Core and Layer 3 Switch Configuration

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 <ID> 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:

interface 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:

ip 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:

interface 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:

interface 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:

ip 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:

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

For trunk ports connecting to other switches, configure:

interface 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:

show ip routeshow ip interface briefping <host in another VLAN>

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.