How do you troubleshoot router subinterface configuration issues in router-on-a-stick?

Use show ip interface brief to check subinterface status and IPs (Internet Protocol addresses), and use show interfaces | include Gig|802.1Q to verify the VLAN (Virtual Local Area Network)-to-subinterface mapping.

Common router subinterface problems:

1. Wrong VLAN ID in encapsulation dot1q — subinterface responds to wrong VLAN's traffic
2. Wrong IP address or subnet mask — doesn't match the VLAN's subnet
3. Physical interface is shut down — all subinterfaces go down with it
4. Missing no shutdown on physical interface — easy to forget since subinterfaces don't need it individually

Quick verification:

R1# show interfaces | include Gig|802.1Q
GigabitEthernet0/0/1 is up, line protocol is up
  Encapsulation 802.1Q Virtual LAN, Vlan ID 1., loopback not set
GigabitEthernet0/0/1.10 is up, line protocol is up
  Encapsulation 802.1Q Virtual LAN, Vlan ID 10.
GigabitEthernet0/0/1.20 is up, line protocol is up
  Encapsulation 802.1Q Virtual LAN, Vlan ID 20.

In this example, if VLAN ID for .10 showed 100 instead of 10, that would be the misconfiguration.

Tip: The | include filter is very powerful for troubleshooting — it strips out hundreds of lines of irrelevant output and shows only what matters.

Go deeper:

• Troubleshoot Inter-VLAN Routing (Cisco Press) — wrong subinterface VLAN-ID/IP and bad encapsulation dot1Q as the router-side failure modes.