CompTIA Network+ (N10-009)Network TroubleshootingMedium
A network technician is setting up VLANs on a new switch. After configuring VLAN 10 for the sales department and VLAN 20 for the marketing department, users in VLAN 10 can communicate with each other, and users in VLAN 20 can communicate with each other. However, users in VLAN 10 cannot communicate with users in VLAN 20. The switch is connected to a router that handles inter-VLAN routing. Which of the following is the MOST likely cause of the communication failure between VLANs?
- AThe default gateway on the client workstations is incorrect.
- BThe router has not been configured with subinterfaces for each VLAN.
- CSpanning Tree Protocol (STP) is blocking the inter-VLAN link.
- DThe switch ports for VLAN 10 and VLAN 20 are configured in half-duplex.
Show answer & explanationAnswer & explanation
Correct answer: B. The router has not been configured with subinterfaces for each VLAN.
Inter-VLAN communication requires a Layer 3 device (router) to route traffic between different VLANs. If users within their own VLANs can communicate but not across VLANs, it indicates that the routing function is missing. Specifically, the router needs to be configured with subinterfaces (or VLAN interfaces) for each VLAN, with an IP address acting as the default gateway for that VLAN, to enable routing between them.
Why the other options are wrong
- A. If the default gateway on client workstations were incorrect, they wouldn't be able to communicate with any devices outside their own subnet, including the router for inter-VLAN routing. However, the problem states they can communicate within their own VLAN.
- C. STP blocks redundant paths to prevent loops; it would not block a single, necessary link for inter-VLAN routing unless there was a misconfiguration causing it to incorrectly see it as a loop.
- D. Half-duplex would cause severe performance issues within VLANs but wouldn't specifically prevent inter-VLAN routing.
Inter-VLAN Routing
The process of forwarding network traffic between different VLANs using a Layer 3 device, typically a router or a Layer 3 switch.
- Each VLAN needs a unique IP subnet and a default gateway.
- Router or Layer 3 switch must have an interface/subinterface in each VLAN.
- Essential for communication between logically segmented networks.
Memory trick: VLANs are like rooms; a 'door' (router) is needed to go between them.