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Understanding How Multicasting Works
Enabling the Installation of Directly Connected Subnets
In PIM sparse mode, a first-hop router that is the designated router (DR) for the interface may need to
encapsulate the source traffic in a PIM register message and unicast it to the rendezvous point (RP). To
prevent the new sources for the group from being learned in the routing table, the (*,G) flows should
remain completely hardware-switched flows. The (subnet/mask, 224/4) entries that are installed in the
hardware FIB allow both (*,G) flows to remain completely hardware-switched flows, and new, directly
connected sources to be learned correctly. The installation of directly connected subnets is enabled
globally by default. One (subnet/mask, 224/4) is installed per PIM-enabled interface.
Enter the show mls ip multicast connected command to view these FIB entries.
To enable the installation of the directly connected subnets, perform this task:
Task
Enable the installation of the directly connected
subnets.
This example shows how to enable the installation of the directly connected subnets:
Router(config)# mls ip multicast connected
Router(config)#
Understanding IGMP Querier
IGMP querier enables IGMP snooping within a VLAN where PIM and IGMP are not configured because
the multicast traffic does not need to be routed.
You must enable IGMP querier for IGMP snooping to work correctly in a VLAN in which no multicast
Note
routers are present.
When you configure IGMP querier for a VLAN, the switch sends out IGMP general query messages
every 125 seconds and listens for the general query messages from the other switches. If the switch
receives a general query, a querier election starts. A querier election across the switches is based either
on an IP address or a MAC address. For an inbound query, if the source IP address is nonzero, the
election is based on the IP address, and the switch with the lower source IP address becomes the querier.
If the source IP address is zero for an inbound query, then the election is based on the source MAC
address, and the switch with the lower MAC address wins the election and becomes the querier. The
switch that becomes the nonquerier maintains an "other querier interval" timer. When this timer expires,
the switch elects itself as the querier.
For information on enabling IGMP querier, see the
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
51-8
Chapter 51
Command
Router(config) # mls ip multicast connected
"Enabling the IGMP Querier" section on page
Configuring Multicast Services
51-16.
OL-8978-04
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Troubleshooting
