18
{
ALLOW
—The Source Address fields in this group record contain a list of the additional sources
that the system wants to obtain data from, for packets sent to the specified multicast address. If
the change was to an Include source list, these sources are the addresses that were added to the
list. If the change was to an Exclude source list, these sources are the addresses that were
deleted from the list.
{
BLOCK
—The Source Address fields in this group record contain a list of the sources that the
system no longer wants to obtain data from, for packets sent to the specified multicast address.
If the change was to an Include source list, these sources are the addresses that were deleted
from the list. If the change was to an Exclude source list, these sources are the addresses that
were added to the list.
IGMP SSM mapping
The IGMP SSM mapping feature enables you to configure static IGMP SSM mappings on the last-hop
router to provide SSM support for receiver hosts that are running IGMPv1 or IGMPv2. The SSM model
assumes that the last-hop router has identified the desired multicast sources when receivers join multicast
groups.
•
When a host that is running IGMPv3 joins a multicast group, it can explicitly specify one or more
multicast sources in its IGMPv3 report.
•
A host that is running IGMPv1 or IGMPv2, however, cannot specify multicast source addresses in its
report. In this case, you must configure the IGMP SSM mapping feature to translate the (*, G)
information in the IGMPv1 or IGMPv2 report into (G, INCLUDE, (S1, S2...)) information.
Figure 13
IGMP SSM mapping
As shown in
, on an SSM network, Host A, Host B, and Host C run IGMPv1, IGMPv2, and
IGMPv3, respectively. To provide SSM service for all the hosts if IGMPv3 is not available on Host A and
Host B, you must configure the IGMP SSM mapping feature on Router A.
With the IGMP SSM mapping feature configured, when Router A receives an IGMPv1 or IGMPv2 report,
it checks the multicast group address G carried in the message and does the following:
•
If G is not in the SSM group range, Router A cannot provide the SSM service but can provide the
ASM service.
•
If G is in the SSM group range but no IGMP SSM mappings that correspond to the multicast group
G have been configured on Router A, Router A cannot provide SSM service and drops the message.