| Exam 400-101 | Question id=1278 | Layer 3 Technologies |
You issue the show ip route command on RouterA and receive the following partial output:
O 10.20.0.0/28 [110/64] via 192.168.10.1, 00:02:38, Serial0/1
D 10.20.0.0/26 [90/2809856] via 192.168.10.4, 00:02:14, Serial0/4
R 10.20.0.0/24 [120/3] via 192.168.10.3, 00:33:38, Serial10/3
S 10.20.0.0/22 [120/3] via 192.168.10.2
RouterA receives a packet that is destined for 10.20.0.14.
Which of the following routes will RouterA use to send the packet?
| A. |
the static route, because static routes are preferred over dynamic routes | |
| B. |
the EIGRP route, because it has the lowest administrative distance | |
| C. |
the RIP route, because it has the highest administrative distance | |
| D. |
the OSPF route, because it is the route with the longest prefix match |
RouterA will use the Open Shortest Path First (OSPF) route, because it is the route with the longest prefix match. When a packet is sent to a router, the router checks the routing table to see whether the nexthop address for the destination network is known. If multiple routes to a destination are known, the most specific route is used. Therefore, the following rules apply on RouterA:
- Packets sent to the 10.20.0.0/28 network use the OSPF route. This includes destination addresses from 10.20.0.0 through 10.20.0.15.
- Packets sent to the 10.20.0.0/26 network, except those sent to the 10.20.0.0/28 network, use the Enhanced Interior Gateway Routing Protocol (EIGRP) route. This includes destination addresses from 10.20.0.16 through 10.20.0.63.
- Packets sent to the 10.20.0.0/24 network, except those sent to the 10.20.0.0/26 network, use the Routing Information Protocol (RIP) route. This includes destination addresses from 10.20.0.64 through 10.20.0.255.
- Packets sent to the 10.20.0.0/22 network, except those sent to the 10.20.0.0/24 network, use the static route. This includes destination addresses from 10.20.1.0 through 10.20.3.255.
- Packets sent to any destination not listed in the routing table are forwarded to the default gateway, if one is configured.
Because the most specific route to 10.20.0.14 is the route toward the 10.20.0.0/28network, RouterA will forward a packet destined for 10.20.0.14 to the Serial0/1 interface.
RouterA will not use the EIGRP route to send a packet that is destined for 10.20.0.14. Administrative distance (AD) values are used only to determine which route is placed in the routing table when multiple routes to a destination are known. However, a router considers routes with different prefix lengths as separate routes. If OSPF, EIGRP, and RIP had each advertised routes to 10.20.0.0/28, the EIGRP route would have been selected because EIGRP has the lowest AD.
RouterA will not use the RIP route to send a packet that is destined for 10.20.0.14. Routes with longer prefix lengths are preferred over routes with shorter prefix lengths, and routes with lower ADs are preferred over routes with higher ADs.
RouterA will not use the static route to send a packet that is destined for 10.20.0.14. If the static route were configured so that the destination network was 10.20.0.0/28, the static route would be preferred over the OSPF route because static routes have a lower AD than dynamic routes.