HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 511

at the helper address considers the upstream interface to be a multicast host
that is a member of every group to which at least one host in the stub networks
belongs.
Group 99
Group 98
Figure 10-5. IGMP Proxy
When an upstream interface receives a multicast packet, the router forwards
it through the downstream interfaces on networks for which the relevant
group is active. This function is called multicast forwarding.
Figure 10-5 illustrates how IGMP reports and multicast messages flow.
Configuring Multicast Support for a Stub Network
IGMP report
Group 99
Switch
Network 1
Downstream
interfaces
IGMP
Router
queries
Upstream
interface
Switch
Network 2
IGMP report
Group 98
IGMP report
Group 98, 99
Source 10.1.1.1
Multicast
10.1.1.1
Overview
Router
10-9