Configuring Standby Interfaces

You can configure multipathing groups with standby interfaces. As the name implies, the interface is considered a standby interface. The standby interface is not used unless some other interface in the group fails. A standby interface has an IFF_INACTIVE flag when the interface is not hosting any failover IP address. Consequently, when an active interface fails, the standby interface is always chosen for failover. After the standby interface is chosen, the IFF_INACTIVE flag is cleared on the standby interface. From that instant, the active standby interface is treated the same as other active interfaces. Some failures might not choose a standby interface. Instead, some failures might choose an active interface that hosts fewer IP addresses than the standby interface.

The standby interface is not used to send normal data packets. Consequently, limited traffic flows on a standby interface. You must configure standby interfaces with a test address to ensure that probes are sent to determine if the interface is functional. If you do not configure standby interfaces with a test address, the interface is not chosen for failovers when another interface in the group fails. A standby interface might carry traffic under the following conditions:

• If another host on the network communicates with a host by using the standby interface address, the standby interface is subsequently used for incoming packets.

• Applications can bind to an address by using bind or IP_ADD_MEMBERSHIP. Applications that bind to the address that is hosted on the standby interface might continue to generate traffic by using the standby interface.

Thus, the system does not normally select a standby interface, except for probes, unless the standby interface is explicitly chosen by an application. If some interface in the group fails, all network access is switched to the standby interface. To configure a standby interface, you use the ifconfig command's new standby parameter by using the following syntax:

# ifconfig interface-name standby group group-name

How to Configure a Multipathing Group With One of the Interfaces a Standby Interface shows the steps that you perform.

The in.mpathd daemon sends probes on the standby interface after a test address is configured on the standby interface. You should configure only test addresses on a standby interface. If any other address is added on the standby, the addition of this address fails. If a standby interface already has addresses other than test addresses, automatic failover of these addresses occurs to a different interface in the group. Only the test address remains, if a test address exists. Avoid configuring non-test address on a standby interface.

You need to mark the address as a test address by using the ifconfig command's deprecated and -failover options before setting standby or setting up.

To configure a test address on a standby interface, use the following syntax:

# ifconfig interface-name plumb ip-address
   <other-parameters> deprecated -failover standby up

For <other-parameters>, use the parameters that are required by your configuration. See the ifconfig(1M) man page for descriptions.

Standby interfaces are not used for failover if no test address is configured on that interface.

For example, if you want to create a test address with the following configuration:

• Physical interface hme2 as a standby interface

• Address of 19.16.85.22

• With deprecated and -failover set

• Sets the netmask and broadcast address to the default value

You type the following command line:

# ifconfig hme2 plumb 19.16.85.22 netmask + broadcast + deprecated -failover standby up

The interface is marked as a standby interface only after the address is marked as a NOFAILOVER address.

How to Configure a Multipathing Group With One of the Interfaces a Standby Interface shows the steps that you perform.

You can clear a standby interface by using the following syntax:

# ifconfig interface-name -standby