Gratuitous ARP(GARP)

Overview

GARP (Gratuitous ARP) is an ARP message sent outside of the normal ARP operation of resolving MAC addresses from IP addresses. This section explains the purpose of GARP and how it works.

What is Gratuitous ARP (GARP)?

Gratuitous ARP (GARP) is an ARP message sent outside of the normal ARP process. GARP is used primarily for two purposes

• Duplicate IP address detection
• Update ARP cache and MAC address table

In the following sections, the GARP mechanism for these two purposes will be explained.

Related articles

For the normal ARP process of resolving MAC addresses from IP addresses, see the following article.

N-Study

ARP(Address Resolution Protocol) | N-Study

https://www.n-study.com/en/tcp-ip/arp/

ARP (Address Resolution Protocol) is the protocol for associating an IP address with a MAC address. The mapping of IP addresses to MAC addresses is called address resolution, hence the name of the protocol, ARP.

Duplicate IP address detection

IP addresses must not be duplicated. When assigning a new IP address to an interface, GARP can be used to detect duplicate IP addresses.

An ARP request is sent with the IP address configured on the interface set as the Target IP address. This is a Gratuitous ARP that is sent at a different timing from the normal ARP process. When an ARP reply to this ARP request is returned, it indicates that the IP address is already in use.

Update ARP cache and MAC address table

GARP can be used to promptly update the ARP cache of other devices when IP addresses are changed. Broadcast ARP reply so that the ARP caches of other devices can be updated promptly.

GARP is also sent to update the MAC address table of the intervening Layer 2 switch when switching active/master in HSRP or VRRP. The newly active/master router broadcasts an ARP reply. Its source MAC address is a virtual MAC address, and the virtual MAC address entry is updated in the Layer 2 switch’s MAC address table.

Summary