Fault Tolerance with Virtual IP Using Proxy Arp

Resolving The Problem

Beginning with V520, Virtual IP Addresses (VIPAs) can be configured with Proxy ARP. This enables the VIPA to reside on the same subnet as the Physical Interface and, more importantly, eliminates the need for RIP. Additional information on Virtual IP Addresses and Fault Tolerance can be found in the IBM eServer iSeries IP Networks: Dynamic! IBM Redbooks at the following Web site:

http://www.redbooks.ibm.com/pubs/pdfs/redbooks/sg246718.pdf

One use for virtual IP addresses is to provide route fault tolerance. This example shows several different ways a route can be recovered after an outage.

The most reliable connection is when a virtual IP address is defined on the system. With the support of virtual IP addresses, even if an interface fails, the session can still communicate using different interfaces.

What happens if router R1 fails?

o	Connections through R1 are rerouted through R2.
o	The failed gateway will detect R1 recovery, but active connections will continue to run through R2.

What happens if interface 192.168.1.1 fails?

o	Active connections to 192.168.1.1 are lost, but other connections to 192.168.1.2 and 192.168.1.3 remain.
o	Route rebinding: -- The primary virtual IP address of the system remains active. -- The system stays accessible as long as at least one physical interface remains active

Network Configuration

The network configuration to ensure fault tolerance consists of at least two physical network adapters on the iSeries system. Each of these physical adapters is connected to a separate link partner (hub or switch). Each link partner has a physical connection to each router to ensure fault tolerance. This cabling configuration prevents a single hardware failure from interrupting transmissions to or from the iSeries server.

iSeries Configuration

Both physical interfaces and a Virtual IP Address, also known as a Circuitless Interface, can be configured via green screen or through iSeries Navigator. Enabling Proxy Arp must be done through iSeries Navigator. For ease of configuration, we will create all interfaces using green screen, then use iSeries Navigator to enable Proxy Arp for the Virtual IP Address.

IP Address Configuration

For this scenario, two logical IP addresses and one virtual IP address must be configured. Each logical IP address is bound to a individual physical adapter. The virtual IP address is a circuitless logical interface and is not bound to any physical adapter. The virtual IP address must be on the same subnet as the physical interfaces.

Create interfaces using a character-based interface:

ADDTCPIFC INTNETADR('192.168.1.1') LIND(ETHLINE00) SUBNETMASK('255.255.255.0')
ADDTCPIFC INTNETADR('192.168.1.2') LIND(ETHLINE01) SUBNETMASK('255.255.255.0')
ADDTCPIFC INTNETADR('192.168.1.3') LIND(*VIRTUALIP) SUBNETMASK('255.255.255.255')

Enable Proxy Arp on VIPA using iSeries Navigator:
o Select Network -> TCP/IP Configuration -> IPv4 -> Interfaces.
o Right-click on the Virtual IP Address entry, and select Properties from the dropdown menu.
o Select the Advanced tab, and check the Enable Proxy Arp box.

Notes:
1. This interface must not be active when making this change.
2. If iSeries Navigator is not available to use, Proxy ARP can be enabled through the use of an API. For additional information, refer to Rochester Support Center knowledgebase document N1015049, Using API to Enable Proxy ARP: .






3. For information on using the Virtual IP address as the outbound address, you should refer to Rochester Support Center knowledgebase document N1015633, Use the *VIRTUALIP Interface as the Outbound Source IP Address: .