topic Re: External connector not working as expected in DevNet Sandbox

External connector not working as expected

robert-boener — Wed, 14 Jan 2026 01:25:46 GMT

I am trying to use an external connector to allow me to connect from my laptop (connected via AnyConnect VPN to CML) to routers in my lab for automation (Ansible) validation. I shutdown the sample lab so addresses are not being used. I built two routers and addressed the ethernet interfaces as 10.10.20.171/24 and 10.10.20.172/24. I connected the two routers together using a layer 2 switch. I can ping between the two routers as expected. The mac-address table of the switch shows the correct MAC addresses on the correct switch ports (ports 1,2). I then add an external connector to port 3 of the switch in bridge mode. Once I start the external connector I can no longer ping between the two routers. The mac-address table of the switch now shows both MAC addresses of the two routers as connected on port 3 which is the external connector not port 1 and 2 where they should be. Am I doing something wrong? Thanks.

Re: External connector not working as expected

Jesus Illescas — Wed, 14 Jan 2026 09:41:20 GMT

@robert-boener can you share the yml file of you topology to try to reproduce. And to confirm, you are using the Cisco Modeling Labs sandbox, https://devnetsandbox.cisco.com/DevNet/catalog/cml-sandbox_cml right?

Re: External connector not working as expected

robert-boener — Wed, 14 Jan 2026 18:37:39 GMT

Yes, I am using CML Sandbox hosted on devnetsandbox.cisco.com. Attached is the .yaml file.
Thanks

Re: External connector not working as expected

Jesus Illescas — Thu, 15 Jan 2026 07:47:09 GMT

@robert-boener there's an sandbox upgrade happening at the moment. I will test it once is done.

Re: External connector not working as expected

Jesus Illescas — Fri, 16 Jan 2026 11:29:59 GMT

Hi@robert-boener I got it working but I'm not sure exactly how. Let me expand. I replaced the switch in the middle by an unmanaged switch, this is what I usually use to connect to an external connector and what I have seen in most examples.

But it didn't work out of the box. I have ping many times the default GW and from my computer to try to force arp to pass, but it didn't work. So I left the sandbox running during the night and the next day all works. Not sure if there was a timer somewhere on the backend of the sandbox that did the trick.

I also stopped, wiped and deleted the default lab. I know CML maintains some information of the mac addresses behind the scenes, so I wanted a clean state.

I'm attaching the lab I used for testing and below you can see a gif of the connections working. I hope this helps

Re: External connector not working as expected

robert-boener — Sat, 17 Jan 2026 18:33:41 GMT

@Jesus Illescas , thanks for the feedback. I tried the same thing, setup my lab and left it alone overnight. When I first started the lab it was not working, overnight it started working. There is something strange going on with the sandbox. Note I used both unmanaged switch and the "IOL-L2" switch and saw the same behavior. Using the managed switch provided visibility into layer 2 connectivity. What doesn't make any sense is when the external connector is added the MAC addresses for both routers instantly appears as connected via the external connector not the routers themselves. After sitting overnight the switch now shows MAC addresses connected to the correct switch ports and everything is working.