ACI Leaf as Ethernet Hub - Spanning tree handing in ACI

 

🔁 ACI Leaf as Ethernet Hub (Behavioral Analogy)

• ACI leaf switches forward BPDUs transparently between connected devices.
• This behavior mimics a hub, where multiple devices share the same broadcast domain.
• Therefore, STP decisions and transitions are influenced by how the connected switch interprets the topology.

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⚡ P2P Mode (Rapid Convergence)

• When a switch receives a Proposal BPDU on a P2P link:
• It can immediately respond with an Agreement BPDU.
• This allows the sender to transition from Blocking to Forwarding without waiting for timers.
• This is ideal for RSTP-enabled switch-to-switch links.

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🕒 Shared Mode (Delayed Convergence)

• On a Shared link, the receiving switch cannot send an Agreement immediately.
• The sender must wait for the Forward Delay timer to expire before transitioning.
• This introduces latency in STP convergence.

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🔄 Impact Across All ACI Versions

• This behavior is consistent across all ACI firmware versions.
• It’s crucial to explicitly configure STP link-type on external switches connected to ACI leafs to ensure optimal convergence.

 ACI Port Configuration Best Practices for External Switches

1. Determine the Nature of the Connection

Connection Type	Recommended STP Link-Type	Reason
Switch-to-Switch (Trunk or Access)	Point-to-Point (P2P)	Enables rapid STP convergence via RSTP
Switch-to-Hub or Shared Media	Shared	Prevents premature forwarding; slower convergence
Legacy or non-RSTP switch	Shared	Ensures compatibility with older STP implementations

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2. ACI Interface Policy Configuration

In ACI, configure the following under Access Policies:

• Interface Policy Group:
• Enable STP Interface Policy
• Set Link Type to either point-to-point or shared based on the external device
• Attach the Interface Policy Group to the appropriate Leaf Interface Profile

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4. Avoid STP Misconfigurations

• Ensure BPDU Guard is disabled on ACI ports connected to switches.
• Avoid enabling PortFast on external switch ports facing ACI unless it's an edge port.
• Monitor STP topology changes to detect misbehaving devices.

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5. Use LLDP/CDP for Visibility

Enable LLDP/CDP on both ACI and external switches to:

• Verify connectivity
• Identify misconfigured ports
• Assist in troubleshooting