How to interconnect two core switches

The optimal way to connect two core switches is to use a combination of Layer-3 point-to-point links for routing and Layer-2 trunks for VLAN traffic, ensuring redundancy and proper spanning tree configuration.

Connection Methods

1. Layer-3 Point-to-Point Links: For core switches, it is recommended to establish Layer-3 connections using dedicated VLAN interfaces (SVIs) or routed ports. Each link should have its own subnet and IP addresses, allowing the switches to route traffic efficiently without creating Layer-2 loops. This approach reduces STP complexity and avoids broadcast storms between cores (Cisco community guidance) . 2. Layer-2 Trunks: In addition to Layer-3 links, a Layer-2 trunk can be configured between the core switches to carry multiple VLANs. This trunk allows VLAN traffic to traverse both switches while maintaining VLAN separation. It is common to create a dedicated VLAN for the point-to-point link to simplify management and avoid conflicts with production VLANs . 3. Redundancy and Spanning Tree:

• Use Rapid-PVST or MSTP to manage Layer-2 loops.
• Define primary and secondary root bridges for each VLAN to control traffic paths.
• Align HSRP or VRRP active gateways with the spanning tree topology to ensure optimal routing for end devices . 4. Stacking vs Cascading:
• Stacking: If the switches support stacking, use stacking ports and cables to create a single logical switch. This provides high bandwidth and simplified management but requires switches from the same vendor .
• Cascading: If stacking is not supported, connect switches using multiple Ethernet links (port channels) to increase bandwidth and provide redundancy. Each switch remains independently managed .

Best Practices

• Use link aggregation (LACP) for multiple physical connections to increase throughput and provide failover.
• Separate management VLANs from data VLANs to improve security and monitoring.
• Ensure consistent firmware and configuration across core switches to prevent compatibility issues.
• Monitor traffic and STP states to detect potential bottlenecks or misconfigurations early. By combining Layer-3 routing for core connectivity with Layer-2 trunks for VLAN traffic, and implementing redundancy and proper STP/HSRP alignment, you can achieve a resilient, high-performance core network that minimizes downtime and maximizes bandwidth utilization .