Cisco CCNA (200-301)IP ConnectivityMedium
An engineer configures a recursive static route on R1: 'ip route 192.168.10.0 255.255.255.0 172.16.1.1'. R1 also has a route to 172.16.1.0/24 reachable out interface GigabitEthernet0/0. When R1 receives a packet destined for 192.168.10.5, what must it do before forwarding the packet?
- ADrop the packet because the next-hop address is not directly connected
- BForward the packet immediately out GigabitEthernet0/0 without any additional lookup
- CPerform a second lookup in the routing table to resolve 172.16.1.1 to an outgoing interface
- DSend an ARP request for 192.168.10.5 before forwarding
Show answer & explanationAnswer & explanation
Correct answer: C. Perform a second lookup in the routing table to resolve 172.16.1.1 to an outgoing interface
Because the static route's next hop (172.16.1.1) is not a directly connected address, the router must perform a recursive lookup, searching the routing table again to find which interface can reach 172.16.1.1 (here, via the 172.16.1.0/24 connected route out Gi0/0), before it can forward the original packet.
Why the other options are wrong
- A. Recursive next hops are valid as long as a route to reach that IP exists elsewhere in the table.
- B. A single lookup only resolves the next-hop IP, not the actual exit interface, when the next hop isn't directly connected.
- D. ARP resolution happens after the exit interface is determined, not as a substitute for the routing lookup.
Recursive Static Route
A static route whose next hop is an IP address (not an exit interface) requires the router to look up that next-hop IP again in the routing table to determine the actual outgoing interface.
- Contrasts with exit-interface static routes, which need no recursive lookup
- Adds slight CPU overhead due to the extra table lookup
- Next hop must be reachable via another route entry, or the static route stays inactive
Memory trick: Next-hop IP? Look it up twice before you leap.