Two VLANs on the core switch are interconnected

Two VLANs on a core switch can communicate through inter-VLAN routing, typically using Switch Virtual Interfaces (SVIs) or routed ports on a Layer 3 switch.

Understanding the Concept

By default, VLANs are isolated broadcast domains, meaning devices in VLAN 10 cannot communicate with devices in VLAN 20 without routing. Inter-VLAN routing allows traffic to pass between VLANs while maintaining the segmentation benefits of VLANs, such as improved security and reduced broadcast traffic .

Configuration Methods

1. Switch Virtual Interfaces (SVIs) On a Layer 3 switch, you can create an SVI for each VLAN. Each SVI acts as the default gateway for devices in that VLAN. Once IP routing is enabled on the switch, traffic can flow between VLANs at wire speed. For example, VLAN 10 could have SVI 10.0.10.1/24 and VLAN 20 could have SVI 10.0.20.1/24 .
2. Routed Ports Individual switch ports can be converted into Layer 3 routed ports, which are assigned IP addresses and can route traffic between VLANs. This method is suitable for point-to-point connections or when only a few devices need inter-VLAN communication .
3. Router-on-a-Stick (RoAS) If the switch is Layer 2 only, a router with subinterfaces can be used. A trunk port carries traffic for multiple VLANs to the router, which routes packets between VLANs. Each subinterface on the router is assigned an IP address corresponding to a VLAN .

Trunking Between Switches

If multiple switches are involved, the VLANs must be interconnected via trunk ports using 802.1Q tagging. Trunk ports allow multiple VLANs to traverse a single physical link, ensuring that VLAN traffic reaches the core switch for routing .

Best Practices

• Enable IP routing on the Layer 3 switch to allow inter-VLAN communication.
• Assign unique subnets to each VLAN to avoid IP conflicts.
• Avoid using VLAN 1 as the native VLAN on trunks; choose an unused VLAN ID to prevent traffic leaks.
• Use HSRP or VRRP for redundancy if multiple core switches are present .
• Limit unnecessary trunk ports to reduce security risks and potential loops. By following these steps, devices in different VLANs on a core switch can communicate efficiently while maintaining network segmentation and security.