What is wrong with vlan?

Question

Computer networks

The bottom line:
switch root is a provider from which the Internet came through switch 3 to router 1 and router 2 to the common switch 2 network. Internet on switch 1. According to the attached scheme, in my opinion, everything should work, but when the red link is turned on, switch root catches the loop and disables the port.
What did I do wrong and how should I?

Answer 1

catches the loop and disables the port

Are you sure that the port is disabled due to the formation of a loop (i.e. it goes into blocking/alternate mode), and not due to the operation of, for example, root guard?
In addition, it is worth checking if the settings of the routers (router 1, router 2) contain any frame forwarding (bridging, for example, or BPDU forwarding) between the ports indicated in the diagram (i.e., is one of the routers the cause of , by which root thinks there is a loop).

Answer 2

Judging by the scheme and description, you do not need a "red line" at all.
By the situation, it looks like switch1 and switch2 are connected.
Metaphysical question: "Why vlans with letters?"

Answer 3

zxc80 - letters are probably logical to understand as taget and antaget.
And as in my opinion, the loop is rather not on switch root, but on switch 3 on vlan 0 in the first block diagram.
Try disabling switch root and connecting directly to switch3 with the red line on and see how it works. Use RSTP.
Vlan Loop Decoupling