Connecting Four VLANs to Two Fast Ethernet Interfaces: Optimal Network Performance

Configuring Multiple VLANs on a Router with Two Fast Ethernet Interfaces

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A router has two Fast Ethernet interfaces and needs to connect to four VLANs in the local network.

How can you accomplish this task, using the fewest physical interfaces and without decreasing network performance?



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A. B. C. D.


The best option to connect a router with two Fast Ethernet interfaces to four VLANs in the local network is to use the router-on-a-stick configuration. This configuration enables a single physical interface on the router to connect to multiple VLANs, effectively increasing the number of networks that can be connected to the router without the need for additional physical interfaces.

The router-on-a-stick configuration involves configuring a single physical interface on the router as a trunk interface that carries traffic for multiple VLANs. The trunk interface tags the traffic with the appropriate VLAN ID as it travels between the router and the switch, allowing the router to identify and process the traffic for each VLAN.

To configure router-on-a-stick, the following steps should be taken:

  1. Configure the physical interface on the router as a trunk interface using the "switchport mode trunk" command. This enables the interface to carry traffic for multiple VLANs.

  2. Create the VLANs on the switch and assign the appropriate ports to each VLAN.

  3. Configure the switchport connected to the router interface as a trunk port using the "switchport mode trunk" command.

  4. Configure sub-interfaces on the router for each VLAN, using the "interface fa0/0.1" command. The ".1" notation indicates that this is a sub-interface for VLAN 1. Similarly, ".2" would be used for VLAN 2, ".3" for VLAN 3, and so on.

  5. Assign an IP address to each sub-interface using the "ip address x.x.x.x y.y.y.y" command.

Once the router-on-a-stick configuration is complete, the router can communicate with all four VLANs using a single physical interface. This solution is cost-effective as it requires the least number of physical interfaces while ensuring optimal network performance.

The other options presented in the question are not ideal. Adding two more FastEthernet interfaces (option A) would increase the number of physical interfaces required and therefore increase costs. Adding a second router (option B) would also increase costs and complicate the network architecture unnecessarily. Using a hub (option C) is not recommended as it does not provide the same level of security, performance, and manageability as a router-on-a-stick configuration.