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Interconnection between core switches and routers

Fiber and communications infrastructure knowledge brief Published 21 Jul 2024

Prepared for preliminary research by the METRA knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

Core switches and routers are interconnected using a combination of Layer 2 trunks and Layer 3 routed links to ensure high-speed, redundant, and efficient network traffic flow.

Core Concepts

Core switches operate primarily at Layer 2 but often include Layer 3 capabilities, enabling them to handle inter-VLAN routing and high-speed switching within the network backbone. Routers operate at Layer 3, directing traffic between different networks and managing IP-based routing decisions. The interconnection between these devices is critical for maintaining network performance, redundancy, and scalability.

Typical Interconnection Strategies

Layer 3 Routed Links Between Core Switches Core switches are often connected using Layer 3 point-to-point links with unique VLAN interfaces (SVIs) and IP addresses. This approach reduces broadcast traffic and avoids spanning tree protocol (STP) issues while allowing efficient routing between cores. Each link can be dedicated to a specific VLAN for routing purposes, ensuring isolation and simplified troubleshooting.

Layer 2 Trunk Connections In addition to Layer 3 links, Layer 2 trunks are used between core switches to carry multiple VLANs, enabling communication for access VLANs and providing a path for VLAN traffic that requires Layer 2 adjacency. Rapid-PVST or MSTP can be implemented to manage STP and prevent loops, with primary and secondary root bridges defined for stability. Redundant Connections to Routers Each core switch should connect to WAN or edge routers using separate Layer 3 links.

This ensures high availability and allows routing protocols like OSPF or BGP to dynamically select optimal paths.

HSRP or VRR

P can be deployed to provide a virtual gateway IP for hosts, aligning with the spanning tree topology to optimize traffic flow.

VLAN and I

P Planning Assigning dedicated VLANs for point-to-point links between cores and routers simplifies routing and reduces the risk of broadcast storms. Inter-VLAN routing can be handled either on the core switches (if Layer 3 capable) or on dedicated routers, depending on network design and performance requirements.

Best Practices

Use Layer 3 links for core-to-core connections to minimize STP complexity and improve convergence times. Maintain redundant paths to routers and between core switches for fault tolerance. Implement routing protocols (OSPF, EIGRP, or BGP) to dynamically manage traffic and failover.

Align HSRP/VRRP configurations with the Layer 2 topology to ensure optimal gateway availability. Monitor and segment traffic using VLANs to maintain security and performance.

Summary

The interconnection between core switches and routers combines Layer 3 routed links for backbone routing and Layer 2 trunks for VLAN traffic, supported by redundancy and high-availability protocols. This design ensures efficient data flow, minimizes broadcast issues, and provides a scalable and resilient network infrastructure suitable for enterprise environments.

Interconnection between core switches and routers - METRA FIBER MARKET [PDF]

Interconnection between core switches and routers

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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