Cisco+lab+162 May 2026

Even seasoned engineers get stuck here. Here are the top 5 failure points in Lab 162:

1. The "Incomplete ARP" Issue

2. Native VLAN Mismatch

3. VLANs Not Allowed on Trunk

4. IP Routing Disabled

5. Host Firewall (Soft Issue)

Now comes the moment of truth. Let’s verify that our configuration works.

1. Verify Router Interfaces: Check if the sub-interfaces are "up/up" and have the correct IPs.

R1# show ip interface brief

You should see g0/0.10 and g0/0.20 with status "up".

2. Verify Trunking on Switch: Ensure the link to the router is indeed trunking.

S1# show interfaces trunk

You should see VLANs 10 and 20 allowed on the trunk. cisco+lab+162

3. The Ping Test: From PC1, attempt to ping PC2.

C:\> ping 192.168.20.10

If the ping is successful (replies received), congratulations! You have successfully configured Inter-VLAN routing. The first ping might time out due to ARP processes, but subsequent pings should succeed.

Without trunks, SW1’s VLAN 10 traffic cannot reach SW2’s VLAN 10. Trunks using IEEE 802.1Q add a VLAN tag to each frame.

On SW1 (uplink to Multilayer SW):

interface gigabitEthernet 0/1
 switchport trunk encapsulation dot1q   (Required on older switches)
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 no shutdown

On SW2 (uplink to Multilayer SW):

interface gigabitEthernet 0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport trunk allowed vlan 10,20
 no shutdown

Verification: Run show interfaces trunk. You should see the ports listed with Native VLAN "1" and allowed VLANs "10,20". If this output is blank, the trunk did not form—check for mismatched encapsulation or cabling.

This is the core of "Router-on-a-Stick." We will configure sub-interfaces on a single physical interface. Each sub-interface acts as the default gateway for a specific VLAN.

1. Enable the Physical Interface: Do not assign an IP address to the physical interface itself; just turn it on.

R1> enable
R1# configure terminal
R1(config)# interface gi0/0
R1(config-if)# no shutdown
R1(config-if)# exit

2. Configure Sub-Interface for VLAN 10: We create a logical interface gi0/0.10. We must specify the encapsulation (dot1q) and the VLAN ID (10).

R1(config)# interface gi0/0.10
R1(config-subif)# encapsulation dot1q 10
R1(config-subif)# ip address 192.168.10.1 255.255.255.0
R1(config-subif)# exit

3. Configure Sub-Interface for VLAN 20: Similarly, create gi0/0.20 for the Engineering VLAN. Even seasoned engineers get stuck here

R1(config)# interface gi0/0.20
R1(config-subif)# encapsulation dot1q 20
R1(config-subif)# ip address 192.168.20.1 255.255.255.0
R1(config-subif)# exit

Note: The IP addresses assigned here (.1) will be the Default Gateway addresses configured on your PCs.

Students often write an ACL that permits everything they need, but forget that at the end of every ACL is an invisible deny any any.