Cisco ACI Fabric Discovery and Nodes registration [Explained]

Cisco ACI Fabric Discovery process:

The APIC is a central point of automated provisioning and management for all the switches that are part of the ACI fabric.

• The APIC discovers new switches that are directly connected to any switch it currently manages. Each APIC instance in the cluster first discovers only the leaf switch to which it is directly connected.
  

• After the leaf switch is registered with the APIC, the APIC discovers all spine switchesthat are directly connected to the leaf switch.
  
• As each spine switch is registered, that APIC discovers all the leaf switches that are connected to that spine switch.

This cascaded discovery allows the APIC to discover the entire fabric topology in a few simple steps.

APIC Cluster Discovery Process:

1- LLDP Neighbors discovery:

• APIC start sending out LLDP packets from its Inband Interfaces.
• The Leaf start sending LLDP packets on all operational ports.
• If the Leaf is running ACI code (Nexus 9000 In ACI mode), it should form LLDP neighborship with the APIC and send its Serial number and other hardware details to the APIC.

2- DHCP and TEP Address assignment:

• The unregistred ACI nodes send DHCP Discover packet on all its operational ports.
• When APIC Discover the node via LLDP and process the DHCP Discover Packet, the Fabric node (Leaf or Spine) will show up in the fabric memebership tab in the APIC GUI.
• Once the administrator register the new node, the APIC will repond to the DHCP Requests and send a DHCP OFFER from the TEP Pool introduced in the Fabric Intial setup Interface.

2- IFM (Intra-Fabric Messaging) Policy and Heartbeat:

A fabric node (Leaf or Spine) can pass to active when the APIC and node can exchange heartbeats through the Intra-Fabric Messaging (IFM) process. This process is also used by the APIC to push policy from APIC to the Fabric.

Fabric Node Discovery States:

• Unknown: Node discovered but no Node ID policy configured
• Undiscovered: Node ID configured but not yet discovered
• Discovering: Node discovered but IP not yet assigned
• Unsupported: Node is not a supported model
• Disabled: Node has been decommissioned
• Inactive: No IP connectivity
• Active: Node is active

Register Fabric node (Leaf or Spine) to APIC via GUI:

Step 1: In the Fabric Membership work pane, click the Nodes Pending Registration tab. Switches in the Nodes Pending Registration tab table can have the following conditions:

• A newly discovered but unregistered node has a node ID of 0 and has no IP address.

• A manually entered (in APIC) but unregistered switch has an original status of Undiscovered until it is physically connected to the network. Once connected, the status changes to Discovered.

Step 2: In the Nodes Pending Registration table, locate a switch with an ID of 0 or a newly connected switch with the serial number you want to register.

Step 3: Right-click the leaf switch row, select Register, and perform the following actions: Verify the displayed Serial Number to determine which switch is being added.