IBM MQ Multicast topic topology

Use this example to understand the IBM® MQ Multicast topic topology.

IBM MQ Multicast support requires that each subtree has its own multicast group and data stream within the total hierarchy.

The classful network IP addressing scheme has designated address space for multicast address. The full multicast range of IP address is 224.0.0.0 to 239.255.255.255, but some of these addresses are reserved. For a list of reserved address either contact your system administrator or see https://www.iana.org/assignments/multicast-addresses for more information. It is recommended that you use the locally scoped multicast address in the range of 239.0.0.0 to 239.255.255.255.

In the following diagram, there are two possible multicast data streams: 

DEF COMMINFO(MC1) GRPADDR(239.XXX.XXX.XXX
)

DEF COMMINFO(MC2) GRPADDR(239.YYY.YYY.YYY)
where 239.XXX.XXX.XXX and 239.YYY.YYY.YYY are valid multicast addresses.
These topic definitions are used to create a topic tree as shown in the following diagram: 

DEFINE TOPIC(FRUIT) TOPICSTRING('Price/FRUIT') MCAST(ENABLED) COMMINFO(MC1)

DEFINE TOPIC(FISH) TOPICSTRING('Price/FISH') MCAST(ENABLED) COMMINFO(MC2)
Image of a multicast topic tree

Each multicast communication information (COMMINFO) object represents a different stream of data because their group addresses are different. In this example, the FRUIT topic is defined to use COMMINFO object MC1 , the FISH topic is defined to use COMMINFO object MC2 , and the Price node has no multicast definitions.

IBM MQ Multicast has a 255 character limit for topic strings. This limitation means that care must be taken with the names of nodes and leaf-nodes within the tree; if the names of nodes and leaf-nodes are too long, the topic string might exceed 255 characters and return the 2425 (0979) (RC2425): MQRC_TOPIC_STRING_ERROR reason code. It is recommended to make topic strings as short as possible because longer topic strings might have a detrimental effect on performance.