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Novell Business Continuity Clustering 1.1 for NetWare Administration Guide
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Physical IP network
: An IP network that a physical IP address is a part of. In practical terms, a
physical IP network identifies a logical IP network that is configured over a physical hardware
wire.
5.1.2 Characteristics
Virtual IP addresses are unique in that they are bound to a virtual “ether” medium instead of to a
“physical” network medium such as Ethernet or token ring. In other words, the virtual IP address
space is exclusive from the physical IP address space. As a result, virtual IP network numbers need
to be different from physical IP network numbers. However, this mutual exclusivity of the IP
address space for the physical and virtual networks doesn’t preclude the possibility of configuring
multiple virtual IP networks in a single network domain.
5.2 Virtual IP Address Benefits
In spite of their simplicity, virtual IP addresses offer two main advantages over their physical
counterparts:
“High Availability” on page 74
“Unlimited Mobility” on page 77
These advantages exist because virtual IP addresses are purely virtual and are not bound to a
physical network wire.
5.2.1 High Availability
If a virtual IP address is defined on a multihomed server with more than one physical NIC, a virtual
IP address is a highly reachable IP address on the server when compared to any of the physical IP
addresses. This is especially true in the event of server NIC failures. This assumes that the server 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.
Physical IP addresses might not be reachable because:
TCP/IP protocols use link-based (network-based) addressing to identify network nodes. As a
result, the routing protocols preferentially deliver a packet to the server through the network
that the target IP address is part of.
Dynamic routing protocols are extremely resilient to intermediate link and router failures, but
they do not adapt well to failures of links at the last hop that ultimately delivers a packet to its
destination.
This is because the last hop link is typically a stub link that does not carry any routing
heartbeats. Therefore, if one of the physical cards in a server fails, the server as well as any
service that it hosts on the corresponding physical IP address can become inaccessible. This
could occur in spite of the fact that the server is still up and running and can be reached through
the other network card.
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 can be expected to always be 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.
