| Exam 400-101 | Question id=1241 | Layer 3 Technologies |
In which of the following situations does a router use AD values to determine route selection?
| A. |
when multiple routes to the same destination network are received, and all of these routes are received from the same routing protocol | |
| B. |
when multiple routes to the same destination network are received, and each of these routes is received from a different routing protocol | |
| C. |
when multiple routes to different destination networks are received, and all of these routes are received from the same routing protocol | |
| D. |
when multiple routes to different destination networks are received, and each of these routes is received from a different routing protocol |
A router uses administrative distance (AD) values to determine route selection when multiple routes to the same destination network are received, and each of these routes is received from a different routing protocol. Lower ADs are preferred over higher ADs.
A router uses metrics to determine route selection when multiple routes to the same destination network are received, and all of these routes are received from the same routing protocol. Each routing protocol uses different metrics. For example, Routing Information Protocol (RIP) uses hop count as a metric, Open Shortest Path First (OSPF)uses cost as a metric, and Enhanced Interior Gateway Routing Protocol (EIGRP) uses bandwidth and delay by default. When a routing protocol contains multiple routes to the same destination network, a router prefers the route with the lowest metric.
A router uses prefix lengths to determine route selection when multiple routes to different destination networks are received, regardless of the routing protocol. When multiple routes to overlapping networks exist, a router will prefer the most specific route, which is the route with the longest prefix match. For example, if a router has a packet destined to 10.1.1.1, it will prefer a route to 10.1.1.0/24 over a route to 10.1.1.0/16, and it will prefer a route to 10.1.1.0/30 over a route to 10.1.1.0/24.