First, thank you very much for your response. Unfortunately this isn't a solution to my proposed set-up (however flawed it may be). Sorry for not explaining the scenario more completely. This isn't an uplink to a single switch. It's an ASR aggregating multiple switches and or other network devices, some of which I don't control. For simplicity sake, I only included 2 interfaces in the example, but the topology would actually include 5+ ports in the same theoretical Layer 2 Segment. The goal is to allow Spanning Tree to run to detect possible loops in these respective switches as well as loops caused by them possibly becoming interconnected somehow. The somewhat obvious solution would be to use tagged VLANs and be done with it or introduce a switch for aggregation below it, but that also isn't an option unfortunately. The simple way to think of the topology is an office or a small apartment complex where people are allowed to hook up switches and additional devices. I'm concerned the answer is that spanning tree won't run without the presence of tagged VLANs because that's the conclusion I'm drawing from what I've read, but I figured I would present the question in the hopes someone with more experience than I with IOS-XE would correct me. I realize split horizon would help minimize some of my concerns, but I'm still interested in understanding if the proposed idea is possible.
