Fabric Manager Users Guide, Version 6.1, Revision A - QLogic

2–Advanced Fabric Manager Capabilities
SM Loop Test
As part of loop test the SM sets up a large number of these loop routes so as to
cover all ISLs in the fabric. The SM associates a LID with each of the loop routes
in such a way that a packet sent to that LID will enter the loop and spin around the
loop utilizing the ISLs on that loop. The SM injects a packet to into each loop by
sending a packet to each of the LIDs associated with the loop routes. Those
packets loop around the loops and continuously pass traffic on the ISLs.
Once loop test is stopped, the SM invalidates the loop LIDs, which in turn would
cause the loop packets to be dropped by the switches and stop the utilization of
the switch ISLs.
Important loop test parameters
• Loop path length – The number of hops the SM checks to find a loop that
leads back to the same switch is termed as the loop path length.
• Number of packets injected – The number of 256-byte packets the SM
injects into each of the loops. These packets would loop around on the ISLs.
These parameters can be configured as part of the loop test.
Loop Test Fast Mode
QLogic recommends the customers use the fast mode for ISL validation and link
integrity testing. In the fast mode, the loop test doesn't attempt to include each ISL
in all possible loops, but includes it in at least the specified number of loops (this
value is controlled using the MinISLRedundancy parameter). When using fast
mode the computations are less expensive and finish faster which allows the loop
test to be started quickly. The loop test fast mode uses a MinISLRedundancy
value of four by default (for example, each ISL is included in at least four loops).
There is an important reason for including an ISL in multiple loops – when an ISL
goes down in the fabric, the loop that this ISL is part of, is broken and therefore
other ISLs in the loop will no longer be tested. But if each ISL is included in
multiple loops, the other ISLs in the broken loop are also part of other loops and
will continue to see traffic and therefore continue to get tested (albeit at a slightly
lower utilization).
In typical fast mode operations (with the default MinISLRedundancy of 4),
injecting four packets into each loop is sufficient to get a high utilization on the
ISLs.
Loop Test Default Mode
By default the loop test runs in default mode. In the default mode, the SM uses an
exhaustive approach to setup loop routes and will include each ISL in as many
loops as possible. This ensures that each ISL is exactly in the same number of
loops and hence will see the same amount of utilization. But finding all possible
loops is computationally intensive and can take a long amount of time.
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