Lab: VLANs that stay apart until the router joins them

Two VLANs on a single switch cannot talk to each other. That is not a misconfiguration — it is the definition. This lab builds the standard way to join them.

What you are given

A switch with PCs in VLAN 10 and VLAN 20, and a router with one physical link to it.

Do this

• Confirm the two VLANs exist and the access ports are in them.
• Make the switch port facing the router a trunk, allowing both VLANs.
• On the router, create a sub-interface per VLAN, each with an 802.1Q
• Point each PC at its own gateway.
• Ping across.

R1(config)# interface GigabitEthernet0/0.10
R1(config-subif)# encapsulation dot1Q 10
R1(config-subif)# ip address 192.168.10.1 255.255.255.0

The point

One cable, two VLANs, one router. The tag is what keeps them apart on the wire; the sub-interface is what tells the router which gateway to answer as. Both ends have to agree about the tag or the frames arrive in the wrong VLAN — which does not fail, it just goes somewhere wrong.

Try breaking it

• Remove VLAN 20 from the trunk's allowed list and find the symptom **from
• Put it back with switchport trunk allowed vlan add 20 and then, on
• Set the native VLAN differently on each end and watch where the untagged