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Router on a stick

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Router on a stick is an inter-VLAN routing method that uses one router interface with multiple VLAN-tagged virtual subinterfaces over a trunk link.

Router on a stick (ROAS) routes between VLANs through one router interface connected to a switch by a trunk. Multiple virtual subinterfaces share that physical interface, and each subinterface provides an IP gateway for one VLAN. section 12.4.1

A basic ROAS configuration follows this sequence:

  • Assign each subinterface an IP address from its VLAN’s subnet; the subinterface number need not equal the VLAN ID. section 12.5

VLAN 20 to VLAN 10

A host in VLAN 20 sends a tagged frame to its default gateway on the router’s G0/0.20 subinterface. The router routes the packet and sends a new frame through G0/0.10 toward the host in VLAN 10.

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains trunking, subinterfaces, 802.1Q encapsulation, addressing, native VLANs, and representative Cisco configuration.
Examples
Uses VLAN 20-to-VLAN 10 traffic, G0/0.10, G0/0.20, G0/0.30, and Cisco IOS configuration listings.
What it assumes
Assumes VLAN segmentation, trunking, IP subnets, and default gateways while defining the required router commands.
What you can do after
Build it from scratch by configuring a switch trunk, router subinterfaces, VLAN encapsulation, and gateway IP addresses.
How it connects
Connects VLANs, trunk links, subinterfaces, native VLANs, inter-VLAN routing, and multilayer switches.
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Related concepts

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