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Adding routes to the RIB

After exchanging the service messages, the routers add the received routes to the RIB. In this case, some routes are filtered out. To filter the routing information, the devices are guided by the following principles:

• only one route to the destination network can be added to the RIB;
• if there are two routes to the same network, a the route with a lower metric value is added to the RIB;
• a route with a higher metric value can be added to the RIB if it is received from the same source;
• if there are two routes to the same destination network with the same metric values, then the route received first will be added to the RIB.

Let's assume that router R4 received a service message from router R2 earlier than from R3. Then, in accordance with according to the described principles, routes the routers add the following routing information to the RIB:

Adding routes

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to the FIB

Routes export from RIB to FIB is go along with the analysis of distance values assigned to the route source. Distance value 120 is assigned to RIP, so some routes added to the RIB at the previous step will be filtered out. For example, all routes with metric 1 will be discarded because these are routes to directly connected networks and have 0 distance value.

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