IBM RS/6000 SP Handbook page 201

The N+1 feature of these switches allows for concurrent replacement of any
failed power supplies or cooling fans. The supervisor can also be replaced while
the switch is operating. PSSP level 3.2 software is required to use these
switches.
This section describes some of the most important hardware characteristics of
the SP Switch2 hardware.
TOD synchronization
The SP Switch2 is designed with multiple clock sources in the system. There is
no master clock, unlike HPS or SP Switch. Therefore, instead of having clock
selection logic, SP Switch2 switch has a separate oscillator for each switch chip.
The TOD (time of day) logic on the SP Switch2 has also been significantly
redesigned to simplify both coding and system requirements, as well as to
improve the accuracy of the TOD across the system.
J-TAG interface
An additional interface, J-TAG interface, is added to the SP Switch2. This new
interface will allow the supervisor to perform new functions such as writing
initialization data to the switch chips and reading error status information back
from the switch chip.
Adaptive routing
To support the double-port, a number of routing enhancements were added in
SP Switch2, namely adaptive routing and multicast packets. Adaptive routing will
allow the switch chip to determine which output port to route the packet to based
on the route nibble. Multicast packets give the switch the ability to replicate and
distribute packets to predefined groups of nodes.
When you order F/C 4012, you receive one SP Switch2 and, according to your
SP system configuration, all of the switch-to-node cables you need to connect up
to 16 POWER3 High Nodes. These nodes can be within the switch-equipped
frame and in any non-switched expansion frames. You must specify the length of
all switch-to-switch cables that make the connections between switch-equipped
frames and to an SP Switch2 Frame (F/C 2032).
Each occupied switch port in the SP Switch2 contains an interposer card (F/C
4032). Interposer cards can be changed or added while the switch is operating.
Any unused switch ports must have blank interposer cards (F/C 9883) installed,
which prevent contamination of the connector and ensure a proper cooling air
flow.
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Chapter 5. The Switch