Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
In the Cisco Application Centric Infrastructure (ACI), each leaf node must connect to each spine node.
Cisco ACI is a data center technology that uses switches, categorized as spine and leaf nodes, to dynamically implement network application policies in response to application-level requirements. Network application policies are defined on a Cisco Application Policy Infrastructure Controller (APIC) and are implemented by the spine and leaf nodes.
The spine and leaf nodes create a scalable network fabric that is optimized for east-west data transfer, which in a data center is typically traffic between an application server and its supporting data services, such as database or file servers. Each spine node requires a connection to each leaf node? however, spine nodes do not interconnect nor do leaf nodes interconnect. Despite its lack of fully meshed connections, this physical topology enables nonlocal traffic to pass from any ingress leaf interface to any egress leaf interface through a single, dynamically selected spine node. By contrast, local traffic is passed directly from an ingress interface on a leaf node to the appropriate egress interface on the same leaf node.
Because a spine node has a connection to every leaf node, the scalability of the fabric is limited by the number of ports on the spine node, not by the number of ports on the leaf node. In addition, redundant connections between a spine and leaf pair are unnecessary because the nature of the topology ensures that each leaf has multiple connections to the network fabric. Therefore, each spine node requires only a single connection to each leaf node.
Redundancy is also provided by the presence of multiple APICs, which are typically deployed as a cluster of three controllers. APICs are not directly involved in forwarding traffic and are therefore not required to connect to every spine or leaf node. Instead, the APIC cluster is connected to one or more leaf nodes in much the same manner that other endpoint groups (EPGs), such as application servers, are connected.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 4, ACI, pp. 135
Cisco: Application Centric Infrastructure Overview: Implement a Robust Transport Network for Dynamic Workloads