A DHCP address conflict occurs when an address in a server’s leasable range is already configured or used by another host. This can happen when a manually assigned address was not excluded with ip dhcp excluded-address. section 4.1.2
Cisco IOS detects and handles a conflict before completing the lease:
- After receiving DHCPDISCOVER, the server selects an address from the DHCP pool. section 4.1.2
- Before sending DHCPOFFER, the server pings the selected address. section 4.1.2
- A reply indicates that another host is using the address; IOS marks it conflicted and removes it from the pool. section 4.1.2 section 4.4
- The server selects another address and repeats the uniqueness check. section 4.1.2
Detecting and verifying 10.0.0.8
PC3 requests an address, and R1 selects 10.0.0.8. R1 pings it before sending an offer; SRV1 replies because it already uses 10.0.0.8. IOS reports "%DHCPD-4-PING_CONFLICT: DHCP address conflict: server pinged 10.0.0.8." Use show ip dhcp conflict to verify the entry, which displays 10.0.0.8 with detection method Ping.
A conflicted address remains unavailable until the underlying conflict is resolved and the entry is cleared. Use clear ip dhcp conflict {* | ip-address} to clear every conflict or a specified address. section 4.4
