Why Router Interfaces Cannot Share One Subnet
Refer to the exhibit. A network technician is asked to design a small network with redundancy. The exhibit represents this design, with all hosts configured in the same VLAN. What conclusions can be made about this design? [sampleXML/xmlfile-37_1.jpg]
Community Insight
The question tests IPv4 interface addressing rules: a router will not accept two active interfaces configured in the same subnet, and learners trap themselves by jumping to STP or trunking because the topology looks redundant.
This Redundant CCNA design question tests whether two router interfaces can be configured with addresses inside the same 192.168.1.0/24 network. The page confirms that the router rejects the overlapping addressing scheme, making option C the correct conclusion rather than spanning tree, trunks, or 802.1Q encapsulation.
Option B (Spanning-tree will need to be used) is the most common wrong pick, because redundancy instinctively signals a Layer 2 loop; but the design fails before loop control matters, since the router refuses two interfaces addressing the same 192.168.1.0/24 network.
Community Discussion (28 comments)
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Expert Analysis
Why the Answer Is Correct
The exhibit places both router interfaces inside one IPv4 network (for example 192.168.1.1/24 and 192.168.1.2/24), so the router raises an overlap error and will not commit the second address. A router's fundamental role is to forward between distinct networks, so each interface must terminate a different subnet — that is why the addressing scheme itself is rejected. As Fakecon demonstrated in Packet Tracer, the CLI literally answers "% 192.168.1.0 overlaps with GigabitEthernet0/0", confirming option C as the concrete, verifiable outcome. Until the technician re-addresses the links into separate subnets, the redundant design cannot function as intended, so no other option describes a valid conclusion.Why the Other Options Are Wrong
Option A is false because the router refuses the second interface address, so the design fails immediately. Option B is tempting, but the two switch-facing ports terminate on routed interfaces, and as lordnano noted, "The router does not forward layer 2 broadcast on routed interfaces" — spanning tree would not cure an addressing overlap anyway. Option D is incorrect because all hosts share a single VLAN and each router interface acts as a Layer 3 gateway link, so a trunk between the switches adds nothing. Option E misapplies 802.1Q: router-on-a-stick encapsulation is only needed when one physical link carries multiple VLANs, which is not the defect shown here.Community Comment Notes
Commenters converge on C, with marcojmnez summarizing that "Both Rs interfaces overlaping" and JSMM noting that 192.168.1.1/24 and 192.168.1.2/24 belong to the same network. CCNaKicker openly asked "why it's C and not B?", which is exactly the trap this page resolves: STP addresses loops, not duplicate subnet assignment. nebacudnezzer added that the router "would need minimum 2 subnets to be able to differentiate WAN from LAN", and agazi joked that the designer deserves scrutiny while conceding the design would not work "as intended".Exam Strategy
When a design question shows a router with two interfaces, immediately check whether the interface addresses fall in the same subnet — an overlap is a configuration rejection, not a theoretical concern. Rule out structural options (trunk, 802.1Q, STP) only after the addressing is valid, since Layer 2 remedies cannot fix a Layer 3 addressing error.
Frequently Asked Questions
Why is spanning tree (B) not the right answer for this redundant design?
The two switch-facing ports land on routed interfaces, which stop Layer 2 broadcast propagation, so no loop needs breaking. The design fails earlier because the router rejects two interfaces in one subnet.
Can a router really refuse an IP address because it overlaps?
Yes. Cisco IOS rejects a second interface address that falls inside an already-used subnet with an "% ... overlaps
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