Blog Posts in February 2026

Interface MAC Address in IOS Layer-2 Images

Here’s another “You can’t make this up, but it sounds too crazy to be true” story: Cisco IOS layer-2 images change the interface MAC address when you change the interface switchport status.

Let me start with a bit of background:

  • IOL Layer 2 image starts with interfaces enabled and in bridged (switchport) mode (details)
  • netlab has to run a normalize script (applicable to IOLL2, IOSv L2, and Arista EOS) before configuring anything else to ensure all interfaces are shut down.
  • The IOLL2 normalize Jinja template had a bug – when setting the interface MAC address, it checked l.mac_address instead of intf.mac_address. Nevertheless, everything worked because the MAC addresses were also set during the initial device configuration.
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Fast FRR Container Configuration

After creating the infrastructure that generates the device configuration files within netlab (not in an Ansible playbook), it was time to try to apply it to something else, not just Linux containers. FRR containers were the obvious next target.

netlab uses two different mechanisms to configure FRR containers:

  • Data-plane features are configured with bash scripts using ip commands and friends.
  • Control-plane features are configured with FRR’s vtysh

I wanted to replace both with Linux scripts that could be started with the docker exec command.

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