A Virtual IP address is an IP address that is bound to the Virtual NIC (VNIC) and is a purely virtual card that has no real physical counterpart. Conceptually, a Virtual NIC can be thought of as a loopback interface with an added external visibility. Similarly, Virtual IP addresses can be thought of as loopback address with the 127.0.0.0 IP network constraint removed. Virtual IP addresses are different from the regular primary and secondary IP addresses in a very subtle way. They are not bound to a physical IP network on the wire. Therefore they are assigned from an exclusive IP network number, different from all IP network numbers that are currently assigned to physical LANs.
Virtual IP addresses can be used in two different High Availability configurations:
Redundant LAN configuration: In such a topology, a server hosting a mission critical application is configured to reside on two or more different IP networks/subnets. All critical services are then configured on the Virtual IP address. The server is also made to run a routing protocol that advertises the reachability information to the Virtual NIC address to the other machines on the network.
Hosting of critical applications on the Virtual NIC address removes the dependency of client applications to a specific network attachment on the server. As a result, these critical services are uniformly accessible even if of one of the LAN attachments on the server fails.
The failover of the connections to the active LAN segments is ensured by the routing protocol, and the failover time is dependent on specific routing protocol used. This is typically of the order of 180s in case of RIP.
Virtual Server farms: Virtual Server farms configured behind an L4 redirector: In such a topology, the individual servers in the farm share a single Virtual IP address. Typically, a non-ARPable secondary IP address (See Section 2.11, Non-ARPable Secondary IP Address) is used to configure such a Virtual IP address on each of the servers in the farm. Alternatively, even Virtual NIC can be configured to host such a Virtual IP address.
One of the serious limitations of Virtual IP addresses is that each one of them consumes an additional IP network address space. This is a serious constraint for a fixed-length subnetted environment where a whole chunk of a subnet address space allocated to the Virtual NIC address space remains unused. However, for environments that use either Private address space or that use Variable Length Subnetting, this should not be a problem. Virtual IP addresses are supported only on NetWare 6.5 and are not available in earlier versions. Also, the current release of NetWare 6.5 supports OSPF, RIP-I and RIP-II as routing protocols for advertising reachability information to the Virtual IP networks.
For more information, see Section 5.16, Virtual IP Address.