Update route table

The fact that r I p records only one route for each destination requires r I P to actively maintain the integrity of the route table. This is done by requiring all active r I p routers to broadcast the contents of their route tables to adjacent r I P routers at regular intervals, all received updates automatically replace the information already stored in the route table.

R I P 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 r I P 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 enable the r I P node to maintain the integrity of the route and recover the network from the fault through time-based triggering.

1. initialize table update
The r I P router triggers a table update every 3 0 seconds. The update timer is used to record the time amount. Once the time reaches, the r I P node will generate a series of packets containing all of its route tables.
These packets are broadcast to each adjacent node. Therefore, each r I p router receives an update from each adjacent r I P node every 3 0 seconds.

Note that in a larger r I P-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 r I P automatically completes the update. Each update timer is reset, and 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 invalidate a series of routes because Update packets are lost (Remember, r I P updates packets using 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, r I P 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 r I P router needs to change the route table to indicate that the given route is no longer accessible.

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 1 8 0 seconds. When the route changes to active or updated, the clock is initialized.

1 8 0 s is the approximate estimated time, this time is enough for a router to receive six route table Update packets from its neighboring routers (assuming they send route updates every 3 0 s). If 1 8 0 s disappears, the r I p router does not receive an update about the route. The r I p 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 an r I P node, 1 6 is infinite. Therefore, a route entry can be discarded if the measurement value is 1 6.

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: r I P 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, the route refresh timer is initialized for 1 8 0 seconds. This timer is usually set to 9 0 seconds.

If the route update is not received after 2 7 0 s (1 8 0 s timeout plus 9 0 s route refresh time), remove the route from the route table (that is, refresh ). The timer that refreshes the descending count of a route is called a route refresh timer. This timer is absolutely required for r I P's ability to recover from network faults.

Active and passive sites
Note that in order for the r I P network to work normally, it is important that 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 users and forward their route table items 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 the route table is particularly useful in the days before the appearance of the hardware router. At that time, the route is a background program running under the u n I X processor, this minimizes the routing overhead on the u n I X host.
Article entry: csh responsible editor: csh