IP Conflicts DHCP

IP address conflicts occur when two or more devices attempt to use the same IP address on the same network segment, causing network connectivity issues and packet delivery failures.

Understanding IP Conflicts

  • Static-to-Static Conflicts: Manual configuration errors where administrators assign duplicate static IPs
  • DHCP-to-Static Conflicts: DHCP server assigns an IP that’s already statically configured on another device
  • DHCP-to-DHCP Conflicts: Multiple DHCP servers assign the same IP, or DHCP database corruption occurs
  • Stale ARP Entries: Old MAC-to-IP mappings cause confusion even after IP reassignment

Key Point: The device that comes online first typically “wins” the IP address, while subsequent devices experience intermittent connectivity

DHCP Conflict Prevention Mechanisms

Mechanism Function Default Behavior
ICMP Echo (Ping) Tests IP before assignment 2 ping attempts, 1-second timeout
ARP Request Checks for existing MAC mapping Broadcasts “Who has this IP?”
Gratuitous ARP Client announces its new IP Sends unsolicited ARP reply
Conflict Detection Client monitors for duplicate responses Reports conflicts back to server
  • DHCP servers should ping-test addresses before assignment (enabled by default on most enterprise equipment)
  • Lease database maintains IP-to-MAC bindings to prevent immediate reassignment conflicts

Common Troubleshooting Scenarios

Scenario 1: Mixed Static/DHCP Environment

  • Problem: Server configured with static IP 192.168.1.100, DHCP pool includes 192.168.1.1-254
  • Solution: Exclude static ranges from DHCP scope using ip dhcp excluded-address commands
  • Best Practice: Reserve lower addresses (1-50) for infrastructure, higher addresses (51-254) for DHCP

Scenario 2: Multiple DHCP Servers

  • Problem: Rogue DHCP server or misconfigured secondary server
  • Detection: Clients receive unexpected gateway/DNS configurations
  • Solution: Enable DHCP snooping on switches to authorize legitimate DHCP servers only

Scenario 3: Lease Database Corruption

  • Problem: DHCP server assigns already-active IPs after database reset
  • Symptoms: Sudden spike in IP conflicts across multiple clients
  • Solution: Clear DHCP bindings, restart service, allow natural lease renewal

Diagnostic Commands and Tools

Windows Clients:

  • ipconfig /all - View current IP configuration and DHCP server
  • ipconfig /release - Release current DHCP lease
  • ipconfig /renew - Request new IP from DHCP server
  • arp -a - Display ARP table for duplicate MAC entries

Cisco IOS DHCP Server:

  • show ip dhcp binding - Display active lease assignments
  • show ip dhcp conflict - Show detected IP conflicts
  • clear ip dhcp conflict * - Clear conflict database
  • debug ip dhcp server packet - Monitor DHCP packet exchanges

Vocabulary

  • ARP (Address Resolution Protocol): Layer 2 protocol that maps IP addresses to MAC addresses within broadcast domains
  • Gratuitous ARP: Unsolicited ARP reply announcing IP-to-MAC binding, used for duplicate IP detection
  • DHCP Snooping: Switch security feature that validates DHCP messages and maintains binding database
  • Lease Time: Duration a DHCP client can use assigned IP before renewal (typically 24 hours to 8 days)
  • DHCP Reservation: Permanent IP assignment based on client MAC address
  • Exclusion Range: IP addresses within DHCP scope that server will never assign

Notes

  • Always exclude statically assigned IP ranges from DHCP scopes - this prevents 80% of common conflicts
  • DHCP conflict detection adds ~2-3 seconds to lease assignment but significantly reduces network issues
  • In enterprise environments, use DHCP reservations instead of static IPs for servers requiring consistent addresses
  • Rogue DHCP servers are often introduced by users connecting personal routers - implement DHCP snooping as standard practice
  • Windows behavior: When conflict detected, client auto-configures APIPA address (169.254.x.x) and continues checking every 5 minutes
  • Cisco routers can act as DHCP servers but lack advanced conflict resolution features found in Windows Server or ISC DHCP
  • Lease renewal occurs at 50% of lease time - clients attempt to renew with original DHCP server first, then broadcast renewal at 87.5% of lease time