User Manual

OpenSM – Subnet ManagerRev 2.2-1.0.1
Mellanox Technologies
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because they cannot be used to construct a loop encircling T. The hop I-r uses a separate VL, so
it cannot contribute to a credit loop encircling T. Extending this argument shows that in addition
to being capable of routing around a single switch failure without introducing deadlock, torus-
2QoS can also route around multiple failed switches on the condition they are adjacent in the last
dimension routed by DOR. For example, consider the following case on a 6x6 2D torus:
Suppose switches T and R have failed, and consider the path from S to D. Torus-2QoS will gen-
erate the path S-n-q-I-u-D, with an illegal turn at switch I, and with hop I-u using a VL with bit 1
set.
As a further example, consider a case that torus-2QoS cannot route without deadlock: two
failed switches adjacent in a dimension that is not the last dimension routed by DOR; here the
failed switches are O and T:
In a pristine fabric, torus-2QoS would generate the path from S to D as S-n-O-T-r-D. With failed
switches O and T, torus-2QoS will generate the path S-n-I-q-r-D, with illegal turn at switch I, and
with hop I-q using a VL with bit 1 set. In contrast to the earlier examples, the second hop after
the illegal turn, q-r, can be used to construct a credit loop encircling the failed switches.
8.5.7.2 Multicast Routing
Since torus-2QoS uses all four available SL bits, and the three data VL bits that are typically
available in current switches, there is no way to use SL/VL values to separate multicast traf
fic
from unicast traffic. Thus, torus-2QoS must generate multicast routing such that credit loops can-