timer maintain one for each route. In this case, different timeout and route refresh timers can be combined in each route table entry. Together, these timers allow the RIP node to maintain the integrity of the route and recover the network from the fault through time-based triggering.

1. initialize table update

The RIP Router triggers a table update every 3 0 seconds. The update timer is used to record the time amount. Once the time reaches, the RIP node will generate a series of packets containing all of its route tables. These packets are broadcast to each adjacent node. Therefore, each RIP Router receives an update from each adjacent RIP node every 3 0 seconds. Note that in a larger RIP-based autonomous system, these periodic updates generate unacceptable traffic. Therefore, it is more ideal for a node to be updated in a node-by-node manner. The RIP automatically completes the update. Each time the timer is updated, it is reset. A small and arbitrary time value is added to the clock. If the update does not appear as expected, it indicates that a fault or error occurs somewhere in the Internet. A fault may be caused by dropping the packet containing the updated content. Faults may also be serious, such as router faults, or between these two extremes. Obviously, taking appropriate measures will be very different for different faults. It is unwise to invalidate a series of routes because of the loss of update packets. RIP Update packets use unreliable transmission protocols to minimize overhead ). Therefore, it is reasonable not to take corrective actions when an update is lost. To help identify the importance of faults and errors, RIP uses multiple timers to identify invalid routes.

2. identify invalid routes

There are two ways to make the route invalid: Route termination. The router learns from other routers and the route is unavailable. In any situation, the RIP Router needs to change the route table to indicate that the given route is no longer reachable. If a route does not receive an update within a given time period, it is aborted. For example, the router timeout timer is usually set to 180 seconds. When the route changes to active or updated, the clock is initialized. 180 seconds is the approximate estimated time. This time is enough for a router to receive six route table Update packets from its neighboring routers. Suppose they send route updates every 30 seconds ), if the RIP Router does not receive any update about the route after 180 seconds, the RIP Router considers that the destination I P address is no longer reachable. Therefore, the router will mark the route table as invalid. You can set its route metric value to 1 6 and set the route change flag. This information can be exchanged with neighboring routers through periodic route table updates.

Note: For nodes of the RIP routing protocol, 16 is infinite. Therefore, a route entry can be discarded if the consumption metric value is set to 16.

Neighboring nodes that receive a new notification of invalid routing use this information to update their route tables. This is the second method in which the route becomes invalid. An invalid entry exists in the routing table for a short time. The router determines whether to delete it. Even if the table item is kept in the routing table, the packet cannot be sent to the destination address of the table item: RIP cannot forward the packet to an invalid destination.

3. Delete invalid routes

Once the router realizes that the route is invalid, it will initialize a second Timer: Route refresh timer. Therefore, after the last timeout timer is initialized for 180 seconds, the route refresh timer is initialized. This timer is usually set to 90 seconds. If the route update does not receive the timeout of 270 seconds and the refresh time of the route is 90 seconds after 180 seconds), remove the route from the route table, that is, refresh the route ). The timer that refreshes the descending count of a route is called a route refresh timer. This timer is absolutely required for the ability of RIP to recover from network faults.

Active and passive sites

Note that in order for the RIP network to work normally, every gateway in the network must be involved. Participation can be active or passive, but all gateways must be involved. Active nodes are those that actively share route information. They receive updates from neighboring nodes and forward route table entries to those adjacent nodes. Passive sites receive updates from neighboring users and use those updates to maintain their route tables. However, passive nodes do not actively publish copies of their own route table items. The ability to passively maintain route tables is particularly useful in the days before the appearance of a hardware router. At that time, a route is a background program running on a UNIX processor, this minimizes the routing overhead on UNIX hosts.