Released
PMC-Sierra, Inc.
PM9311/2/3/5 ETT1™ CHIP SET
Data Sheet
PMC-2000164
ISSUE 3
ENHANCED TT1™ SWITCH FABRIC
The information carried on the links is protected by checksums. For example, if the EPP receives
information with a checksum error on one link, then it simply ignores that link and uses the information from
the other link. All errors are reported to the local CPU via interrupts.
The redundant core is tolerant of such single errors, provided that they are detectable. In general, all errors
that occur on the links or within the Scheduler or Crossbar will be detectable.
It should be clear from Figure 6 that only the Dataslice and EPP have the redundant connections. An
individual Crossbar or Scheduler cannot compensate for information that is received with a checksum
error. The following discussion reviews how these two types of devices are affected by receiving corrupted
information.
If a Crossbar detects a checksum error then it simply marks the cell as being corrupted. This information is
passed to the egress Dataslice which will ignore that link and use the information from the other Crossbar.
The Scheduler is more complicated because it must maintain accurate state information on the occupancy
of all queues as well as the backpressure status of some of the egress queues. If a Scheduler receives a
checksum error then it must effectively remove itself from the system.
Consider the simple configuration illustrated in Figure 7. Scheduler 0 sees a checksum error on
information it receives from EPP 1. Scheduler 0 now has incorrect state information. It immediately
disables all of its outbound links so that EPP 1 and EPP 2 know that Scheduler 0 should be ignored. The
EPPs now only accept information from Scheduler 1.
Figure 7. Simple Redundant Scheduler Configuration
EPP 1
EPP 2
Scheduler 0
Scheduler 1
If a new Scheduler-0 board is inserted in the system, then it must have its internal state updated to match
that of Scheduler-1. This is done using a sequence of actions that are controlled by the ETT1 CPU. In
essence, the linecards are temporarily stopped from sending new cells, and the state within each EPP is
downloaded into the Schedulers, and then the linecards are restarted. The entire process is very rapid
(much less than 1ms), but does cause best-effort traffic to be suspended for a brief time.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
27
PMC [ PMC-SIERRA, INC ]
