Router-on-a-stick checklist
| Check | Command | Look for |
|---|---|---|
| Physical port up | show ip interface brief | Gi0/0 up/up (subinterfaces follow it) |
| Right VLAN per subinterface | show running-config | section interface | encapsulation dot1Q matches the VLAN of that subnet |
| Switch port is a trunk | show interfaces trunk | The port listed as trunking, with the VLANs allowed |
| Gateways | ipconfig / ip route on the PCs | The subinterface address of their own VLAN |
Multilayer switch checklist
| Check | Command | Look for |
|---|---|---|
| Routing on | show ip route | "Default gateway is ..." means ip routing is off |
| SVIs up | show ip interface brief | Vlan10 up/up; up/down means the VLAN is missing or has no active ports |
| Access ports | show vlan brief | PC ports in the right VLANs |
Here R1's Gi0/0.20 says encapsulation dot1Q 21: frames for VLAN 20 never reach it. Spot it in the configuration:
In NetLab
▶ PC2 student@PC2:~$ ping -c 2 192.168.20.1 PING 192.168.20.1 (192.168.20.1) 56(84) bytes of data. From 192.168.20.10 icmp_seq=1 Destination Host Unreachable From 192.168.20.10 icmp_seq=2 Destination Host Unreachable --- 192.168.20.1 ping statistics --- 2 packets transmitted, 0 received, 100% packet loss, time 1001ms ▶ R1 R1>enable R1#show running-config | section interface GigabitEthernet0/0. interface GigabitEthernet0/0.10 encapsulation dot1Q 10 ip address 192.168.10.1 255.255.255.0 interface GigabitEthernet0/0.20 encapsulation dot1Q 21 ip address 192.168.20.1 255.255.255.0
Tip: Packet Walk is excellent here: it shows the router dropping a frame tagged for a VLAN none of its subinterfaces handles.