Cisco routing decision scenario: The Corporate router receives an IPv4 packet with source 192.168.214.20 and destination 192.168.22.3. Based on the following 'show ip route' output, what action will the router take? Corp#sh ip route [output cut] R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0 R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0 R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0 C 192.168.20.0 is directly connected, Serial0/0 C 192.168.214.0 is directly connected, FastEthernet0/0

Difficulty: Medium

The packet will be discarded.
The packet will be routed out the S0/0 interface.
The router will broadcast looking for the destination.
The packet will be routed out the Fa0/0 interface.

Correct Answer: The packet will be discarded.

Explanation:

Introduction / Context:
This question tests how Cisco routers perform a routing table lookup and decide the forwarding action when no matching route exists. Understanding connected routes, learned routes, and default routes is essential for troubleshooting reachability.

Given Data / Assumptions:

Routing table shows connected networks: 192.168.214.0/24 on FastEthernet0/0 and 192.168.20.0/24 on Serial0/0.
Routed (RIP) networks: 192.168.215.0/24, 192.168.115.0/24, 192.168.30.0/24 via 192.168.20.2.
No default route is displayed.
Packet destination is 192.168.22.3 (network 192.168.22.0/24).

Concept / Approach:
A router forwards a packet only if it finds a longest-prefix match in the RIB (routing table). If no specific route or default route (0.0.0.0/0) exists, the router drops the packet. The router does not broadcast to discover destinations.

Step-by-Step Solution:

Identify destination network: 192.168.22.0/24.Check connected routes: 192.168.214.0/24 and 192.168.20.0/24 (no match).Check learned routes: 192.168.215.0/24, 192.168.115.0/24, 192.168.30.0/24 (no match).Check for a default route: none shown.Conclusion: No matching route → drop the packet.

Verification / Alternative check:
A quick mental match of 192.168.22.0/24 against the entries confirms there is no 192.168.22.0/24 route and no 0.0.0.0/0, so per Cisco behavior the packet is discarded.

Why Other Options Are Wrong:
Routed out S0/0 or Fa0/0: No route covers 192.168.22.0/24, so no interface selection is possible.
Broadcast for the destination: Routers do not broadcast to locate unknown destination networks; they use the RIB.

Common Pitfalls:
Assuming a router will “try the next-hop for everything” or ARP beyond connected prefixes; without a default route or specific entry, packets are dropped.

More Questions from IP Routing

Discussion & Comments