DHCP Assigning Router IP to Client — No Exclusion Configured
Presenting Symptom
A DHCP client receives an IP address that belongs to the router's LAN interface instead of a valid host address from the pool.
Network Context
A small branch office with a Cisco 4321 ISR router acting as the DHCP server for the 192.168.1.0/24 LAN. The router's GigabitEthernet0/0/0 interface has IP address 192.168.1.1/24. The DHCP pool is configured to serve addresses from 192.168.1.1 to 192.168.1.254 without any excluded addresses. A single Windows 10 client is connected via a switch and requests an IP address.
Diagnostic Steps
Check the IP address assigned to the client
On the client: ipconfig /all (Windows) or ip addr show (Linux)IPv4 Address: 192.168.1.1 Subnet Mask: 255.255.255.0 Default Gateway: 192.168.1.1
The client has received the router's own interface IP address (192.168.1.1). This is abnormal because the router should not assign its own IP to a client.
Examine the DHCP pool configuration on the router
show ip dhcp poolPool LAN-POOL : Utilization mark (high/low) : 100 / 0 Subnet size (first/next) : 0 / 0 Total addresses : 254 Leased addresses : 1 Pending event : none 1 subnet is currently in the pool : Current index IP address range Leased addresses 192.168.1.1 192.168.1.1 - 192.168.1.254 1
The pool includes the entire subnet range starting at 192.168.1.1, which is the router's own interface IP. There is no excluded address configured, so the router can lease out its own IP.
Check the DHCP bindings to see which address was leased
show ip dhcp bindingBindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.1.1 0063.6973.636f.2d30. Mar 01 1993 12:01 AM Automatic
3030.302e.3030.3030.
2e30.3030.302d.4769.
30The binding shows that IP address 192.168.1.1 has been leased to a client. This confirms the router assigned its own interface IP.
Verify the router's interface IP address
show ip interface briefInterface IP-Address OK? Method Status Protocol GigabitEthernet0/0/0 192.168.1.1 YES NVRAM up up
The router's interface has IP 192.168.1.1, which is the same address that was leased to the client. This confirms the conflict.
Root Cause
The DHCP pool is configured with a network range that includes the router's own interface IP address (192.168.1.1) and no excluded address is configured. As a result, the DHCP server can lease out the IP address that is statically assigned to its own interface, causing an IP address conflict.
Resolution
Verification
1. Verify the excluded addresses: show ip dhcp excluded-address Expected output: IP excluded addresses: 192.168.1.1 to 192.168.1.10 2. Clear the current binding to force the client to get a new address: clear ip dhcp binding * 3. On the client, release and renew the IP address (ipconfig /release then ipconfig /renew on Windows). 4. Verify the new client IP: show ip dhcp binding Expected output: The client should now have an IP address outside the excluded range, e.g., 192.168.1.11.
Prevention
["Always configure an ip dhcp excluded-address command for the router's interface IP and any other statically assigned addresses before configuring the DHCP pool.","Use a subnet design that reserves the first few addresses (e.g., .1-.10) for static assignments and excludes them from the DHCP pool.","Implement IP address management (IPAM) to track static and dynamic assignments."]
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests the candidate's ability to troubleshoot DHCP misconfigurations. The exam may present a multiple-choice question asking why a client received an IP address that conflicts with the router's interface, or a drag-and-drop task to order the troubleshooting steps. The key fact is that the DHCP server can assign any address in the pool unless explicitly excluded, including the router's own IP.
Exam Tips
Remember that the DHCP server does not automatically exclude its own interface IP; you must manually configure the exclusion.
The show ip dhcp pool command displays the current index, which indicates the next address to be assigned. If it starts at the router's IP, that's a red flag.
The clear ip dhcp binding * command is useful for resetting leases during troubleshooting, but be aware it disrupts all DHCP clients.
Commands Used in This Scenario
ip dhcp excluded-address [start] [end]
Excludes one or more IP addresses from the DHCP pool so they are not automatically assigned to clients, typically used for static assignments like servers or routers.
show ip dhcp binding
Displays the current DHCP binding table, showing which IP addresses have been leased to clients, along with their MAC addresses, lease expiration, and type of binding.
show ip dhcp pool
Displays the configuration and utilization statistics of a DHCP pool, used to verify pool settings and address allocation status.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions