Trigger a Gratuitous ARP (GARP) from a Palo Alto Networks Device

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Created On 09/26/18 13:51 PM - Last Modified 09/09/20 18:45 PM

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Deployment

Resolution

Overview

Gratuitous ARP (GARP) is used to update an ARP table of the hosts in a Broadcast Domain when the sender's IP address or MAC address has changed. Other usages of GARP include detecting IP conflicts and during HA fail-overs.

Details

In situations where there is no control over the upstream devices, the a command from the CLI can be used to update the ARP table of connected Layer3 devices that are unable to auto-update their ARP table. The following command will send gratuitous ARP for an IP address from a specific interface:

> test arp gratuitous ip <ip/netmask> interface <interface name>

Example

The following is an example of an interface details listing:

> show interface ethernet1/3

---------------------------------------------------------------------------

Name: ethernet1/3, ID: 18

Link status:

Runtime link speed/duplex/state: 1000/full/up

Configured link speed/duplex/state: auto/auto/auto

MAC address:

Port MAC address 00:1b:17:09:21:12

Operation mode: layer3

Untagged sub-interface support: no

---------------------------------------------------------------------------

Name: ethernet1/3, ID: 18

Operation mode: layer3

Virtual router default

Interface MTU 1500

Interface IP address: 10.66.24.67/23

To update the ARP table of connected hosts for the ethernet1/3 IP Subnet (10.66.24.67/23 with MAC address of 00:1b:17:09:21:12), run the following CLI command:

> test arp gratuitous ip 10.66.24.67 interface ethernet1/3

1 ARPs were sent

PCAP details:

Ethernet II, Src: PaloAlto_09:21:12 (00:1b:17:09:21:12), Dst: Broadcast (ff:ff:ff:ff:ff:ff)

Destination: Broadcast (ff:ff:ff:ff:ff:ff)

Source: PaloAlto_09:21:12 (00:1b:17:09:21:12)

Type: ARP (0x0806)

Address Resolution Protocol (request/gratuitous ARP)

Hardware type: Ethernet (1)

Protocol type: IP (0x0800)

Hardware size: 6

Protocol size: 4

Opcode: request (1)

[Is gratuitous: True]

Sender MAC address: PaloAlto_09:21:12 (00:1b:17:09:21:12)

Sender IP address: 10.66.24.67 (10.66.24.67)

Target MAC address: Broadcast (ff:ff:ff:ff:ff:ff)

Target IP address: 10.66.24.67 (10.66.24.67)

NAT Scenario Example

Any IP address in subnet 10.66.24.67/23 can be used with the test arp gratuitous command to forcefully refresh the IP-MAC address mapping on connected Layer-3 devices. An example scenario for the use of the command is for an inbound NAT configuration on a Palo Alto Networks firewall.

The following is the destination NAT rule configured to translate traffic for IP 10.66.24.139, received on interface ethernet1/3, to an internal IP of 192.168.67.5:

> show running nat-policy

Inbound-NAT {

from untrust-L3;

source any;

to untrust-L3;

to-interface ;

destination 10.66.24.139;

service any/any/any;

translate-to "dst: 192.168.67.5";

}

To update the ARP table of connected hosts with IP address 10.66.24.139 (used in above NAT rule) with the ethernet1/3 MAC address 00:1b:17:09:21:12, run the CLI command as follows:

> test arp gratuitous ip 10.66.24.139 interface ethernet1/3

ARPs were sent