Configure VLAN interconnection on the core switch

VLAN interconnection between core switches is best achieved by creating VLANs on Layer 3-capable core switches, configuring SVIs for each VLAN, and enabling inter-VLAN routing to allow communication across VLANs at line speed.

Step 1: Plan VLANs and Subnets

Before configuration, define the VLANs and assign each a unique subnet. For example, VLAN 10 for Engineering (192.168.10.0/24) and VLAN 20 for Finance (192.168.20.0/24). Ensure that each VLAN corresponds to a single IP subnet to simplify routing and management .

Step 2: Configure VLANs on Core Switches

On each core switch, create the VLANs and assign the relevant interfaces:

CodeCopySwitch(config)# vlan 10Switch(config-vlan)# name EngineeringSwitch(config)# vlan 20Switch(config-vlan)# name Finance

Add the interfaces connecting to other switches or devices to the appropriate VLANs. For trunk links between core switches, configure the ports as trunk and allow all necessary VLANs:

CodeCopySwitch(config)# interface GigabitEthernet1/0/1Switch(config-if)# switchport mode trunkSwitch(config-if)# switchport trunk allowed vlan 10,20

Ensure the native VLAN is consistent across trunks to avoid VLAN mismatches .

Step 3: Configure SVIs (Switch Virtual Interfaces)

For Layer 3 routing, create an SVI for each VLAN on the core switch and assign an IP address to act as the default gateway for devices in that VLAN:

CodeCopySwitch(config)# interface vlan 10Switch(config-if)# ip address 192.168.10.1 255.255.255.0Switch(config)# interface vlan 20Switch(config-if)# ip address 192.168.20.1 255.255.255.0

Enable the VLAN interfaces:

CodeCopySwitch(config)# no shutdown

This allows the core switch to route traffic between VLANs internally without requiring an external router .

Step 4: Enable Inter-VLAN Routing

On Layer 3 switches, ensure IP routing is enabled:

CodeCopySwitch(config)# ip routing

This allows the switch to forward packets between VLANs using the SVIs as gateways. For redundancy and scalability, consider using dynamic routing protocols like OSPF if multiple core switches are interconnected .

Step 5: Verify Connectivity

Test inter-VLAN communication by pinging from a host in one VLAN to the SVI of another VLAN. Ensure that firewall rules or ACLs do not block necessary traffic. Use the following commands for verification:

CodeCopySwitch# show ip routeSwitch# ping 192.168.20.1

Confirm that each VLAN can reach its default gateway and other VLANs as required .

Best Practices

• Centralize VLAN management on the core switch to maintain consistency and simplify troubleshooting .
• Use Layer 3 switches for inter-VLAN routing to reduce latency and avoid bottlenecks.
• Separate management VLAN from default VLAN 1 for security.
• Document VLAN IDs, subnets, and SVI IPs to prevent conflicts and simplify future expansion.
• Consider redundancy with protocols like HSRP or VRRP for high availability of default gateways. By following these steps, VLANs across core switches can communicate efficiently while maintaining segmentation, security, and scalability.