Configuring the Subnet ARP Broadcast in Power Virtual Server subnets

IBM Power Virtual Server in IBM data center

The Address Resolution Protocol (ARP) is a network protocol that is used to map IP addresses to physical Media Access Control (MAC) addresses in a network. The IP address mapping must be established to enable the communication between endpoints in subnets or in a local network.

The Subnet ARP Broadcast is a switch that is provided on a subnet page of a Power Virtual Server workspace. You can set Subnet ARP Broadcast to Enabled or Disabled. When you set Subnet ARP Broadcast to Enabled, the ARP request is broadcast across the subnet. You can use the broadcast request for ARP to distribute the ARP traffic in the Power Virtual Server network fabric similar to how ARP is managed in conventional networks. A Gratuitous ARP (GARP) request can update the host ARP caches when an IP address has a different MAC address. To accomplish GARP requests, you must broadcast an ARP request across the subnet. For example, when Subnet ARP Broadcast is used with cluster software, the network traffic is routed to the active cluster node.

The Subnet ARP Broadcast option is not available if you are using Cloud Connections. For more information, see IBM Power Virtual Server Cloud Connections.

Benefits of the Subnet ARP Broadcast in Power Virtual Server subnets

To get the benefits of the Subnet ARP Broadcast, you must set the option to Enabled for a subnet of a Power Virtual Server workspace on the Subnet page. The ARP request is broadcast across the subnet and the following changes in the subnet are determined:

Identify the change in the MAC address of a device that is mapped to an IP address
Identify a MAC address of an IP address that frequently moves from one MAC addresses to another
Identify the current IP address of a silent IP address in the subnet

In the Power Virtual Server network fabric, when an IP address is reassigned to a different host without notifying the network fabric is called as a silent IP address. When you set the Subnet ARP Broadcast to Enabled, the host with the silent IP address responds to the ARP request. The Power Virtual Server network fabric updates the MAC address of the silent IP address with the MAC address of the host that responded to the ARP request.

If the Subnet ARP Broadcast option is disabled, the Power Virtual Server network fabric continues to forward the ARP request to the MAC address that is saved in the network fabric. The network fabric continues to forward the ARP request until the silent IP address endpoint exists. Also, the network traffic flow is optimized as the ARP request is sent to the MAC address associated with the silent IP address. The IP address moves to the new host through GARP or other requests. The endpoint then notifies the network fabric about the change in the current host.

Enabling or disabling the Subnet ARP Broadcast

To enable the Subnet ARP Broadcast option when you create a new subnet, set Subnet ARP Broadcast to Enabled on the Create subnet page. By default, the Subnet ARP Broadcast option is set to Disabled. For more information, see Configuring a private network subnet. You can enable or disable the subnet ARP option by using the UI, CLI, or API interfaces.

To change the status of the Subnet ARP Broadcast option for an existing subnet on the Power Virtual Server page, complete the following steps:

Sign in to the IBM Cloud Portal.
Search for Power Virtual Server in the search box for IBM Cloud catalog.
Select Workspace from the left menu.
Select the workspace for which you want to edit the subnet details.
Select Subnets under Networking from the left menu.
Select the subnet for which you want to enable the Subnet ARP Broadcast option. The Subnet ARP Broadcast field is set as Enabled or Disabled.
Click Edit Subnet in the Subnet details section.
Set Subnet ARP Broadcast to Enabled or Disabled on the Edit Subnets pane.

To change the status of the Subnet ARP Broadcast option for an existing subnet, use the following API or CLI commands:

API: To enable or disable the broadcast of ARP when you create a subnet, use the Create a new Network API. To enable or disable the broadcast of ARP when you update a subnet, use the Update a Network API. Set the arpBroadcast parameter to enable or disable value. By default, the arpBroadcast parameter is set to disable value.
CLI: To enable or disable the ARP broadcast option when you create a subnet, use the command ibmcloud pi subnet create. To enable or disable the ARP broadcast option when you update a subnet, use the command ibmcloud pi subnet update. Set the arpBroadcast parameter to enable or disable value. By default, the arpBroadcast parameter is set to disable value.