IBM AIX Continuous Availability Features - IBM Redbooks

2.4.1 Virtual IP address support (VIPA)
Prior to AIX V5.1, separate applications were required to provide high availability for a service
IP address and its associated interface. If the network interface failed, then the application’s
TPCP/IP session was often lost, resulting in the loss of application availability.
To overcome this, support for virtual IP addresses (VIPA) on both IPv4 and IPv6 was
introduced in AIX V5.1. VIPA allows the application to bind to a system-wide level virtual IP
address, as opposed to a single network interface. VIPA is a virtual device often utilizing
several network interfaces. VIPA can often mask underlying network interface failures by
re-routing automatically to a different one. This allows continued connectivity and is
transparent to the application and processes. VIPA also supports load balancing of traffic
across the available connections.
Another advantage of choosing a virtual device (as opposed to defining aliases to real
network interfaces) is that a virtual device can be brought up or down separately without
having any effect on the real interfaces of a system. Furthermore, it is not possible to change
the address of an alias (aliases can only be added and deleted), but the address of a virtual
interface can be changed at any time.
Since its initial introduction in AIX V5.1, VIPA has been enhanced to make it friendlier, from a
network administration perspective. It has also been enhanced so that failovers are
completed faster, thus further improving availability.
2.4.2 Multipath IP routing
Prior to AIX V5.1, a new route could be added to the routing table only if it was different from
the existing routes. The new route would have to be different by either destination, netmask,
or group ID.
Also, previous AIX releases did not provide any mechanism to associate a specific interface
with a route. When there were multiple interfaces on the same subnet, the same outgoing
interface for all destinations accessible through that network was always chosen.
In order to configure a system for network traffic load balancing, it is desirable to have
multiple routes so that the network subsystem routes network traffic to the same network
segment by using different interfaces.
With the new multipath routing feature in AIX V6.1, routes no longer need to have a different
destination, netmask, or group ID list. If there are several routes that equally qualify as a route
to a destination, AIX will use a cyclic multiplexing mechanism (round-robin) to choose
between them. The benefit of this feature is two-fold:
► It enables load balancing between two or more gateways.
► The feasibility of load balancing between two or more interfaces on the same network can
be realized. The administrator would simply add several routes to the local network, one
through each interface.
Multipath routing is often utilized together with dead gateway detection.
2.4.3 Dead gateway detection
The dead gateway detection (DGD) feature introduced in AIX V5.1 implements a mechanism
for hosts to detect a dysfunctional gateway, adjust its routing table accordingly, and reroute
Chapter 2. AIX continuous availability features 37