no ip dhcp conflict address [ip]
Use this command to clear a specific IP address from the DHCP conflict table, allowing the DHCP server to reassign that address to a client.
Definition: no ip dhcp conflict address [ip] is a Cisco IOS global config command. Use this command to clear a specific IP address from the DHCP conflict table, allowing the DHCP server to reassign that address to a client.
Overview
The `no ip dhcp conflict address [ip]` command is a global configuration command used on Cisco IOS routers and switches to clear a specific IP address from the DHCP conflict table. The DHCP conflict table stores IP addresses that have been detected as conflicting—meaning the DHCP server attempted to lease the address, but discovered that another device on the network was already using it (e.g., via a ping or ARP probe). When a conflict is detected, the address is marked as unavailable and will not be reassigned by the DHCP server until it is manually cleared or the conflict timer expires.
This command is essential for network administrators who need to reclaim a specific IP address that was erroneously flagged as conflicting, perhaps due to a temporary duplicate IP scenario or a misconfigured static host. Without this command, the address would remain in the conflict table for a default period (typically 24 hours) or until the conflict table is cleared entirely with `clear ip dhcp conflict *`. The command provides granular control, allowing the administrator to remove only the problematic address while preserving other conflict entries.
It is particularly useful in environments where IP addresses are scarce, such as in a /24 subnet with many clients, or when a specific address is needed for a critical device like a printer or server that was previously flagged. The command operates at the global configuration level, meaning it does not affect the running configuration permanently—it only clears the conflict from the dynamic table. The change is immediate and does not require a save to startup-config, though the conflict may reappear if the underlying issue (e.g., a rogue DHCP server or static IP conflict) is not resolved.
The command is available in all IOS versions, including IOS-XE, and requires privilege level 15 (enable mode) to execute. It is important to note that the command does not generate any buffered output; it simply returns to the prompt without confirmation. The DHCP conflict table is stored in RAM and is not part of the running configuration, so a reload will clear all conflicts.
This command is often used in conjunction with `show ip dhcp conflict` to identify the conflicting address and then selectively remove it. Alternatives include using `clear ip dhcp conflict *` to remove all conflicts, which is less precise, or adjusting the DHCP conflict detection mechanism (e.g., `no ip dhcp conflict detection`) to disable conflict checking entirely—though that is risky. The command fits into a broader troubleshooting workflow: first, identify the conflicting address via `show ip dhcp conflict`, then investigate the cause (e.g., static IP on a client, duplicate DHCP server), resolve the root issue, and finally clear the conflict to allow the DHCP server to reassign the address.
This command is a targeted tool for surgical removal of a single conflict entry, ensuring minimal disruption to the DHCP pool.
no ip dhcp conflict address [ip]When to Use This Command
- After resolving a duplicate IP address issue, clear the conflict entry so the address can be reused.
- When a statically assigned device is moved or reconfigured, remove its IP from the conflict table.
- During network troubleshooting, clear a conflict entry to test if the address is now available.
- After fixing a misconfigured device that caused a conflict, remove the conflict record to restore normal DHCP operation.
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| ip | A.B.C.D | Specifies the IP address to be removed from the DHCP conflict table. Must be a valid IPv4 address in dotted-decimal notation (e.g., 192.168.1.10). Common mistakes include using an address that is not in the conflict table (the command will silently do nothing) or mistyping the address. The address must exactly match an entry in the conflict table; partial matches are not supported. |
Command Examples
Clear a single IP address conflict
no ip dhcp conflict address 192.168.1.100This command removes the IP address 192.168.1.100 from the DHCP conflict table. No output is displayed if successful. Use 'show ip dhcp conflict' to verify the entry is gone.
Clear all DHCP conflicts
clear ip dhcp conflict *This command clears all entries from the DHCP conflict table. No output is displayed. Use 'show ip dhcp conflict' to confirm the table is empty.
Understanding the Output
The 'no ip dhcp conflict address' command does not produce any output on success. To verify the result, use 'show ip dhcp conflict' which displays a table with columns: 'IP address' (the conflicting IP), 'Detection method' (how the conflict was detected, e.g., Ping or Gratuitous ARP), 'Detection time' (timestamp of detection), and 'VLAN' (VLAN ID if applicable). A clean conflict table indicates no conflicts; any remaining entries need investigation.
