How to configure a Layer 3 core switch

A Layer 3 core switch is configured by enabling routed interfaces, creating VLANs and SVIs, assigning IP addresses, and enabling IP routing to support inter-VLAN and external routing.

Step 1: Configure Management Interface

Begin by configuring a dedicated management interface for switch administration. Assign an IP address in a management VLAN that allows connectivity to network administrators and monitoring systems. Ensure this interface is separate from data or voice VLANs to avoid conflicts .

Step 2: Create VLANs

Define all required VLANs for your network segments. For example, separate VLANs for data, voice, and management traffic. On Cisco switches, use the vlan command to create VLANs and assign names for clarity .

Step 3: Configure Layer 3 Interfaces (SVIs)

For each VLAN, create a Switched Virtual Interface (SVI) to enable inter-VLAN routing. Assign an IP address to each SVI, which will act as the default gateway for hosts in that VLAN. Example:

CodeCopyinterface vlan 10 ip address 192.168.10.1 255.255.255.0 no shutdown

Repeat for all VLANs . Ensure each SVI is in a unique subnet.

Step 4: Enable IP Routing

Enable routing on the switch to allow traffic between VLANs:

CodeCopyip routing

This command activates Layer 3 forwarding on the switch .

Step 5: Configure Routed Ports

Convert uplink or inter-switch ports to routed ports by disabling the switchport feature. Routed ports are used for connections to other Layer 3 devices or upstream routers:

CodeCopyinterface gigabitEthernet 1/1 no switchport ip address 10.10.10.1 255.255.255.0 no shutdown

This allows the core switch to route traffic directly without Layer 2 switching .

Step 6: Configure Port Channels (Optional)

For redundancy and higher bandwidth, create Layer 3 port channels by aggregating multiple physical interfaces. Assign an IP address to the port channel interface:

CodeCopyinterface port-channel 1 no switchport ip address 10.10.20.1 255.255.255.0

Ensure all member interfaces are added to the port channel and are configured as routed ports .

Step 7: Configure Default Routes and Routing Protocols

Set a default route to the upstream router or firewall:

CodeCopyip route 0.0.0.0 0.0.0.0 10.10.10.254

For larger networks, configure dynamic routing protocols such as OSPF or EIGRP to advertise VLAN subnets to other Layer 3 devices .

Step 8: Configure Access Ports

Assign access ports to the appropriate VLANs for end devices:

CodeCopyinterface gigabitEthernet 1/10 switchport mode access switchport access vlan 10

For trunk ports connecting to other switches, configure:

CodeCopyinterface gigabitEthernet 1/2 switchport mode trunk switchport trunk allowed vlan 10,20,30

This ensures VLAN traffic is properly carried across the network .

Step 9: Verification

Verify configuration using commands such as:

CodeCopyshow ip routeshow ip interface briefping

Ensure SVIs are up, routing is enabled, and inter-VLAN connectivity is functional .

Best Practices

• Keep management, data, and voice VLANs separate.
• Use port channels for uplinks to improve redundancy.
• Assign unique subnets to each VLAN.
• Enable routing only on the core or distribution layer to maintain hierarchical design.
• Monitor interface counters and logs for performance and troubleshooting . This configuration provides a scalable, high-performance Layer 3 core switch setup suitable for enterprise networks, supporting inter-VLAN routing, routed uplinks, and redundancy.