What happens when a router cannot determine how to forward a received packet?

What happens when a router cannot determine how to forward a received packet?

• A. Forwards to a default route
• B. Forwards to all interfaces
• C. Sends an ICMP error
• D. Drops the packet

Answer: (D)

When a router decides where to send a packet, it looks up the destination in its routing table to find a next-hop. If there is no route for that destination, there is no defined path to forward the packet. Without a valid next hop and without a configured default route, the router has nothing to forward to, so the safe and standard action is to discard the packet. Note that if a default route were present, the packet would be sent to that default gateway instead of being dropped. Some devices may generate an ICMP Destination Unreachable message to inform the sender in such cases, but the basic behavior when forward information is unavailable is to drop the packet.

When a router decides where to send a packet, it looks up the destination in its routing table to find a next-hop. If there is no route for that destination, there is no defined path to forward the packet. Without a valid next hop and without a configured default route, the router has nothing to forward to, so the safe and standard action is to discard the packet.

Note that if a default route were present, the packet would be sent to that default gateway instead of being dropped. Some devices may generate an ICMP Destination Unreachable message to inform the sender in such cases, but the basic behavior when forward information is unavailable is to drop the packet.