Configuration Scenarios
Clearing a single DHCP conflict after resolving a duplicate IP issue
A network administrator discovers that a printer with IP 192.168.1.50 is not receiving a DHCP lease because the address is in the DHCP conflict table. The conflict was caused by a temporary static IP assignment during maintenance. After removing the static configuration, the administrator needs to clear the conflict to allow the DHCP server to lease the address to the printer.
Topology
DHCP Server (Gi0/0)---192.168.1.0/24---(Gi0/0) Switch---Printer (192.168.1.50)Steps
- 1.Step 1: Enter privileged EXEC mode: Router> enable
- 2.Step 2: Enter global configuration mode: Router# configure terminal
- 3.Step 3: Clear the specific DHCP conflict: Router(config)# no ip dhcp conflict address 192.168.1.50
- 4.Step 4: Exit configuration mode: Router(config)# end
- 5.Step 5: Verify the conflict is removed: Router# show ip dhcp conflict | include 192.168.1.50
! Enter global configuration mode Router# configure terminal ! Clear the conflict for 192.168.1.50 Router(config)# no ip dhcp conflict address 192.168.1.50 ! Return to privileged EXEC Router(config)# end
Verify: Use `show ip dhcp conflict` to verify the address is no longer listed. Expected output: no line containing 192.168.1.50. Alternatively, check the DHCP bindings: `show ip dhcp binding | include 192.168.1.50` should show the address as active if the client has obtained a lease.
Watch out: If the conflict was caused by a rogue DHCP server, clearing the conflict alone will not prevent the address from being flagged again. The root cause must be addressed first.
Clearing a conflict to free up an address for a critical server
A file server with a reserved DHCP reservation (192.168.10.100) is unable to obtain its lease because the address is in the conflict table due to a previous misconfiguration. The administrator needs to clear the conflict immediately to restore server connectivity without waiting for the conflict timer to expire.
Topology
DHCP Server (Gi0/0)---192.168.10.0/24---(Gi0/0) Access Switch---File Server (192.168.10.100)Steps
- 1.Step 1: Enter privileged EXEC mode: Router> enable
- 2.Step 2: Enter global configuration mode: Router# configure terminal
- 3.Step 3: Clear the specific DHCP conflict: Router(config)# no ip dhcp conflict address 192.168.10.100
- 4.Step 4: Exit configuration mode: Router(config)# end
- 5.Step 5: Force the DHCP client to renew (on the server side): Router# clear ip dhcp binding * (or specific binding if known)
! Enter global configuration mode Router# configure terminal ! Clear the conflict for 192.168.10.100 Router(config)# no ip dhcp conflict address 192.168.10.100 ! Return to privileged EXEC Router(config)# end ! Optionally clear the DHCP binding to force renewal Router# clear ip dhcp binding 192.168.10.100
Verify: Use `show ip dhcp conflict` to confirm the address is gone. Then check `show ip dhcp binding` to see if the server has obtained the lease. Expected output: the binding should show the address with a lease time.
Watch out: Clearing the conflict does not automatically trigger the DHCP client to renew. The client must send a DHCPREQUEST, which may require rebooting the client or releasing/renewing the IP. On Cisco IOS DHCP server, you can clear the binding to force the server to respond to new requests.
Troubleshooting with This Command
When troubleshooting DHCP conflicts, the `no ip dhcp conflict address` command is used after identifying a specific conflicting address. The first step is to display the DHCP conflict table using `show ip dhcp conflict`. Healthy output shows no entries or only entries that are expected (e.g., due to known static IPs).
Problem indicators include multiple entries for the same subnet, especially for addresses that should be dynamically assigned. Focus on the 'Detection time' and 'Detection method' fields: a detection method of 'ping' or 'arp' indicates the server actively discovered the conflict. If the conflict table grows rapidly, it may indicate a rogue DHCP server or a device with a static IP overlapping the DHCP pool.
The diagnostic flow: 1) Run `show ip dhcp conflict` to list all conflicts. 2) Identify the specific address causing issues (e.g., a client unable to get an IP). 3) Investigate the cause: check for static IPs on hosts, rogue DHCP servers, or misconfigured reservations. Use `show ip dhcp binding` to see current leases and `show ip dhcp pool` to review pool configuration. 4) Resolve the root cause (e.g., remove static IP, shut down rogue server). 5) Clear the specific conflict with `no ip dhcp conflict address <ip>`. 6) Verify the address is removed from the conflict table. 7) Monitor the client to ensure it obtains a lease. Correlate with `debug ip dhcp server events` to see real-time DHCP transactions.
