Igmp Snooping; Joining A Multicast Group - Cisco Catalyst 3850 series Configuration Manual
Ip multicast routing configuration guide
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Configuring IGMP
IGMP Snooping
IGMP Snooping
Layer 2 devices can use IGMP snooping to constrain the flooding of multicast traffic by dynamically configuring
Layer 2 interfaces so that multicast traffic is forwarded to only those interfaces associated with IP multicast
devices. As the name implies, IGMP snooping requires the LAN device to snoop on the IGMP transmissions
between the host and the router and to keep track of multicast groups and member ports. When the device
receives an IGMP report from a host for a particular multicast group, the device adds the host port number to
the forwarding table entry; when it receives an IGMP Leave Group message from a host, it removes the host
port from the table entry. It also periodically deletes entries if it does not receive IGMP membership reports
from the multicast clients.
For more information on IP multicast and IGMP, see RFC 1112 and RFC 2236.
Note
The multicast router (which could be a device with the IP services feature set on the active device) sends out
periodic general queries to all VLANs. All hosts interested in this multicast traffic send join requests and are
added to the forwarding table entry. The device creates one entry per VLAN in the IGMP snooping IP multicast
forwarding table for each group from which it receives an IGMP join request.
The device supports IP multicast group-based bridging, instead of MAC-addressed based groups. With
multicast MAC address-based groups, if an IP address being configured translates (aliases) to a previously
configured MAC address or to any reserved multicast MAC addresses (in the range 224.0.0.xxx), the command
fails. Because the device uses IP multicast groups, there are no address aliasing issues.
The IP multicast groups learned through IGMP snooping are dynamic. However, you can statically configure
multicast groups by using the ip igmp snooping vlan vlan-id static ip_address interface interface-id global
configuration command. If you specify group membership for a multicast group address statically, your setting
supersedes any automatic manipulation by IGMP snooping. Multicast group membership lists can consist of
both user-defined and IGMP snooping-learned settings.
You can configure an IGMP snooping querier to support IGMP snooping in subnets without multicast interfaces
because the multicast traffic does not need to be routed.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from this port on the VLAN are deleted.
These sections describe IGMP snooping characteristics:
Joining a Multicast Group
When a host connected to the device wants to join an IP multicast group and it is an IGMP version 2 client,
it sends an unsolicited IGMP join message, specifying the IP multicast group to join. Alternatively, when the
device receives a general query from the router, it forwards the query to all ports in the VLAN. IGMP version
1 or version 2 hosts wanting to join the multicast group respond by sending a join message to the device. The
device CPU creates a multicast forwarding-table entry for the group if it is not already present. The CPU also
adds the interface where the join message was received to the forwarding-table entry. The host associated
with that interface receives multicast traffic for that multicast group.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
OL-32598-01
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