Troubleshooting Router-on-a-Stick (ROAS) Issues

In this lab, we will troubleshoot common issues related to Router-on-a-Stick (ROAS) configurations, including incorrect VLAN assignments, missing encapsulation, disabled interfaces, and trunk misconfigurations.

Scenario Description

A company has implemented ROAS for inter-VLAN routing. However, users are reporting that they cannot communicate between VLANs. The network administrator must troubleshoot the router, switch, and host configurations to resolve the issues.

Since this is a troubleshooting lab, it is better to practice with the save-config LAB.

Network Structure

• Device 1: Cisco Router (R1)
• Device 2: Cisco Switch (SW1)
• Device 3: Host A (HR – VLAN 10, 192.168.10.10/24)
• Device 4: Host B (IT – VLAN 20, 192.168.20.10/24)
• Connections:
  • R1 (Ethernet0/1) ↔ SW1 (Trunk Port)
  • SW1 (Access Ports) ↔ Host A (VLAN 10) & Host B (VLAN 20)

Topology Diagram

Prerequisites

• Basic understanding of VLANs, trunking, and inter-VLAN routing
• Access to a Cisco router and a Layer 2 switch
• CLI familiarity

Implementation Steps

Step 1: Checking VLAN Configuration on Switch

SW1# show vlan brief

Expected output:

VLAN Name Status Ports
—- ——————————– ——— ——————————-
1 default active Et0/0, Et0/1, Et0/2, Et0/3
10 HR active
20 IT active
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup
SW1#

Fix: If VLANs are missing, add them:

SW1(config)# vlan 10
SW1(config-vlan)# name HR
SW1(config-vlan)# exit

SW1(config)# vlan 20
SW1(config-vlan)# name IT
SW1(config-vlan)# exit

Why This Step? Ensures VLANs are properly defined on the switch.

Step 2: Checking Access Port Assignments

SW1#show interfaces status

Port         Name               Status       Vlan       Duplex  Speed Type
Et0/0                           connected    1            full   auto 10/100/1000BaseTX
Et0/1                           connected    1            full   auto 10/100/1000BaseTX
Et0/2                           connected    10           full   auto 10/100/1000BaseTX
Et0/3                           connected    20           full   auto 10/100/1000BaseTX
SW1#

Fix: If a port is in the wrong VLAN, reassign it:

SW1(config)# interface ethernet0/2
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 10
SW1(config-if)# exit

SW1(config)# interface ethernet0/3
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 20
SW1(config-if)# exit

Why This Step? Ensures hosts are connected to the correct VLANs.

Step 3: Checking Trunk Port on Switch

SW1#show interfaces trunk

Port           Mode             Encapsulation  Status        Native vlan
Et0/1          on               802.1q         trunking      1

Port           Vlans allowed on trunk
Et0/1          10,20

Port           Vlans allowed and active in management domain
Et0/1          10,20

Port           Vlans in spanning tree forwarding state and not pruned
Et0/1          10,20

Expected output:

Port        Mode         Encapsulation  Status
Et0/1          on               802.1q         trunking      1

Fix: If the trunk is not configured correctly:

SW1(config-if)#switchport trunk encapsulation dot1q
SW1(config-if)#switchport mode trunk
SW1(config-if)#switchport trunk allowed vlan 10,20
SW1(config-if)# exit

Why This Step? Ensures VLAN traffic can traverse the trunk link.