Show IP DHCP Conflict Command on CISCO Router/Switch

Command

Show IP DHCP Conflict

Use

This command shows information about IP conflicts that are detected during the DHCP negotiation. IP conflicts occur when hosts have statically assigned IP addresses that are within the DHCP configured range, but are not excluded.

Syntax

Router#show ip dhcp conflict

Example

Below is an example of show ip dhcp conflict. First, we will configure R2 to provide DHCP services.

R2(config)#ip dhcp pool VLAN
R2(dhcp-config)#dns-server 4.2.2.2
R2(dhcp-config)#network 10.1.1.0 /24

Now we will change R2’s Fa0/0 IP address to be the first address in the range.

R2(config)#int fa0/0
R2(config-if)#ip add 10.1.1.1 255.255.255.0

We then configure R1’s Fa0/0 interface to receive its address via DHCP. Notice how R1 reaches 10.1.1.2 as its address.

R1(config)#int fa0/0
R1(config-if)#ip add dhcp
R1(config-if)#shut
R1(config-if)#no shut
*Mar 1 01:54:31.603: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.2 (FastEthernet0/0) is down: interface down
R1(config-if)#
*Mar 1 01:54:41.947: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.1 (FastEthernet0/0) is up: new adjacency
R1(config-if)#d
*Mar 1 01:54:42.947: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 10.1.1.2, mask 255.255.255.0, hostname R1

*Mar 1 01:39:42.143: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.2 (FastEthernet0/0) is up: new adjacency

When we check R2, we can see that there was a conflict when R2 tried to offer 10.1.1.1 as an address.

R2(config-if)#do show ip dhcp conflict
IP address Detection method Detection time VRF
10.1.1.1 Gratuitous ARP Mar 01 2002 01:54 AM

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