VLAN 10 and VLAN 20 can't talk
Same-VLAN hosts reach each other, but inter-VLAN routing over a router-on-a-stick is broken. Find the L2/L3 gap.
Scenario
One switch, two VLANs (10 and 20), and a router-on-a-stick doing inter-VLAN routing over a trunk with dot1q subinterfaces. Hosts in the same VLAN reach each other fine, but VLAN 10 and VLAN 20 hosts can't reach each other.
Topology
- VLAN 10 =
10.0.10.0/24, gateway10.0.10.1 - VLAN 20 =
10.0.20.0/24, gateway10.0.20.1 - The router reaches both VLANs over a trunk to the switch, via dot1q subinterfaces (
.10and.20)
Your job
Restore inter-VLAN routing without changing the VLAN or IP design.
What "done" looks like
A host in VLAN 10 can ping a host in VLAN 20, and the switch↔router trunk carries both VLANs.
Teaches: trunking, dot1q tagging, and how L2 (the allowed VLANs on the trunk) and L3 (the router's subinterface tags) have to line up for router-on-a-stick to work.
Tooling
Use Arista cEOS (IOS-like VLAN/trunk syntax) for the switch and router. Free fallback: a Linux VLAN-aware bridge plus 802.1Q subinterfaces. Validate syntax on first deploy.
What gets checked
Your solution is verified against each of these:
- A host in VLAN 10 (10.0.10.10) can ping a host in VLAN 20 (10.0.20.10)
- The switch↔router trunk carries both VLAN 10 and VLAN 20
Solve it in your browser
No setup, no install. Write your solution in the editor and hit Check. The in-house engine renders and grades it instantly, then issues your proof the moment every check passes.
Solve in browser →Prefer your own lab?
- Build the fix locally. New to the tooling? See setting up your lab.
- Push your topology file, device configs, and any playbooks to a public repo (GitHub or GitLab).
- Submit the repo link. We review it by hand, confirm it works, and issue your proof page.