Another very simple concept that confuses the heck out of people is the concept of mismatching VLAN’s and merging broadcast domains.
Let’s take the below topology in consideration.
We have 2 switches connected together, and we have 2 computers connected to them. If we didn’t configure VLAN’s this would be one broadcast domain. This means that any broadcast sent on this flat layer 2 network is sent to every host. Now if we configured VLAN’s we can split these broadcast domains, and thus splitting the network in 2. But if we reconfigured them like above, we would be merging two broadcasts domains and thus creating one. The end result of this is traffic from one VLAN falling into the other and vice versa.
Here is the config for the left switch…
Here is the config for the right switch….
Now a ping from one host to the other…
This of course is no bueno.