From the V5.5-2 VAXcluster SPD 29.78.06:
VAXcluster systems are configured by connecting multiple CPUs with a
communication media, referred to as an interconnect. VAXcluster nodes
communicate with each other using the most appropriate interconnect
available. Whenever possible, in the event of interconnect failure,
VAXcluster software will automatically use an alternate interconnect.
VAXcluster Software supports any combination of the following inter-
o Computer Interconnect (CI)
o Ethernet (NI)
o Digital Storage System Interconnect (DSSI)[*]
o Fiber Distributed Data Interface (FDDI)
Ethernet and FDDI are industry-standard general purpose communications
interconnects that can be used to implement a Local Area Network (LAN).
Except where noted, VAXcluster support for both of these LAN types is
The following configuration rules apply to VAXcluster systems:
o The maximum number of CPUs supported in a VAXcluster system is 96.
o Every VAXcluster node must have a direct communication path to ev-
ery other VAXcluster node via any of the supported interconnects.
o VAX 11/7xx, 6000, 8xxx and 9000-series CPUs require a system disk
that is accessed via a local controller or through a local CI or
DSSI connection. VAXcluster satellite booting is not supported for
o CPUs that use an FDDI for VAXcluster communications can concurrently
use it for other network protocols that conform to the applicable
FDDI standards, such as ANSI X3.139-1987, ANSI X3.148-1988, and ANSI
o All LAN bridges must provide a low-latency data path, with approx-
imately 10 megabits per second throughput for Ethernet and 100 megabits
per second throughput for FDDI. Translating bridges must be used
when connecting VAXcluster nodes on an Ethernet to those on an FDDI.
o The maximum number of VAXcluster members that can be directly con-
nected to the FDDI, via the DEC FDDIcontroller 400 (DEMFA), is 16.
o VAXcluster CPUs should be configured using interconnects that pro-
vide appropriate performance for the required system usage. In gen-
eral, use the highest performance interconnect possible. CI, DSSI,
and FDDI are the preferred interconnects between powerful VAX CPUs.