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Figure 1-4 Network diagram for IGMP proxying
Query from Router A
Report from Router B
Ethernet
Router interface
Host interface
Proxy & Querier
Router B
Querier
Router A
Host B
Receiver
Host A
Receiver
Host C
Query from Router B
Report from Host
PIM domain
As shown in Figure 1-4 , two types of interfaces are defined on an IGMP proxy device:
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Upstream interface: Also referred to as the proxy interface. A proxy interface is an interface on
which IGMP proxying is configured. It is in the direction toward the root of the multicast forwarding
tree. An upstream interface acts as a host running IGMP; therefore, it is also called host interface.
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Downstream interface: An interface that is running IGMP and not in the direction toward the root of
the multicast forwarding tree. A downstream interface acts as a router running IGMP; therefore, it is
also called router interface.
A device with IGMP proxying configured maintains a group membership database, which storesthe
group memberships on all the downstream interfaces. Each entry comprises the multicast address,
filter mode, and source list. Such an entry is a collection of members in the same multicast group on
each downstream interface.
A proxy device performs host functions on the upstream interface based on the database. It responds to
queries according to the information in the database or sends join/leave messages when the database
changes. On the other hand, the proxy device performs router functions on the downstream interfaces
by participating in the querier election, sending queries, and maintaining memberships based on the
reports.
Multi-Instance IGMP
While IGMP maintains group memberships on a per-interface base, IGMP in a VPN instance handles
protocol packets based on the VPN instance on the interface. Upon receiving an IGMP packet, the
router determines the instance to which the message belongs and handles the message within the
instance. If IGMP running in a VPN instance needs to exchange information with another multicast
protocol, the router informs the other multicast protocol only within the VPN instance.
Protocols and Standards
The following documents describe different IGMP versions:
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RFC 1112: Host Extensions for IP Multicasting
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RFC 2236: Internet Group Management Protocol, Version 2
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RFC 3376: Internet Group Management Protocol, Version 3