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Chapter 28 Gratuitous ARP Configuration
28.1 Introduction to Gratuitous ARP
Gratuitous ARP is a kind of ARP request that is sent by the host with its IP address as the
destination of the ARP request.
The basic working mode for the switch is as below: The Layer 3 interfaces of the switch can be
configured to advertise gratuitous ARP packets period or the switch can be configured to
enable to send gratuitous ARP packets in all the interfaces globally.
The purpose of gratuitous ARP is as below:
1. To reduce the frequency that the host sends ARP request to the switch. The hosts in the
network will periodically send ARP requests to the gateway to update the MAC address of
the gateway. If the switch advertises gratuitous ARP requests, the host will not have to
send these requests. This will reduce the frequency the hosts’ sending ARP requests for
the gateway’s MAC address.
2. Gratuitous ARP is a method to prevent ARP cheating. The switch’s advertising gratuitous
ARP request will force the hosts to update its ARP table cache. Thus, forged ARP of
gateway cannot function.
28.2 Gratuitous ARP Configuration Task List
1. Enable gratuitous ARP and configure the interval to send gratuitous ARP request
2. Display configurations about gratuitous ARP
1. Enable gratuitous ARP and configure the interval to send gratuitous ARP request.
Command
Explanation
Global Configuration Mode and Interface Configuration Mode.
ip gratuitous-arp <5-1200>
no ip gratuitous-arp
To enable gratuitous ARP and configure the
interval to send gratuitous ARP request.
The no command cancels the gratuitous ARP.
2. Display configurations about gratuitous ARP
Command
Explanation
Admin Mode and Configuration Mode
show ip gratuitous-arp [interface vlan <1-
4094>]
To display configurations about gratuitous
ARP.