DHCPCCNA 200-301

DHCP Assigning Wrong Default Gateway to Clients

Presenting Symptom

Clients in VLAN 10 receive an IP address from DHCP but cannot reach the internet or other subnets because the default gateway assigned is incorrect.

Network Context

A small branch office with a Cisco 4321 ISR router, a Cisco Catalyst 2960 switch, and 50 clients in VLAN 10. The router acts as the DHCP server for VLAN 10 (192.168.10.0/24) and provides inter-VLAN routing. The correct default gateway should be 192.168.10.1, but clients are receiving 192.168.10.254.

Diagnostic Steps

Check client IP configuration

On a client: ipconfig /all (Windows) or ifconfig (Linux/Mac)

IPv4 Address: 192.168.10.50
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.10.254
DHCP Server: 192.168.10.1

The client has an IP from the correct subnet but the default gateway is 192.168.10.254 instead of 192.168.10.1. This indicates the DHCP server is providing the wrong gateway.

Verify DHCP pool configuration on the router

show running-config | section dhcp

ip dhcp pool VLAN10_POOL
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.254
 dns-server 8.8.8.8

The DHCP pool has the default-router set to 192.168.10.254, which is incorrect. The correct gateway should be 192.168.10.1, the router's interface IP in VLAN 10.

Check router interface configuration for VLAN 10

show running-config interface gigabitethernet 0/0/0.10

interface GigabitEthernet0/0/0.10
 encapsulation dot1Q 10
 ip address 192.168.10.1 255.255.255.0

The router's subinterface for VLAN 10 has IP 192.168.10.1, confirming that the correct default gateway should be 192.168.10.1, not 192.168.10.254.

Check if there is another device with IP 192.168.10.254

ping 192.168.10.254

Reply from 192.168.10.254: bytes=32 time<1ms TTL=255

The IP 192.168.10.254 is reachable, possibly another router or misconfigured device. This could cause routing issues if clients use it as gateway.

Root Cause

The DHCP pool configuration on the router has the default-router set to 192.168.10.254 instead of 192.168.10.1. This is a misconfiguration in the DHCP pool parameters.

Resolution

Enter global configuration mode and modify the DHCP pool to correct the default-router: configure terminal ip dhcp pool VLAN10_POOL default-router 192.168.10.1 end write memory

Verification

After the fix, release and renew a client's IP address. On the client, run ipconfig /all and verify: Default Gateway: 192.168.10.1 Then test connectivity: ping 192.168.10.1 (should succeed) and ping an external IP (e.g., 8.8.8.8) to confirm internet access.

Prevention

1. Always double-check DHCP pool parameters, especially default-router, against the actual router interface IP. 2. Use a DHCP configuration template or script to ensure consistency. 3. Implement IP address management (IPAM) to track and avoid conflicts.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify the misconfigured DHCP parameter. The exam tests your ability to read a running-config and spot the incorrect default-router. Key fact: The default-router in a DHCP pool must match the router's interface IP in that subnet.

Exam Tips

Remember that the default-router command in DHCP pool sets the gateway, not the DHCP server address.

The exam may show a configuration with a typo or wrong IP; always compare the default-router to the interface IP.

Know that 'show running-config | section dhcp' quickly displays all DHCP-related configuration.

Commands Used in This Scenario

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

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