Configuring Strict ARP Learning

Context

If many user hosts simultaneously send a large number of ARP packets to a device, or attackers send bogus ARP packets to the device, the following problems occur:

Processing ARP packets consumes considerable CPU resources. The device learns many invalid ARP entries, which exhaust ARP entry resources and prevent the device from learning ARP entries for ARP packets from authorized users. Consequently, communication of authorized users is interrupted.
After receiving bogus ARP packets, the device incorrectly modifies the ARP entries. As a result, authorized users cannot communicate with each other.

To avoid the preceding problems, configure the strict ARP learning function on the gateway. This function allows the gateway to learn only ARP entries for ARP Reply packets in response to ARP Request packets that it has sent. In this way, the gateway can prevent most ARP attacks.

Strict ARP learning can be configured globally or in the interface view.

If strict ARP learning is enabled globally, all interfaces on the device learn ARP entries strictly.
If strict ARP learning is enabled in the interface view, only this interface learns ARP entries strictly.

When strict ARP learning is enabled globally and in the interface view simultaneously, the configuration on the interface takes precedence over the global configuration.

When strict ARP learning is enabled globally:

If you run the arp learning strict force-disable command on a specified interface, strict ARP learning is administratively disabled on the interface.
If you run the arp learning strict trust command on a specified interface, strict ARP learning configured globally takes effect on the interface.

The firewall installed on a PC may prevent the PC from sending ARP Reply packets after the PC receives ARP Request packets, or the network adapter cannot reply with ARP Reply packets. If a device with strict ARP learning enabled triggers an ARP Miss message, the PC does not respond. As a result, the device cannot learn the PC's ARP entry. If this problem occurs on only a few user hosts, configure static ARP entries (for details, see Configuring Static ARP in "ARP Configuration" in the S2720, S5700, and S6700 V200R019C10 Configuration Guide - IP Service) for the user hosts; if the problem occurs on most user hosts, disable strict ARP learning on the device.

Procedure

Configuring strict ARP learning globally
1. Run system-view
  The system view is displayed.
2. Run arp learning strict
  Strict ARP learning is enabled globally.
  
  By default, strict ARP learning is disabled.
Configuring strict ARP learning on an interface
1. Run system-view
  The system view is displayed.
2. Run interface interface-type interface-number
  The interface view is displayed.
3. (Optional) On an Ethernet interface, run undo portswitch
  The interface is switched to Layer 3 mode.
  
  By default, an Ethernet interface works in Layer 2 mode.
  
  Only the S5720-EI, S5720-HI, S5730-HI, S5731-H, S5731-S, S5731S-H, S5731S-S, S5732-H, S6720-EI, S6720-HI, S6720S-EI, S6730-H, S6730S-H, S6730-S, and S6730S-S support switching between Layer 2 and Layer 3 modes.
4. Run arp learning strict { force-enable | force-disable | trust }
  Strict ARP learning on the interface is enabled.
  
  By default, strict ARP learning is disabled on the interface.