In spite of their simplicity, virtual IP addresses offer the following advantages over their physical counterparts:
Improves availability. The virtual IP addresses are bound to virtual adapters instead of physical adapters. The host mask for the virtual adapter allows each cluster resource to have its own entry in the routing tables. The virtual IP addresses are resilient to physical interface failures because we know if a resource destination is reachable.
Improves mobility. The virtual IP addresses are not tied to any IP address range of a physical network segment. A service can be moved between physical segments without the need to change the resource IP address.
Simplifies name resolution. The association of cluster resource names to IP addresses can be maintained independently of the location of the cluster resource in the business continuity cluster.
These advantages exist because virtual IP addresses are purely virtual and are not bound to a physical network component. Each of these advantages is discussed in more detail below.
Each cluster node is running a routing protocol and is advertising its internal virtual IP network—which only it knows about and can reach—to other network nodes. The virtual IP addresses of the cluster resources are highly available because each resource has its own entry in the routing tables of the LAN routers. This allows you to know whether a destination is reachable. However, with secondary IP addresses, you know only whether there is a route to a segment.
The virtual IP address feature circumvents this problem by creating a virtual IP network different from any of the existing physical IP networks. As a result, any packet that is destined for the virtual IP address is forced to use a virtual link as its last hop link. Because it is purely virtual, this last hop link is always up. Also, because all other real links are forcibly made to act as intermediate links, their failures are easily worked around by the dynamic routing protocols.
Generally speaking, if a connection between two machines is established by using a virtual IP address as the end-point address at either end, the connection is resilient to physical adapter failures if the server has multiple adapters.
There are two important benefits that follow from the highly reachable nature of virtual IP addresses:
A multi-homed server with a virtual IP address no longer needs to carry multiple DNS entries for its name in the naming system.
If one of the subnets that a server interfaces to fails completely or is taken out of service for maintenance, the routing protocols can reroute the packets addressed to the virtual IP address through one of the other active subnets.
Unlike physical IP addresses which are limited in their mobility, virtual IP addresses are highly mobile. The degree of mobility is determined by the number of servers that an IP address on a specific server could be moved to. In other words, if you choose a physical IP address as an IP address of a network resource, you are limiting the set of potential servers to which this resource could be transparently failed-over.
If you choose a virtual IP address, the set of servers that the resource could be transparently moved to is potentially unlimited. This is because of the nature of virtual IP addresses; they are not bound to a physical wire and, as a result, carry their virtual network to wherever they are moved. There is an implicit assumption here that the location of a virtual IP address is advertised to the owning server through some routing protocol. The ability to move an IP address across different machines becomes particularly important when it is required to transparently move (or fail over) a network resource that is identified by an IP address (which could be a shared volume or a mission-critical service) to another server.
This unlimited mobility of virtual IP addresses is an advantage to network administrators, offering them more ease of manageability and greatly minimizing network reorganization overhead. For network administrators, shuffling services between different IP networks is the rule rather than the exception. The need often arises to move a machine hosting a particular service to some other IP network, or to move a service hosted on a particular machine to be rehosted on some other machine connected to a different IP network. If the service is hosted on a physical IP address, accommodating these changes involves rehosting the service on a different IP address pulled out from the new network and appropriately changing the DNS entry for the service to point to the new IP address. However, unless everyone accesses a service via its DNS name instead of its IP address, an IP address change can break the service for the IP address users. In contrast, if the service is hosted on a virtual IP address, the necessity of changing the DNS entries for the service is eliminated, and the service is not broken even for those who use the IP address instead of the DNS name.
In any network environment, one of the first obstacles is how clients locate and connect to the services. A business continuity cluster can exacerbate this problem because services can migrate to nodes on a completely different network segment. Although there are many potential solutions to this problem, such as DNS and SLP, none of them offers the simplicity and elegance of virtual IP addresses. With virtual IP addresses, the IP address of the service can follow the service from node to node in a single cluster, as well as from node to node in separate, distinct clusters. This makes the client reconnection problem trivial; the client just waits for the new route information to be propagated to the routers on the network. No manual steps are required, such as modifying a DNS server.