In a router-on-a-stick scenario, what configuration is needed on the switch side?

The switch needs VLANs (Virtual Local Area Networks) created and named, a management interface, access ports assigned to VLANs, and a trunk port connecting to the router.

Four steps to configure the switch:

Step 1 — Create and name VLANs:

S1(config)# vlan 10
S1(config-vlan)# name LAN10
S1(config-vlan)# exit
S1(config)# vlan 20
S1(config-vlan)# name LAN20

Step 2 — Create the management interface:

S1(config)# interface vlan 99
S1(config-if)# ip add 192.168.99.2 255.255.255.0
S1(config-if)# no shut

Step 3 — Configure access ports:

S1(config)# interface fa0/6
S1(config-if)# switchport mode access
S1(config-if)# switchport access vlan 10

Step 4 — Configure the trunk port to the router:

S1(config)# interface fa0/5
S1(config-if)# switchport mode trunk
S1(config-if)# no shut

Don't forget to also set the default gateway on the switch (ip default-gateway 192.168.99.1) so the switch itself can be managed remotely.

Go deeper:

• Router-on-a-Stick Inter-VLAN Routing (Cisco Press) — the switch side: VLAN creation, access-port assignment, router-facing trunk.
• Configure Router-on-a-Stick (Study-CCNA) — hands-on lab ending in a verified inter-VLAN ping.