How To Troubleshoot InterVLAN Routing.
What is Inter-VLAN Routing?
Inter-VLAN routing involves routing network traffic between various VLANs (Virtual Local Area Networks). VLANs are utilized to divide a larger network into smaller, isolated networks to enhance security, improve traffic management, and boost performance. In order for devices in different VLANs to communicate with each other, a router or layer 3 switch must be used to facilitate the routing of traffic between the VLANs.
This routing process allows for the segregation of network resources while still enabling efficient communication between devices in different VLANs. By implementing inter-VLAN routing, organizations can create more secure and efficient network environments that meet their specific needs and requirements.
Here are the key aspects of Inter-VLAN routing:
Purpose of VLANs: VLANs are created to segment a network into smaller parts. Each VLAN functions as a separate network with its own unique broadcast domain. This segmentation helps in reducing broadcast traffic, improving security by isolating sensitive data, and organizing the network based on functional, departmental, or project needs.
Why Inter-VLAN Routing?: In a network with multiple VLANs, there might be a need for devices in different VLANs to communicate with each other. For instance, a device in the sales department VLAN may need to access a server in the IT department VLAN.
Implementation Methods: Inter-VLAN routing can be implemented in several ways:
Router-on-a-Stick: This is a common method where a single physical router interface is configured to route traffic between multiple VLANs. The router interface is connected to a switch port configured as a trunk link, which carries traffic from all VLANs. Sub-interfaces are created on the router, each representing a VLAN, with its own IP address that serves as the default gateway for that VLAN.
Layer 3 Switch: Some advanced network switches have Layer 3 (routing) capabilities. These switches can perform routing functions internally, providing faster performance compared to the Router-on-a-Stick method, as they can route traffic at switch speed.
Configuring Inter-VLAN Routing: The configuration involves setting up VLANs on the switch, configuring trunk ports to carry VLAN traffic, and setting up routing either on a router or a Layer 3 switch. The router or switch routes traffic based on IP addresses and routing protocols.
Benefits: Inter-VLAN routing allows for better control of network traffic, enhances security by keeping different types of traffic separate while still enabling communication, and improves the overall efficiency of the network.
Inter-VLAN routing is essential in modern network infrastructures, enabling efficient communication across different segments of a network while maintaining the isolation and security benefits of VLANs.
How To Troubleshoot InterVLAN Routing.
The most common mistakes administrators make during networking is during the configuration stage, either on the router, switch, or logical subnet addressing.
On this page, we’ll look at the challenges; common issues, and troubleshooting methods related to configuring multiple VLANs on a network.
If you suspect that there is a problem with a switch configuration, use the show interface (interface-id) switchport command for verification.
The show running-config and the show interface (interface-id) switchport commands are useful Cisco IOS troubleshooting tools for identifying VLAN assignment and port configuration issues.
When using the traditional routing model for inter-VLAN routing, ensure that the switch ports that connect to the router interfaces are configured on the correct VLANs. If the switch ports are not configured or assigned correctly to VLANs, network devices configured on the VLANs will not receive or connect to the router interface, which in turn hinders traffic to other VLANs on the network.
Using the Topology above, PC2 and router R1 interface F0/1 are configured to share the same subnet. However, the switch port F0/3 that connects to router R1 interface F0/1 has not been configured and remains in the default VLAN. Because router R1 is on a different VLAN than PC2, they are unable to communicate.
Solution
To solve this problem, use the switchport access vlan 20 interface configuration command on switch port F0/3 on switch SW1. When the switch port is configured or assigned to the correct VLAN, PC2 can communicate with router R1 interface F0/1, which will then enable access to other VLANs connected to the router.
