Systematic No-Connectivity Troubleshooting — OSI Top-Down
Presenting Symptom
A user in the Sales VLAN cannot ping the server in the Server VLAN, while other users in the same VLAN can reach the server.
Network Context
Small branch office with a single Cisco Catalyst 2960 switch (IOS 15.0) and a Cisco 4321 ISR router. The switch has two VLANs: VLAN 10 (Sales, 192.168.10.0/24) and VLAN 20 (Server, 192.168.20.0/24). The router-on-a-stick configuration provides inter-VLAN routing. The affected PC (192.168.10.50) is connected to interface FastEthernet0/1 on the switch, which is configured as an access port in VLAN 10. The server (192.168.20.100) is connected to FastEthernet0/2 in VLAN 20. The router subinterfaces are Gi0/0.10 (192.168.10.1) and Gi0/0.20 (192.168.20.1).
Diagnostic Steps
Check PC IP configuration
ipconfig (on PC) or show running-config interface FastEthernet0/1 (on switch)IPv4 Address: 192.168.10.50, Subnet Mask: 255.255.255.0, Default Gateway: 192.168.10.1
Verify the PC has correct IP, mask, and gateway. If missing or wrong, that is the root cause.
Test connectivity to default gateway
ping 192.168.10.1 (from PC)Reply from 192.168.10.1: bytes=32 time<1ms TTL=255
If ping fails, problem is Layer 2 or Layer 1 between PC and router. Check switch port status and VLAN assignment.
Check switch port VLAN assignment
show interfaces FastEthernet0/1 switchportAdministrative Mode: static access, Access Mode VLAN: 10 (Sales)
Ensure port is in correct VLAN. If VLAN mismatch, reconfigure port.
Check router subinterface configuration
show running-config interface GigabitEthernet0/0.10encapsulation dot1Q 10, ip address 192.168.10.1 255.255.255.0
Verify subinterface matches VLAN and has correct IP. If missing or wrong, fix configuration.
Check trunk between switch and router
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1
Ensure trunk is up and VLANs 10 and 20 are allowed. If trunk is down or VLANs not allowed, fix trunk.
Check routing table on router
show ip routeC 192.168.10.0/24 is directly connected, GigabitEthernet0/0.10 C 192.168.20.0/24 is directly connected, GigabitEthernet0/0.20
Both subnets should be in routing table. If missing, check subinterface configuration.
Root Cause
The trunk interface between the switch and router is down because the switch port GigabitEthernet0/1 is administratively shut down (shutdown command applied).
Resolution
Verification
On the switch, run 'show interfaces trunk' and 'show interfaces GigabitEthernet0/1'. Expected output: trunk status 'trunking', interface status 'up/up'. Then from the PC, ping 192.168.20.100 successfully.
Prevention
1. Always verify interface status after configuration changes. 2. Use 'show running-config' to check for accidental shutdown commands. 3. Implement network monitoring to alert on interface down events.
CCNA Exam Relevance
CCNA 200-301 exam includes troubleshooting inter-VLAN routing scenarios. Questions may present a similar symptom and ask to identify the root cause (e.g., trunk down, VLAN mismatch, wrong gateway). Candidates must know how to verify trunk status and VLAN assignments.
Exam Tips
Remember that 'show interfaces trunk' is the go-to command for trunk issues.
If ping to gateway fails, the problem is likely Layer 2; if ping to gateway succeeds but not to remote VLAN, check routing.
Be careful: 'shutdown' on a trunk port will cause all inter-VLAN traffic to fail.
Commands Used in This Scenario
show interfaces switchport
Displays the administrative and operational status of a switch port, including VLAN membership, trunking mode, and access VLAN, used to verify VLAN configuration and port security settings.
show interfaces trunk
Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.
show running-config
Displays the current active configuration in DRAM, showing all non-default settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions