Novell BUSINESS CONTINUITY CLUSTERING 1.2.1 - ADMINISTRATION, Руководство администратора

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Using Virtual IP Addresses with BCC 1.2 169
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F.2.2 Unlimited Mobility
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.
F.2.3 Automatic Name Resolution
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.
F.3 Planning a Virtual IP Network Implementation
Consider the guidelines in this section when planning your virtual IP network implementation.
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Section F.3.1, “Routing Protocol,” on page 170
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Section F.3.2, “LAN Routers,” on page 170
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Section F.3.3, “Internal Router,” on page 170