No, ACI doesn’t run STP inside the fabric. However, ACI floods every received STP BPDU frame in each encap VLAN within an EPG.

Master ACI and STP Interaction: The Key to Prevent L2 Loops

Understanding the Cisco ACI interaction with the Spanning-Tree protocol (STP) is essential for network professionals considering connecting external switches to the Cisco ACI fabric. In this video article, I explain the ACI and STP interaction to avoid potential layer-2 loops caused by the external switches.

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Summary

ACI doesn’t run STP inside the fabric. However, ACI floods every received STP BPDU frame in each encap VLAN within an EPG.
We don’t need any configuration for the BPDU flooding in ACI.
External switches are responsible for breaking any potential loops.
Interfaces within the EPG that are connected to the external switches must reside in the same physical or L2 external domain.
BPDU flooding is different from data traffic flooding. Data traffic flooding can be turned on or off at the per-bridge domain.
When ACI receives STP TCNs on a VLAN, it flushes the endpoints associated with that VLAN in the BD (Any EPG). Therefore, endpoints in other encap VLANs in the BD will not be impacted.
Fabric-facing ports in the external switches should be configured as “shared” RSTP link-type.
In MST, BPDUs don’t carry a VLAN tag, and they are sent over the native VLAN. Therefore, we must create an EPG for the native VLAN to carry the BPDUs.

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Salman Alhiary Expert Network Consultant

In the networking field since 2010, 2× CCIE (ENT & DC), and founder of LearnWithSalman—specializes in Cisco Data Center networking and automation. A former Cisco TAC engineer and now an Expert Professional Services consultant, he delivers lab-first tutorials, deep templates, and repeatable playbooks through his courses and technical blog, all built from real enterprise projects.

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Alex

3 months ago

what about in multisite? it should work?

Salman Alhiary

Author

Reply to Alex

Yes, it works.

CiscoLearning

8 months ago

Hi, in ACI Multipod, how STP BPDU forward across pods?

Reply to CiscoLearning

Hi,
ACI encapsulates BPDUs in VXLAN and forwards them via IPN through the spine switches. This is handled transparently when needed to maintain compatibility with traditional STP-based networks.

Reply to Salman Alhiary

Hi, when forwarding STP BPDU across pods, ACI still use FD-VLAN instead of BD-VLAN, right?

Hi,
Yes, FD-VLAN has a fabric-wide scope.

7 months ago

valuable information from you, thank u.