Gratuitous ARP

Latest reply: Feb 24, 2019 11:40:01 195 1 0 0

Gratuitous ARP enables a host to send an ARP Request packet using its own IP address as the destination address. Gratuitous ARP provides the following functions:

  • Checks duplicate IP addresses: Normally, a host does not receive an ARP Reply packet after sending an ARP Request packet with the destination address being its own IP address. If the host receives an ARP Reply packet, another host has the same IP address.

  • Advertises a new MAC address. If the MAC address of a host changes because its network adapter is replaced, the host sends a gratuitous ARP packet to notify all hosts of the change before the ARP entry is aged out.

  • Notifies an active/standby switchover in a VRRP backup group: After an active/standby switchover, the master switchsends a gratuitous ARP packet in the VRRP backup group to notify the switchover.


  • x
  • convention:

MHME
Created Feb 24, 2019 11:40:01 Helpful(0) Helpful(0)

Gratuitous ARP Use Cases

Updating ARP Mapping

The first use case is pretty straight forward, a node can use a Gratuitous ARP to update the ARP mapping of the other hosts on the network should the node’s IP to MAC mapping change.

This might happen if a user manually modifies their MAC address – they retain the same IP address, but now have a new MAC address. Therefore, the ARP mapping for all the nodes which are communicating with this user must be updated.

That being said, manually changing the MAC address is pretty rare. However, you do sometimes see this in redundant Cloud or Virtual environments, where a particular Virtual Machine (VM) ‘jumps’ to a new physical box – the same VM’s IP address is now being served by a different physical machine.

Redundancy

Much more substantial is Gratuitous ARP’s use case in situations where redundancy or failover between two devices are used.

With redundancy, you typically have two scenarios: two devices sharing an IP address, but each having their own MAC address. Or, two devices sharing both an IP address and a MAC address.

In both of these cases, Gratuitous ARP is critically important to ensure the continued ability to communicate with the IP address as it shifts between the two redundant devices. Below are examples of each scenario and how Gratuitous ARP is employed.
  • x
  • convention:

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