Configuration Guide Configuring ARP
2.3.3 Gratuitous ARP
Working Principle
Gratuitous ARP packets are a special type of ARP packets. In a gratuitous ARP packet, the source and destination IP
addresses are the IP address of the local device. Gratuitous ARP packets have two purposes:
5. IP address conflict detection. If the device receives a gratuitous packet and finds the IP address in the packet the
same as its own IP address, it sends an ARP reply to notify the peer end of the IP address conflict.
6. ARP update. When the MAC address of an interface changes, the device sends a gratuitous ARP packet to notify
other devices to update ARP entries.
The device can learn gratuitous ARP packets. After receiving a gratuitous ARP packet, the device checks whether the
corresponding dynamic ARP entry exists. If yes, the device updates the ARP entry based on the information carried in the
gratuitous ARP packet.
Related Configuration
Enabling Gratuitous ARP
Run the arp gratuitous-send interval seconds [number] command in interface configuration mode to enable gratuitous
ARP. This function is disabled on interfaces by default. Generally you need to enable this function on the gateway
interface to periodically update the MAC address of the gateway on the downlink devices, which prevents others from
faking the gateway.
2.3.4 Proxy ARP
Working Principle
The device enabled with Proxy ARP can help a host without any route information to obtain MAC addresses of IP users in
other subnets. For example, if the device receiving an ARP request finds the source IP address in a different network
segment from the destination IP address and knows the route to the destination address, the device sends an ARP reply
containing its own Ethernet MAC address. This is how Proxy ARP works.
Related Configuration
Enabling Proxy ARP
Run the ip proxy-arp command in interface configuration mode to enable Proxy ARP.
By default, this function is enabled .
2.3.5 ARP Trustworthiness Detection
Working Principle
The arp trust-monitor enable command is used to enable anti-ARP spoofing to prevent excessive useless ARP entries
from occupying device resources. After ARP trustworthiness detection is enabled on a Layer-3 interface, the device
receives ARP request packets from this interface:
1. If the corresponding entry does not exist, the device creates a dynamic ARP entry and performs NUD after 1 to 5
seconds. That is, the device begins to age the newly learned ARP entry and sends a unicast ARP request. If the
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