IP Address Conflict Detection

Background

IP address conflicts result in route flapping and service interruptions. Devices are required to automatically detect IP address conflicts and record the cause of a conflict, which helps you troubleshoot such issues and minimizes the impact on services.

IP address conflict detection helps you quickly locate and modify conflicting IP addresses and instruct you in properly configuring and managing the IP addresses of devices.

Implementation

ARP IP address conflict detection can be classified as either active or passive detection.

• Active detection

  When the physical status of an interface on a device goes Up, the device actively sends gratuitous ARP packets to detect possible IP address conflicts. When detecting an IP address conflict, the device periodically broadcasts gratuitous ARP Reply packets until the conflict is removed.

• Passive detection

  After a device receives non-gratuitous ARP packets, it considers that IP address conflicts exist on the network and records logs if any of the following conditions are met:

  • The source IP address in an ARP packet is the same as the IP address of the inbound interface that receives the ARP packet, but the source MAC address in the ARP packet is different from the MAC address of the inbound interface.
  • The source IP address in an ARP packet is the same as the IP address in an existing ARP entry, but the source MAC address is different from the MAC address in the ARP entry.
  • The source IP address in an ARP packet is 0.0.0.0 (probe ARP packet), the destination IP address is the same as the IP address of the inbound interface that receives the ARP packet, but the source MAC address in the ARP packet is different from the MAC address of the inbound interface.
  

  In ARP entries of devices, multiple IP addresses can match the same MAC address.