How to configure interconnection between core switches
Core switches are typically interconnected using Layer 3 links with redundancy and link aggregation to ensure high availability, traffic isolation, and efficient routing.
Layer 2 vs Layer 3 Interconnection
Layer 3 links are generally recommended between core switches to limit broadcast domains, improve control over routing, and support advanced protocols like BGP or MPLS. L3 links allow for better QoS management, load balancing, and failover in case of link or device failure. Routing protocols such as OSPF, EIGRP, or BGP can be run over these links to summarize networks and provide high availability . Layer 2 links can be used in specific scenarios, such as connecting clusters or firewalls that require VRRP or shared VLANs. However, L2 links expand VLANs across both core switches, which can increase broadcast traffic and potential spanning-tree loops. If L2 links are used, it is recommended to define SVIs as passive in routing protocols to avoid routing core-to-core traffic through VLANs meant for clusters .
Redundancy and High Availability
To ensure resilience, core switches can be configured as a VSX pair or MC-LAG setup. This involves creating a link aggregation (LAG) interface between the core switches, which serves as the inter-switch link (ISL). This configuration allows loop-free forwarding, efficient load balancing, and continuous connectivity even if one core switch loses external network peering .
VLAN and Routing Considerations
- VLAN segmentation: Assign VLANs for different traffic types (e.g., education network, ISP traffic) and configure corresponding VPN instances or SVIs on the core switches to isolate traffic .
- Routing: Configure routes to upstream devices (e.g., BRAS or distribution switches) and default routes for outbound traffic. Return routes should also be defined to ensure proper traffic flow .
- Distribution layer integration: Core switches should connect to distribution switches via L3 links, allowing the distribution layer to handle L2 boundaries and prevent broadcast storms from reaching the core .
Best Practices
- Use Layer 3 interconnections between core switches for scalability and control.
- Implement link aggregation (LAG/MC-LAG) for redundancy and load balancing.
- Segment traffic using VLANs and SVIs, and isolate critical traffic with VPN instances if needed.
- Run dynamic routing protocols over core-to-core links to ensure high availability and efficient path selection.
- If Layer 2 links are required for clusters or VRRP, configure SVIs as passive in routing protocols to avoid unnecessary routing loops . By following these practices, core switches can efficiently handle high volumes of traffic, maintain network stability, and provide seamless connectivity to distribution and access layers.
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