Configuring Port Profiles in Cisco ACI (NX-OS Style CLI) – Uplink to Downlink port

Up until Cisco ACI 3.1, fabric ports on
leaf switches were hard-coded as fabric (iVXLAN) ports and could connect only
to spine switches. Starting with Cisco ACI 3.1, you can change the default
configuration and make ports that would normally be fabric links, be downlinks,
or vice-versa. 

🧩 The Discovery Catch

Fabric discovery in
ACI is only supported through native fabric ports. These are
typically high-speed interfaces—like the 40/100G QSFP ports—designed
specifically for spine-leaf connectivity.

If you’re planning to
connect via 10G SFP, you’ll need to use a CVR-QSFP-SFP10G
adapter on one of the native QSFP ports. This allows the switch to
initiate discovery even when using lower-speed optics.

🔄 Post-Discovery Flexibility

Once the switch is
successfully discovered:

• You can reassign downlink ports
  (1–48) to act as fabric ports.
• This gives you flexibility in how you
  architect your connections post-onboarding.

However, this
flexibility only comes after the switch has been discovered.
Until then, you’re dependent on the native fabric ports.

⚠️ Why You Should Always Keep One Native Fabric
Port Active

Even after discovery
and configuration:

• Always reserve at least one native fabric
  port for fabric
  connectivity.
• If the switch is wiped or reset, and no
  native fabric port is active, it won’t be able to rejoin the fabric.
• This can leave you in a bind—unable to
  rediscover or reconfigure the switch without manual intervention.

🧠 Pro Tip

Think of native fabric
ports as your lifeline to the ACI fabric. Use adapters if
needed, but never fully rely on converted downlink ports for long-term fabric
access.

————————

Cisco ACI allows administrators to
configure port profiles using a familiar NX-OS style CLI. This method is
especially useful for those who prefer command-line workflows over GUI-based configuration.
Below is a step-by-step guide to help you set up a port profile on a leaf
switch.

Pre-Requisites

– The ACI fabric is fully deployed, with
APIC controllers online and the cluster in a healthy state.
– You have access to an APIC administrator account with permissions to
configure fabric infrastructure.
– The leaf switches you intend to configure are registered and visible in the
fabric.

Configuration Steps

Step 1: Enter Global Configuration Mode

Start by accessing the APIC CLI and
entering configuration mode:

Step 2: Select the Leaf Node

Specify the leaf switch you want to
configure by its node ID:

Step 3: Define the Interface

Identify the interface you want to
configure. For Ethernet ports, use the format ethernet slot/port:

Step 4: Set the Port Direction

Assign the port role as either uplink or
downlink. In this example, we configure it as a downlink:

Note: On certain models like the
N9K-C9336C-FX, changing port roles from uplink to downlink is not supported.

Step 5: Clean and Reload the Leaf Configuration

Log into the leaf switch directly and run
the following commands to apply the configuration cleanly:

Final Tip

Always verify hardware compatibility before
attempting port role changes. Some switches have fixed port roles that cannot
be modified, and skipping the clean reload step may result in incomplete
configuration.