Why do I need explicit vlans configured for switches not connected directly to end devices?

[danielchenofficial1]

Hi everyone,

I was looking through a packet tracer lab (troubleshooting w/ vlans), and I was perplexed as to why I needed to configure specific access vlans (in this case, 10, 20, and 30) on a switch that was only connected to other switches by trunks. Obviously, switches connected to access devices must have specific vlans created for each of those devices. But why should the switch connected to those access switches need it?

I have attached the lab I was looking at to this post. Communications will not work, 100%, if I do not configure vlan10 and vlan56 (this is speaking in terms of vlan10 devices only) on S1. But why is that? Can someone explain it for me, please?

[Georg Pauwen · Accepted solution]

Hello,

the VLAN databases on all your switches need to be consistent, that is, they need to be the same. The trunk ports will only carry the VLANs the switch knows about.

There are two ways to achieve this:

a) configure VTP - set S1 as VTP Server and S2/S3 as VTP Client. Create all VLANs on S1, and they will be propagated to S2 and S3 through VTP

b) create all VLANs manually on all switches

Also, all your trunk ports need to be configured as:

switchport mode trunk

switchport trunk native vlan 56

You can use the 'interface range' command to assign your ports to the access VLANs as per your requirement.

I have attached the revised file.

[danielchenofficial1]

Thank you! That explanation was exactly what I was looking for.