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

2–Advanced Fabric Manager Capabilities
Fabric Multicast Routing
To implement multicast, the SM must construct spanning tree routes throughout
the fabric which will deliver exactly one copy of each sent packet to every
interested node. The SM must configure such routes within the limitations of the
hardware. Namely:
• Most switch chips have a limitation of only 1024 multicast LIDs
• There may be varied MTU and speed capabilities for different switches and
Inter Switch Links.
• The fabric topology and hardware may change after applications have joined
multicast group
To support efficient yet dependable routing of multicast, the SM allows the user to
configure and control Multicast Routing.
Handling Fabric Changes
The InfiniBand standard has a limitation in that the SM must make the realizable
decision for a multicast group at the time an application creates/joins a multicast
group. This means the SM must determine if there is a path with the appropriate
speed and MTU to meet the requested capabilities of the multicast group.
However later fabric changes could make the multicast group unrealizable. For
example, removal or downgrade of high speed links, loss of switches, changes to
switch MTU or speed configuration, to name a few. Unfortunately, in this case
there is no standard way to notify end nodes that the multicast group is no longer
viable.
To address this situation, the SM performs stricter multicast checking at
Join/Create time. This means a multicast join/create is rejected if there are any
switch-to-switch links which do not have at least the MTU or rate requested for the
multicast group, reducing the chance that a simple fabric failure or change (for
example, loss one-of-one link) could make the group unrealizable.
The DisableStrictCheck parameter controls this capability. When 1, this
parameter disables the strict checking and accepts Join/Create request for which
at least one viable fabric path exists. By default the parameter is 0, which allows
for more strict checking.
In addition, the MLIDTableCap parameter is used to configure the maximum
number of Multicast LIDs available in the fabric. This must be set to a value less
than or equal to the smallest Multicast forwarding table size of all the switches
which may be the in the fabric. It defaults to 1024, which is the typical capability of
switches.
IB0054608-01 B 2-17