For the BGP Route Reflector policy to take effect, the components that must be configured are the leaf fabric interface overrides and profiles.  These configurations are necessary to establish the route reflector relationships within the BGP protocol on the leaf switches

The question asks which switch type is discovered first in the Cisco ACI fabric discovery process. The ACI fabric consists of spine and leaf switches, and the discovery process is a critical initial step to establish the fabric topology. Let’s analyze this based on Cisco ACI documentation.

Requirement Analysis

The ACI fabric discovery process involves the Application Policy Infrastructure Controller (APIC) identifying and registering switches to form the network topology.

The process relies on protocols like Link Layer Discovery Protocol (LLDP) and Cisco Discovery Protocol (CDP) to detect connected devices.

The sequence of discovery is determined by the physical connectivity and the role of switches in the fabric.

Option Evaluation

A. leaf :

In the Cisco ACI fabric, the discovery process begins with the leaf switches. When the APIC is powered on and connected to the fabric, it first discovers the leaf switches directly attached to it or other initial points of connectivity. Leaf switches are the access layer devices that connect endpoints and are the starting point for fabric topology discovery. Once leaf switches are identified, they assist in discovering the spine switches.

With unicast routing enabled on the bridge domain, the two conditions that enable the Cisco ACI leaf to learn a source IP as a local endpoint are:

Through Ethernet traffic received in a bridge domain (Option A) : The leaf switch learns the source IP address as a local endpoint when it receives Ethernet traffic within the bridge domain.

Through ARP received on an SVI (Option E) : The leaf switch also learns the source IP address as a local endpoint when it receives an ARP request or reply on a Switched Virtual Interface (SVI) associated with the bridge domain.

These conditions allow the ACI fabric to maintain an accurate endpoint database for efficient routing and forwarding of traffic within the fabric.

https://www.cisco .com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739989.html

To configure leaf and spine switches in a Cisco ACI fabric to use out-of-band management for NTP protocol, an Override Policy with NTP Out-of-Band must be created. This policy specifies that NTP traffic should be directed through the out-of-band management network, separate from the in-band management traffic, ensuring dedicated management connectivity for time synchronization.

https://www.cisco.com/c/en/us/support/docs/cloud-systems-management/application-policy-infrastructure-controller-apic/200128-Configuring-NTP-in-ACI-Fabric-Solution.html

The import mode that must be used to ensure that the import process terminates if the imported configuration is incompatible with the existing system is the ‘atomic’ mode. This mode ignores shards that contain objects that cannot be imported while proceeding with shards that can be imported.  If the incoming configuration’s version is incompatible with the existing system, the import process will terminate 2 .

In Cisco ACI, the mgmt0 interface is used for out-of-band (OOB) management, and its IP address must be explicitly assigned through the Node Management Address Policy under the mgmt tenant. When new leaf switches are registered in the fabric, they do not automatically receive an IP address for their mgmt0 interfaces unless they are added to this policy.

To enable communication between EPGs and BDs from any tenant without the need for contracts or VRF route leaking, the best approach is to implement an unenforced VRF within the common tenant. By doing so, all bridge domains that are associated with this VRF will be able to communicate with each other without additional configurations.  This method simplifies the network architecture and reduces the complexity associated with contract management and VRF route leaking 1 .

References  :=

Cisco ACI Contract Guide White Paper 1

In Cisco ACI, Intra-EPG Isolation is a feature that prevents communication between endpoints within the same EPG. By default, endpoints in the same EPG can communicate freely without requiring contracts. To disable communication between two backup servers within the same EPG (e.g., in the " Backup EPG " ), you need to enforce Intra-EPG Isolation.

MisCabling Protocol (MCP) detects loops from external sources (i.e., misbehaving servers, external networking equipment running STP, etc.) and will err-disable the interface on which ACI receives its own packet. Enabling this feature is a best practice, and it should be enabled globally and on all interfaces, regardless of the end device. For MCP to be enabled, you need to have it enabled globally and on a per-interface basis. While MCP is enabled on all interfaces by default, it is not turned “on” until you also enable it globally. The global configuration knob for MCP can be enabled by configuring the global settings here: Fabric > Access Policies > Global Policies > MCP Instance Policy default. https://www.cisco.com/c/dam/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/aci-guide-using-mcp-mis-cabling-protocol.pdf