Interface Missing IP Address — Connectivity Fails
Presenting Symptom
PCs in VLAN 10 cannot ping the default gateway (router interface 192.168.10.1), and the router interface shows 'line protocol is down'.
Network Context
A small branch office with a Cisco 4321 router (IOS XE 16.9) connected to a Cisco 2960 switch via a trunk link. The router has subinterfaces for VLANs 10 and 20. The engineer notices that devices in VLAN 10 cannot reach the internet or other VLANs.
Diagnostic Steps
Check interface status
show interfaces gigabitethernet 0/0/0.10GigabitEthernet0/0/0.10 is up, line protocol is down (notconnect)
The line protocol is down, indicating Layer 1 is up but Layer 2 is not operational. This suggests a missing IP address or encapsulation mismatch.
Verify IP address configuration
show running-config interface gigabitethernet 0/0/0.10interface GigabitEthernet0/0/0.10 encapsulation dot1Q 10 no ip address
The subinterface has no IP address configured. Without an IP address, the interface cannot route traffic.
Check VLAN encapsulation on switch
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-1005 Port Vlans allowed and active in management domain Gi0/1 1,10,20
The trunk is correctly configured and VLAN 10 is allowed. This rules out a switch-side issue.
Check CDP neighbors
show cdp neighbors gigabitethernet 0/0/0Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID Local Intrfce Holdtme Capability Platform Port ID
Switch Gig 0/0/0 122 S I WS-C2960 Gig 0/1CDP shows the switch is connected, confirming Layer 1 and 2 connectivity. The problem is isolated to the router subinterface configuration.
Root Cause
The router subinterface GigabitEthernet0/0/0.10 is missing the IP address configuration. Without an IP address, the interface cannot forward traffic for VLAN 10, causing connectivity failure.
Resolution
Verification
Run 'show interfaces gigabitethernet 0/0/0.10' and verify: GigabitEthernet0/0/0.10 is up, line protocol is up Internet address is 192.168.10.1/24 Also test ping from a PC in VLAN 10 to 192.168.10.1 — should succeed.
Prevention
1. Use configuration templates or automation to ensure all subinterfaces have IP addresses. 2. Implement interface-level monitoring (e.g., SNMP traps for line protocol down). 3. Perform a 'show running-config | section interface' after any VLAN or routing changes to verify completeness.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions (multiple-choice or drag-and-drop) where you must identify why a host cannot ping its default gateway. The exam tests your ability to use 'show interfaces' and 'show running-config' to spot missing IP addresses on subinterfaces.
Exam Tips
Remember that 'line protocol is down' often indicates a Layer 2 issue, but on subinterfaces it can also mean missing IP address or encapsulation mismatch.
Always check the subinterface configuration first — a missing 'ip address' is a common CCNA trap.
Know that 'show interfaces' output includes 'Internet address' only if an IP is configured; otherwise it shows 'no ip address'.
Commands Used in This Scenario
show cdp neighbors
Displays information about directly connected Cisco devices discovered via CDP, used to verify neighbor relationships and gather device details.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
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