Question 90

CCNA Question 90 - Answer and Explanation

Correct Answer: B

Detailed Explanation: This is a troubleshooting question that tests your understanding of inter-VLAN routing on Layer 3 switches and common causes of connectivity issues in such environments. Let's analyze each option to determine the most probable cause: Analyzing the Scenario and Options: Scenario Summary: Inter-VLAN routing between VLAN 10 and VLAN 20 is failing on a Layer 3 switch. We are given that: Both VLANs exist and are configured. VLAN interfaces (SVI - Switched Virtual Interfaces) are configured with IP addresses (gateway for each VLAN subnet). VLAN interfaces are administratively up (no shutdown command). Routing is enabled on the switch (likely ip routing command is configured globally). Option A. Spanning Tree Protocol (STP) blocking ports within VLAN 10 or VLAN 20. While STP is crucial for loop prevention in Layer 2, it's less likely to be the direct cause of inter-VLAN routing failure in this scenario. STP operates within a VLAN (a broadcast domain). If STP were blocking ports within VLAN 10 or 20, it might disrupt intra-VLAN communication within those VLANs, but it wouldn't typically prevent routing between VLANs if the Layer 3 interfaces and routing are correctly configured. STP problems usually manifest as network loops or instability, not just isolated inter-VLAN routing failure when other configurations seem correct. While STP issues can indirectly affect routing in complex scenarios, it's not the most likely direct cause given the other information. Option B. An Access Control List (ACL) configured on the VLAN 10 or VLAN 20 interface is blocking inter-VLAN traffic. This is a highly probable cause. ACLs are often used on Layer 3 interfaces (including VLAN interfaces on Layer 3 switches) to filter traffic. If an ACL is mistakenly or intentionally configured on the VLAN 10 or VLAN 20 interface to deny traffic between these VLAN subnets, it would directly prevent inter-VLAN communication. ACLs are a common source of routing and connectivity problems because they can explicitly block traffic that would otherwise be routed. This option aligns well with the given symptoms – VLAN interfaces are up, routing is enabled, but traffic is blocked – suggesting a filtering mechanism like an ACL. Option C. The VLAN interfaces on the Layer 3 switch do not have IP addresses configured. This is incorrect based on the problem description. The problem statement explicitly says "VLAN interfaces are configured with IP addresses on the Layer 3 switch." If VLAN interfaces lacked IP addresses, there would be no Layer 3 gateway for devices in those VLANs, and inter-VLAN routing would not even be attempted. Since the problem description mentions routing failure between VLANs, it implies that IP addresses are configured on the VLAN interfaces, otherwise there would be no routing context at all. Option D. The VLANs are not configured on any trunk ports, so inter-VLAN routing is not possible. This is incorrect in the context of a Layer 3 switch. Trunk ports are primarily relevant for Layer 2 switches that need to carry traffic for multiple VLANs across inter-switch links. On a Layer 3 switch, inter-VLAN routing occurs internally within the switch itself. VLAN interfaces (SVIs) on a Layer 3 switch act as the Layer 3 gateways for their respective VLANs, and the Layer 3 switch's routing engine handles routing between these VLANs. Trunk ports are not directly required for basic inter-VLAN routing on a single Layer 3 switch – the routing happens via the SVIs and the switch's routing processes. Trunking becomes important when you need to extend VLANs across multiple switches. In this scenario, with just one Layer 3 switch described, trunking configuration is less relevant to the described inter-VLAN routing failure. Why Option B is the Most Likely Cause: ACLs are Common Filtering Mechanisms: ACLs are frequently used to control traffic flow in networks, including inter-VLAN traffic. Misconfigured or overly restrictive ACLs are a very common cause of unexpected blocking of traffic. Symptoms Align with ACL Blocking: The symptoms – VLAN interfaces up, routing enabled, but no connectivity between VLANs – are highly indicative of an ACL blocking the traffic. ACLs are designed to selectively permit or deny traffic based on various criteria. Other Options Less Probable Given Context: The other options, while technically possible in some network scenarios, are less likely to be the most likely cause given the specific details provided in the question. STP issues are less direct, Option C contradicts the given info, and Option D is less relevant in a single Layer 3 switch inter-VLAN routing scenario. In Conclusion: Option B, the presence of a blocking ACL on the VLAN interfaces, is the most likely root cause of the inter-VLAN routing failure in this scenario. Troubleshooting inter-VLAN routing often involves checking for ACLs as a primary step. Understanding how Layer 3 switches route between VLANs and how ACLs can control this routing is a core CCNA concept. This question highlights the practical troubleshooting aspect of network administration and the importance of considering security mechanisms like ACLs when diagnosing connectivity issues.

This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.