A detailed explanation of RIP routing update mechanism

8.1.3 RIP Routing update mechanism

RIP protocol has two kinds of update mechanism: one is to update regularly, and the other is to trigger the update. Periodic update sends a regular RIP routing advertisement based on the set update timer. The notification packet carries all the RIP routing information in the local router except for the RIP route suppressed by the horizontal split mechanism. The trigger update is the RIP router notification that is sent only when there is a change in the routing table entry, carrying only the changed routing information in the local routing table. Once the RIP router perceives a change in the network, it sends an update message as soon as possible, rather than waiting for the update cycle to end. As long as the trigger update fast enough, you can greatly prevent the "count to infinity" of the occurrence, but this phenomenon may occur.

Whether you are updating regularly or triggering updates, the update rules for RIP routes are as follows:

L Add the Routing table entry directly in the routing table if an updated routing table entry is not in the routing table;

L Update its routing table unconditionally based on the latest routing information if the routing table entry for the same destination network already has a route list with the same source port;

L If you have routing table entries for the same destination network in the routing table, but the source ports are different, compare their measurements and use one of the smaller measures as their own routing table entries;

L If the routing table entries for the same destination network are already in the route table and the measures are equal, leave the original routing table entries.

The following describes the periodic update mechanism for RIP routing.

1.RIP Routing Periodic update mechanism

RIP routers are always every 30 seconds (this is the default value, a route update package that can be modified, and may also be biased against the set value, is sent to a neighbor router via UDP 520 port in a RIP broadcast response, including the complete routing table on this router (in addition to being "split horizontally" A mechanism-suppressed routing table entry that provides routing updates to neighboring routers and is used to prove their existence to neighboring routers. The routing table for RIP mainly includes three fields: Destination network, next hop address, and distance, see figure 8-3.

If a router does not receive a routing update from a neighboring router in 180 seconds (which is also the default value, which can be modified), the router marks the neighbor router as an unreachable router, which causes the neighbor router to be in a suppression cycle. When the router is in a suppressed cycle, it is still used to forward packets forwards, but other routers in the network do not learn to reach the router's network routing information, unless it is a better route to reach the network of the router information, such as the original 3 jump, in the suppression cycle learned a 2-hop routing information. But after the suppression cycle, even poor routing information is accepted.

If you do not receive a routing update for this router in a continuous 240 second (which is also the default value, which can be modified), the local router deletes the routing table entries associated with that neighbor router in the routing table.

Thus, this routing update affects not only the most table-wide updates on routers across the RIP network and all the required arrivals, or the routing of packets through the router, but also the presence of other neighboring routers. Imagine if there was a packet that was being sent to a host on a network connected to one of the RIP routers. But this rip router just happened to fail, without this router update mechanism, the other routers do not know that it is currently faulted, still in the original routing path to transmit packets, The result, of course, is that the packet always fails to reach the destination host, although it may be tried several times.

2.RIP Routing Periodic update mechanism resolution example

To better understand the update mechanism of the RIP Protocol routing table, the following is an example of a simple interconnection network shown in Figure 8-6 to discuss how routing tables are established in each router in the diagram.

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Figure 8-6 Rip routing table to establish a network example

(1) In the beginning, the routing table in all routers has only the routing table entry information for the network to which they are directly connected. Instead of the RIP routing table entry, which is a direct-attached routing table entry without the next hop (in "--"), the metric "distance" is also 0, and the initial routing table for each router, shown in Figure 8-7, has only two direct-connected network routing table entries.

(2) Next, routers will send routing updates to the neighboring routers on the set cycle (default is 30 seconds). Which router will send the route update first depends on which router is open first. It is assumed that router R2 first receive routing updates from routers R1 and R3, and then updates its own routing table, as shown in Figure 8-8. As you can see, it adds a new route table entry that reaches the 10.0.0.0 network and 30.0.0.0 network through R1 and R3 respectively, with a measure of 1, because it's only a hop.

Figure 8-7 Initial routing table for R1, R2, and R3

(3) R2 updates its own routing table and sends the complete routing table to neighboring routers R1 and R3. The router R1 and R3 are updated separately. According to the rules that are updated on the RIP routing table described earlier, R1 first adds 1 to each metric in the routing table of Figure 8-8 that is received from R2, as shown in Figure 8-9.

Figure 8-8 R2 The routing table after the updated routing table 8-9 R1 to the metric plus 1 that was received from the R2 routing table