User Manual

OpenSM – Subnet ManagerRev 2.1-1.0.6
Mellanox Technologies
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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-
tion, if none of the above spanning tree branches crosses a dateline used for unicast credit loop
avoidance on a torus, and if multicast traffic is confined to SL 0 or SL 8 (recall that torus-2QoS
uses SL bit 3 to differentiate QoS level), then multicast traffic also cannot contribute to the "ring"
credit loops that are otherwise possible in a torus. Torus-2QoS uses these ideas to create a master
spanning tree. Every multicast group spanning tree will be constructed as a subset of the master
tree, with the same root as the master tree. Such multicast group spanning trees will in general
not be optimal for groups which are a subset of the full fabric. However, this compromise must
be made to enable support for two QoS levels on a torus while preventing credit loops. In the
presence of link or switch failures that result in a fabric for which torus-2QoS can generate
credit-loop-free unicast routes, it is also possible to generate a master spanning tree for multicast
that retains the required properties. For example, consider that same 2D 6x5 torus, with the link
from (2,2) to (3,2) failed. Torus-2QoS will generate the following master spanning tree: