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

2–Advanced Fabric Manager Capabilities
Fabric Multicast Routing
Conserving Multicast LIDs
Most InfiniBand switches have a limitation of 1024 multicast LIDs (MLIDs).
However IPv6 (and possibly other applications) can create numerous multicast
groups. In the case of IPv6, there is one Solicited-Node multicast group per
Channel Adapter port. This results in an excessively large number of multicast
groups. Also in large fabrics, this quickly exceeds MLIDTableCap. For example,
a 2000 node fabric with IPv6 would need over 2000 multicast groups.
To address this situation, the SM can share a single MLID among multiple
Multicast groups. Such sharing means both the routes, and destinations will be
shared. This may deliver some unrequested multicast packets to end nodes,
however unneeded packets will be silently discarded by the InfiniBand transport
layer in the Host Channel Adapter/Target Channel Adapter and will have no
impact on applications.
The SM allows the administrator to configure sets of multicast groups which will
share a given pool of Multicast LIDs. This is accomplished using the MLIDShare
sections in the configuration file.
MLID sharing can conserve the hardware MLID tables so other uses of multicast
can be optimized/efficient.
By default the SM will share a pool of 500 LIDs among all IPv6 solicited-node
multicast groups. Thus in fabrics of 500 nodes or less, a unique LID will be used
for every multicast group. However in larger fabrics, LIDs will be shared so that
there are still over 500 unique LIDs available for other multicast groups, such as
the IPoIB broadcast group and other multicast groups which may be used by
applications.
Precreated Multicast Groups
The first end node which joins a multicast group will also create the multicast
group. When a multicast group is created, critical parameters such as the MTU
and speed of the multicast group are also established. The selection of these
values must carefully balance the performance of the multicast group against the
capabilities of the hardware which may need to participate in the group in the
future. For example if an application on a DDR Host Channel Adapter with a 4K
MTU creates a 4K DDR multicast group, it will prevent subsequent joins of the
group by SDR or 2K MTU Host Channel Adapters.
Some ULPs and applications, such as IPoIB, require key multicast groups, such
as the IPv4 broadcast group, to be pre-created by the SM.
Pre-created multicast group configurations are specified in the MulticastGroup
sections of the SM configuration files. When the multicast groups are pre-created,
their MTU and speed are defined by the SM configuration file, allowing the
adminstrator to be able to account for anticipated hardware capabilities and
required performance.
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