3.2 Determine how a router makes a forwarding decision by default.
Objective 3.2 sits in IP Connectivity, which carries 25% of the CCNA exam. The questions below are original, written from the official objective title above, and each explanation cites the Cisco page it rests on.
Objective title verbatim from the official objectives. Cisco exam page ↗
A worked example
Shown solved, with the whole explanation open: this is what every question here carries.
A packet arrives destined for 10.40.9.200 and the router holds this table.
10.0.0.0/8 is variably subnetted, 5 subnets, 5 masks
C 10.0.0.0/30 is directly connected, GigabitEthernet1/0/1
L 10.0.0.1/32 is directly connected, GigabitEthernet1/0/1
O 10.40.0.0/16 [110/40] via 10.0.0.2, 00:22:15, GigabitEthernet1/0/1
D 10.40.8.0/21 [90/3072] via 10.0.0.6, 01:15:03, GigabitEthernet1/0/2
S 10.40.9.128/25 [1/0] via 10.0.0.10Which next hop is used?
The concept
Forwarding and installation are two separate decisions. Once several entries are in the table, the router picks between them on how many bits of the destination each one pins down, and nothing else in the line matters at that moment.
Why this answer
10.40.9.200 falls inside 10.40.9.128 through 10.40.9.255, inside 10.40.8.0 through 10.40.15.255, and inside 10.40.0.0 through 10.40.255.255. The /25 pins down the most bits, so it wins even though its distance and metric were never consulted.
- AThe /16 covers the address but pins down the fewest bits of the three matches.
- BThe /21 covers the address too, and it would win if the /25 entry were withdrawn.
- Correct. The /25 is the longest match for this destination.
- DA drop happens only when nothing in the table covers the destination, and here three entries do.
Now you: objective 3.2 questions
No account needed. The explanation opens when you answer.
Sample question 1 of 3
RIP and internal EIGRP each offer 10.60.0.0/16 to the same router. Only one of them ends up in the table. What settles it?
Sample question 2 of 3
R1 has learned 10.70.0.0/16 from OSPF alone, over two separate paths.
+---- R2 ----+
R1 -----+ +---- R4 ---- 10.70.0.0/16
+---- R3 ----+
path via R2 cost 30
path via R3 cost 22Which path is installed?
Sample question 3 of 3
An engineer sees that R1 learned 10.80.0.0/16 from EIGRP over two neighbors, and both paths report a composite metric of 3072. What does R1 do with the second path?
That’s 3 of the full CCNA bank.
Keep going free: 10 questions per certification in bank practice, with no account.
Continue practicingRead the sources
These are the official pages the questions above cite. Reading them is studying the objective from the primary source, which is what the explanations point you toward anyway.