How to find vlans on the network that are displayed on the router but not on the switch?

Question

Computer networks

Hello, the question will probably be amateurish, but as I see there are enough of them, so do not strangle too much :)

The context is as follows, there is a network built on ubiquiti. The main router and 6 switches, they are all managed. On the router in interfaces, I see two vlans -
eth1.20 (192.168.2.1/24)
eth2.50 (10.12.12.1/24)

So, vlan 50 is present on one of the switches. There is no Vlan 20, there is only one switch in its subnet, it shows that the vlans are not unbuilt, everything is by default.
One could assume that this is some kind of old entry and the vlan is not used / it is not in reality, but looking at the traffic on the interface I see that it is there.

What am I missing?

Answer 1

Most likely you are missing the port setting, which connects the switch to the eth1.20 router interface. If there is suddenly tagged vlan 20 (or maybe 50 on it), and on the ports to which the other switches are connected, access vlan 20 (i.e. port without vlan support), the rest of the switches will not know about vlan 20, for them the entire network will be vlan0.
In general, the vlan setup is desirable end-to-end, so as not to catch a random loop in the backbone or not to connect networks on L1 that should be decoupled, IMHO the admin who set it up was too lazy to make the same vlans everywhere, and set the default vlan display of the rest of the switches in vlan 20 to be where the vlans are still configured.

Answer 2

Draw a complete network backbone diagram. All active L2 equipment, all connections between them with port numbers on each side. On each piece of equipment, for each trunk port, check that it has only tagged VLANs, and for each client (including unused) - only one untagged VLAN.
If this is not the case (and, judging by the description, this is exactly the state of things that will be found out) - draw up the necessary VLAN plan and a plan for reconfiguring the equipment to this scheme without stopping the service. And implement. And you will put the network in order, and you will know what and where it runs.
Yes, don't forget a separate control VLAN to control the equipment. And don't use VLANID=1.