User Manual

Features Overview and ConfigurationRev 2.3-1.0.1
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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.
3.2.2.5.6.2 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 traffic
from unicast traffic. Thus, torus-2QoS must generate multicast routing such that credit loops can-
not arise from a combination of multicast and unicast path segments. It turns out that it is possi-
ble to construct spanning trees for multicast routing that have that property. For the 2D 6x5 torus
example above, here is the full-fabric spanning tree that torus-2QoS will construct, where "x" is
the root switch and each "+" is a non-root switch:
For multicast traffic routed from root to tip, every turn in the above spanning tree is a legal DOR
turn. For traffic routed from tip to root, and some traffic routed through the root, turns are not
legal DOR turns. However, to construct a credit loop, the union of multicast routing on this span-
ning tree with DOR unicast routing can only provide 3 of the 4 turns needed for the loop. In addi-