As you are training a new junior technician, the trainee is examining the routing table. He tells you that there are four different routes to the same network in different routing databases. He asks you which of the routes will be used to populate the routing table.
What will your answer be, assuming that all routing protocols are set at the default administrative distance?
A. The route with an R next to it
B. The route with an S next to it
C. The route with a C next to it
D. The route with an I next to it
Correct Answer: C
Explanation:
The route with a C next to it is a directly connected route and has an administrative distance of 0, which means it will be preferred over any routes with a larger value for administrative distance. Each routing protocol has a default administrative distance assigned. Administrative distance is used by the router to determine the preferred route when a route is learned from different routing protocols. This process can be manipulated by the administrator by using the distance command to alter the default assignments.
It is significant to note that routers with no static routes and no routing protocols enabled will populate all directly connected routes to the routing table with no action on the part of the administrator. Routes that are NOT directly connected will not be in the routing table unless one of two things occurs:
– A static route is created by the administrator
– A routing protocol is enabled that allows the router to learn about the network and its route from another router running the same routing protocol
For example, in the diagram below, R3 will have routes to the 192.168.3.0/24 ,192.168.1.0/24 and the 192.168.0.0/24 networks in its routing table by default. It will only have routes to the 192.168.2.0/24, 192.168.5.0/24, and 192.168.6.0/24 networks if a routing protocol is used or if an administrator creates static routes for each network.
When a packet is received by a router interface, the router de-encapsulates the frame or removes the layer two information (MAC data for Ethernet or DLCIs for frame relay) and then performs a lookup for the network ID of the network in which the destination IP address resides. When multiple routes exist, it will choose the one with the lowest administrative distance. The router only places the route with the lowest distance in the table.
The route with an R next to it is a route learned from Routing Information Protocol (RIP). It has a default administrative distance of 120, so it will not prefer over a directly connected route.
The route with an S next to it is a static route or one configured manually. It has an administrative distance of 1, so it will not be preferred over a directly connected route.
The route with an I next to it is a route learned from Internal Gateway Routing Protocol (IGRP). It has an administrative distance of 100, so it will not be preferred over a directly connected route.