If the conflict reappears, the root cause is still present. Note that the command does not produce output; success is indicated by the absence of the address in subsequent `show` commands. Common symptoms this command helps diagnose include: clients receiving APIPA addresses (169.254.x.x), DHCPNAK messages, or duplicate IP warnings.
The command is a surgical tool; for bulk clearing, use `clear ip dhcp conflict *`.
CCNA Exam Tips
CCNA exam expects you to know that DHCP conflicts are detected via ping or gratuitous ARP, and that clearing conflicts is done with 'clear ip dhcp conflict *' or 'no ip dhcp conflict address <ip>'.
Remember that 'no ip dhcp conflict address' is a global config command, not an exec command. You must be in global configuration mode.
The exam may test that after clearing a conflict, the DHCP server can reassign the address immediately if it is still in the pool and not excluded.
Be aware that 'clear ip dhcp conflict *' is an exec mode command, while 'no ip dhcp conflict address' is a global config command. Know the difference.
Common Mistakes
Mistake: Using 'no ip dhcp conflict address' in privileged exec mode instead of global config mode. Consequence: Command not recognized.
Mistake: Forgetting to verify with 'show ip dhcp conflict' after clearing. Consequence: Unaware that conflict still exists.
Mistake: Using 'clear ip dhcp conflict *' in global config mode. Consequence: Command not recognized; must be in exec mode.
no ip dhcp conflict address [ip] vs ip dhcp excluded-address [start] [end]
Both commands influence which IP addresses the DHCP server can assign, but they operate differently: one dynamically resolves conflicts while the other statically reserves addresses. They are commonly confused because both can prevent an address from being leased, yet they serve distinct purposes in conflict management versus static assignment.
| Aspect | no ip dhcp conflict address [ip] | ip dhcp excluded-address [start] [end] |
|---|---|---|
| Scope | Single IP address | Range of addresses (start/end) |
| Effect | Clears address from conflict table, allowing reassignment | Prevents assignment to any client |
| Persistence | Not saved to config; cleared on reload when conflict table is rebuilt | Saved to running-config and startup-config |
| Precedence | Overrides exclusion? No; exclusion still blocks assignment if address is excluded | Takes priority over conflict resolution; excluded addresses cannot be leased |
| Typical Use | After resolving a duplicate-address issue to free an address | To reserve addresses for static devices like servers, routers, or printers |
Use no ip dhcp conflict address [ip] when you have confirmed an IP conflict is resolved and want to return that address to the DHCP pool for reassignment.
Use ip dhcp excluded-address [start] [end] when you need to ensure specific addresses are never automatically assigned by DHCP, typically for devices requiring static IP configuration.
Platform Notes
In IOS-XE (e.g., Catalyst 9000 switches), the syntax and behavior are identical to classic IOS. The command is `no ip dhcp conflict address <ip>` in global config mode. Output from `show ip dhcp conflict` is similar, though some platforms may include additional fields like 'VLAN' or 'Interface'.
In NX-OS (Cisco Nexus switches), the DHCP conflict table is managed differently. The equivalent command is `clear ip dhcp conflict <ip>` in privileged EXEC mode (not global config). For example: `switch# clear ip dhcp conflict 192.168.1.50`.
NX-OS also uses `show ip dhcp conflict` to display conflicts. There is no ASA equivalent; ASA devices do not run a DHCP server in the same sense (they can act as DHCP client or server for limited scenarios, but conflict detection is not a feature). In IOS-XR (Cisco ASR 9000, etc.), the DHCP server implementation is different; conflict detection is not typically used, and the command does not exist.
Instead, administrators manage DHCP pools via configuration and use `clear dhcp ipv4 binding` to clear bindings. For IOS versions: the command has been available since at least IOS 12.0 and remains unchanged in 15.x and 16.x. There are no syntax differences between versions.
The command does not affect the running configuration; it only clears the in-memory conflict table. After a reload, all conflicts are cleared automatically. On some platforms, the conflict table may be stored in NVRAM if the DHCP database agent is configured, but the `no ip dhcp conflict address` command still only clears the in-memory entry.
Related Commands
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 conflict
Displays IP address conflicts detected by the DHCP server, helping administrators identify and resolve duplicate IP assignments on the network.
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