How Igmp Snooping Works - HP MSR2003 Configuration Manual

NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.

How IGMP snooping works

The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "Configuring static
IGMP messages types include general query, IGMP report, and leave message. An IGMP
snooping-enabled Layer 2 device performs differently depending on the message types.
General query
To check for the existence of multicast group members, the IGMP querier periodically sends IGMP
general queries to all hosts and routers on the local subnet. All these hosts and routers are identified by
the address 224.0.0.1.
After receiving an IGMP general query, the Layer 2 device forwards the query to all ports in the VLAN
except the port that received the query. The Layer 2 device also performs one of the following actions:
• If the receiving port is a dynamic router port in the router port list, the Layer 2 device restarts the
aging timer for the port.
If the receiving port does not exist in the router port list, the Layer 2 device adds the port to the router
•
port list. It also starts an aging timer for the port.
IGMP report
A host sends an IGMP report to the IGMP querier for the following purposes:
Responds to queries if the host is a multicast group member.
•
Applies for a multicast group membership.
•
After receiving an IGMP report from the host, the Layer 2 device forwards it through all the router ports
in the VLAN. It also resolves the address of the reported multicast group, and looks up the forwarding
table for a matching entry as follows:
If no match is found, the Layer 2 device creates a forwarding entry for the group with the receiving
•
port as an outgoing interface. It also marks the receiving port as a dynamic member port and starts
an aging timer for the port.
If a match is found but the matching forwarding entry does not contain the receiving port, the Layer
•
2 device adds the receiving port to the outgoing interface list. It also marks the receiving port as a
dynamic member port and starts an aging timer for the port.
If a match is found and the matching forwarding entry contains the receiving port, the Layer 2
•
device restarts the aging timer for the port.
In an application with a group filter configured on an IGMP snooping-enabled Layer 2 device, when a
user requests a multicast program, the user's host initiates an IGMP report. After receiving this report, the
Layer 2 device resolves the multicast group address in the report and performs ACL filtering on the report.
If the report passes ACL filtering, the Layer 2 device creates an IGMP snooping forwarding entry for the
multicast group with the receiving port as an outgoing interface. Otherwise, the Layer 2 device drops this
report, in which case the multicast data for the multicast group is not sent to this port, and the user cannot
retrieve the program.
A Layer 2 device does not forward an IGMP report through a non-router port because of the IGMP report
suppression mechanism. For more information about the IGMP report suppression mechanism, see
"Configuring IGMP."
ports."
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