Cabling two access ports on the same switch from different VLANs

Question

Theoretically would I be able to connect two switch ports together with a simple straight/cross cable on the same switch. If the two ports are in different VLANs (eg. 2 and 22) and ports are in access mode?

BPDU guard and STP are active. The addresses and subnet is the same on both ports 10.9.0.0/24. (I mean there I have assets in VLAN 22, but I have route from the client side to VLAN2, default gw., if asset is connected to access port member in VLAN2, and has the address 10.9.0.2 it can be reached).

I would like to accomplish: make my assets only reachable when switch ports are connected with a cable (and the asset is connected to an access port member of VLAN22), and cut the connection as soon as I pull the cable out? Theoretically. In practice ports are in err-disabled with Cisco Nexus 31xx. It is easy if the cable connects two switches, but what if only one device is present?

Thank you!

Answer 1

If you directly connect two different VLANs, you will have created a single broadcast domain from two separate broadcast domains. That means that the separate VLANs are unnecessary, or you have made a mistake that can cause you problems.

If you are using the same network addressing on both of the VLANs, you need to make sure that all the addresses used by the hosts are unique, otherwise you will have some serious trouble. Also, you cannot route traffic between the VLANs because a router will not route traffic from one network to the same network, and you cannot even set up two router interfaces in the same network.

It actually makes no sense to have separate VLANs that are in the same network that are connected to create a single VLAN. Simply use the same VLAN for both.

The switch interfaces will not err-disable unless you are running bpduguard on the interfaces.