N10-009 Question 7
Single answerLayer 3 - NetworkA network administrator is troubleshooting connectivity between two IPv4 subnets in a small branch office. Hosts on VLAN 10 (192.168.10.0/24) can reach their default gateway at 192.168.10.1, and hosts on VLAN 20 (192.168.20.0/24) can reach their default gateway at 192.168.20.1. However, users on VLAN 10 cannot access a file server at 192.168.20.50, and users on VLAN 20 cannot reach any devices on VLAN 10. The Layer 3 switch has SVIs configured for both VLANs, and the administrator confirms both interfaces are up/up. Which of the following is the MOST likely cause of the problem?
- A
IP routing is disabled on the Layer 3 switch
- B
The file server is using the wrong DNS server address
- C
A duplex mismatch exists between the switch and the file server
- D
The hosts in VLAN 10 need a default gateway in a different subnet
Show answer and explanation
Correct answer: A
Explanation
This question tests practical Layer 3 troubleshooting in an inter-VLAN routing scenario. When each VLAN can reach its own SVI but cannot communicate with other VLANs, one of the first checks is whether Layer 3 routing is actually enabled on the multilayer switch. Best practice when troubleshooting is to verify: 1) host IP configuration and subnet mask, 2) default gateway reachability, 3) SVI status, and 4) routing functionality between directly connected networks. Vendor documentation for multilayer switches commonly notes that creating VLAN interfaces alone does not automatically enable IPv4 routing between them; the device must be operating with Layer 3 forwarding enabled. This aligns with standard Network+ expectations for diagnosing Layer 3 connectivity issues.
- A. Correct.
Correct. On a multilayer switch, having SVIs configured and operational is not enough by itself to route traffic between VLANs. Inter-VLAN communication requires Layer 3 forwarding to be enabled, typically with an 'ip routing' command or equivalent platform feature. If hosts can reach their own default gateway but cannot reach devices in the other subnet, disabled IP routing is a very plausible root cause.
- B. Incorrect.
Incorrect. DNS configuration affects name resolution, not basic Layer 3 reachability by IP address. In this scenario, users cannot reach devices across subnets even when the destination is identified by its IPv4 address (192.168.20.50), so DNS is not the likely cause.
- C. Incorrect.
Incorrect. A duplex mismatch can cause poor performance, late collisions, frame errors, and intermittent connectivity on a specific link, but it would not typically prevent all hosts in one VLAN from reaching all devices in another subnet through otherwise healthy SVIs. The issue described is broader and points to a routing problem rather than a single access-port performance issue.
- D. Incorrect.
Incorrect. Hosts in VLAN 10 should use a default gateway within their own subnet, such as 192.168.10.1. A default gateway in a different subnet would be invalid for normal host configuration unless special mechanisms are in place. Since the hosts can already reach 192.168.10.1, their gateway placement appears correct.