Detailed analysis of the route table of the RIP Router

RIP routers are widely used and are believed to have been used by most administrators. How can we analyze the route table content of the RIP Router? The following describes the route table of the RIP Router. 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. Only one update timer is used for each RIP node. 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 enable the RIP node to maintain the route integrity 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. Once the time arrives, the RIP node generates a series of packets containing all 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 Autonomous System Based on RIP, these periodic updates will 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. Every 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. by

It is unwise to discard a series of routes when the update packet is lost. RIP uses 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 that 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 route timeout timer is usually set to 180 seconds. When the route becomes 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 that for a RIP node, 16 is infinite. Therefore, you can simply set the consumption measurement value to 16 to invalidate a route.

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 routing 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 items are 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 still does not receive the timeout of 270 seconds and the refresh time of 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 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.