Cisco Certified Support Technician (CCST) NetworkingIP ConnectivityEasy
A network administrator is troubleshooting an issue where a router is not forwarding traffic to a newly added subnet. The routing table shows no entry for the destination subnet. Which of the following is the most likely reason for this issue?
- AThe routing protocol has not converged, or the static route is missing/incorrect.
- BThe router's interface connected to the subnet is administratively down.
- CThe Time-to-Live (TTL) value in the IP packet header is too low.
- DThe router has an incorrect administrative distance configured.
Show answer & explanationAnswer & explanation
Correct answer: A. The routing protocol has not converged, or the static route is missing/incorrect.
If a router's routing table has no entry for a destination subnet, it indicates that the router does not know how to reach that subnet. This is typically due to a missing or incorrect static route, or the dynamic routing protocol has not learned the route yet (not converged or misconfigured).
Why the other options are wrong
- B. An interface being down would prevent direct connectivity, but the question states no entry in the routing table, which is a routing logic issue.
- C. Low TTL would prevent packets from reaching the destination, but the problem here is the router doesn't even know where to send them.
- D. Administrative distance determines route preference, not route presence in the table.
Routing Table Lookup Failure
When a router receives a packet for a destination network, it consults its routing table. If no matching entry is found, the router discards the packet or forwards it to a default route if one exists.
- A routing table entry is essential for forwarding packets.
- Missing entries can be due to misconfigured static routes or dynamic routing protocol issues.
- Without an entry, traffic cannot be forwarded to that destination.
Memory trick: No map, no path, just a dead end.