Technical data

Each node (as a member of the host configuration in the cluster) retains a separate
IP address. This is beneficial for troubleshooting the individual node because you
can
ping
the specific node to see whether it is running.
All of the TCP/IP services support automatic failover and can run on multiple
nodes in an OpenVMS Cluster. For example, if more than one node in the cluster
is running the NFS server, the cluster can appear to the NFS client as a single
host. For more information about configuring a specific service for cluster failover,
refer to the particular service in the Compaq TCP/IP Services for OpenVMS
Management guide.
3.2.2 Connection Load Balancing
Load balancing using the TCP/IP Services is defined by the load broker.Theload
broker is a configurable, calculated, load-balancing mechanism for distributing the
work load among DNS (Domain Name System, which maintains and distributes
information about Internet hosts) cluster members. For more information about
DNS, see Chapter 5.
Unlike round-robin scheduling (the default method used by most DNS name
servers, in which each individual node in the cluster is polled in a continual,
specific order), the load broker takes into account the load on all DNS cluster
participants. The load broker polls DNS cluster members and updates the metric
server accordingly.
When the load broker starts, it reads its configuration file and starts polling DNS
cluster members. The load broker exchanges messages with DNS cluster members
that run the metric server, which calculates the current load on a DNS cluster
host by using a specific equation. The metric server calculates the current rating
and reports it when polled by the load broker. Periodically, the load broker sorts
the list of addresses based on metric rating reports, drops the systems that do not
respond after being polled three times, and compares a subset of the list with
thenameserverinformation.
To do the comparison, the load broker sends a host lookup request to the specified
name server. If the lists are the same, the load broker does not make changes. If
the lists are different, the load broker updates the name server data by sending
a dynamic update request to the specified name server. The name server uses
round-robin scheduling to further balance the load across the members of a DNS
cluster. Thus, every consecutive request for translating the DNS cluster name
results in a returned list, is rotated by one.
For specific information about configuring the load broker, starting and stopping
the metric server, and troubleshooting, refer to the Compaq TCP/IP Services for
OpenVMS Management guide.
3.3 Multihoming and Multiple Interfaces
Although host computers can have several network interface cards (NICs)
installed, you can configure the host through a single, primary interface. This
section introduces the following concepts:
• Multihomed computers
• Primary interface
• Pseudointerfaces
3–4 OpenVMS Server and Network Configurations