TCP/IP Tutorial and Technical Overview - IBM Redbooks

2. Subset A is currently included. However, only subset B is desired. To do this,
first an ALLOW_NEW_SOURCES message is issued to add all of subset B
except for those addresses already included in A. This is followed by a
BLOCK_OLD_SOURCES message to exclude all of subset A except for
those addresses which also belong to B.
3. Now only subset B is included. However, we want to change the filter to
EXCLUDE, and to specify only subset A. This is done with one
CHANGE_TO_EXCLUDE_MODE specifying subset A.
4. After step 3, only subset A is excluded. Now, we want to exclude only
subset B. First issue an ALLOW_NEW_SOURCES message to remove all of
subset A from the excluded list except those addresses also in subset B.
Then add all of the addresses in subset B to the exclude list except for those
also in subset A.
5. Now only subset B is excluded. We want to change the filter to INCLUDE and
add A to the list. Use the CHANGE_TO_INCLUDE_MODE specifying
subset A.
Multicast router operations
When a host attempts to join a group, multicast routers on the subnet receive the
membership report packet and create an entry in their local group database. This
database tracks the group membership of the router’s directly attached networks.
Each entry in the database is of the format [group, attached network]. This
indicates that the attached network has at least one IP host belonging to the
group. Multicast routers listen to all multicast addresses to detect these reports.
The information in the local group database is used to forward multicast
datagrams. When the router receives a datagram, it is forwarded out each
interface containing hosts belonging to the group.
To verify group membership, multicast routers regularly send an IGMP query
message to the all hosts’ multicast address. Each host that still wants to be a
member of a group sends a reply. RFC 3376 specifies this verification should by
default occur every 125 seconds. To avoid bursts of traffic on the subnet, replies
to query messages are sent using a random delay. Because routers do not track
the number of hosts in each group, any host that hears another device claim
membership cancels any pending membership replies. If no hosts claim
membership within the specified interval, the multicast router assumes no hosts
on that network are members of the group.
IGMP snooping switches
One potential problem when implementing IGMP is the flooding of a network
segment with multicast packets, even though there might not be any nodes on
that segment that have any interest in receiving the packets. Although the
Chapter 6. IP multicast 249