How to relay between two core switches

To interconnect two core switches, configure a dedicated transport VLAN, use trunk ports, and optionally implement stacking or VSS for redundancy and simplified routing.

Step 1: Establish a Transport VLAN

Create a dedicated VLAN on both core switches to serve as the transport or inter-switch link. Assign unique IP addresses to each switch within this VLAN to enable Layer 3 communication. For example, CS1 could use 10.0.0.1/24 and CS2 10.0.0.2/24 . This VLAN should be used exclusively for inter-core traffic to avoid conflicts with other VLANs.

Step 2: Configure Trunk Ports

Set the ports connecting the two core switches to trunk mode and allow the transport VLAN. On Cisco switches, the configuration would look like:

interface GigabitEthernet1/1 switchport mode trunk switchport trunk allowed vlan <transport_vlan_id>

This ensures that multiple VLANs can traverse the link if needed, while the transport VLAN carries routing traffic .

Step 3: Avoid MAC Address Conflicts

If using virtual switching technologies like VSS or stacking, ensure that each switch has a unique MAC address for the transport VLAN. Identical MAC addresses can prevent successful ping tests and routing between switches .

Step 4: Consider Redundancy and Aggregation

For high availability, you can either:

• Stack or configure VSS: This makes two physical switches appear as a single logical device, allowing you to create a port channel from downstream devices to both cores simultaneously .
• Separate links with STP: If not using VSS, connect multiple links but rely on Spanning Tree Protocol to block redundant paths and prevent loops .

Step 5: Routing Between Core Switches

Once the transport VLAN is active and trunked, enable routing on the VLAN interface. This allows the two core switches to exchange routes and forward traffic between their respective VLANs. For Layer 3 switches, configure static routes or a dynamic routing protocol (e.g., OSPF or EIGRP) over the transport VLAN.

Step 6: Verification

• Use ping to test connectivity between the transport VLAN IPs.
• Check show vlan and show interface trunk to ensure the VLAN is active and allowed on the trunk.
• Use show cdp neighbor to verify that the switches recognize each other on the interconnect port . By following these steps, you can successfully relay traffic between two core switches, maintain redundancy, and ensure proper routing across your network.