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Networking Fundamentals · Intermediate

Gratuitous ARP (GARP)

An unsolicited ARP message in which a host announces its own IP-to-MAC mapping.

Definition

Hosts send gratuitous ARPs at startup to detect address conflicts and to update other devices' caches after a MAC change. Redundancy protocols such as HSRP and VRRP use them during failover. Attackers abuse unsolicited ARPs in ARP spoofing.

Example

After a firewall cluster fails over, the new active unit sends gratuitous ARPs for the shared IP.

Exam tip

GARP is legitimate for failover and conflict detection but is also the mechanism behind ARP poisoning.

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