ADTRAN Stub Routing - Manual (Page 9)

IP Multicast Stub Routing in AOS IP Multicast in Stub Topologies
61200890L1-29.3A Copyright © 2005 ADTRAN, Inc. 9
to the multicast-enabled network (the backbone in Figure 2). The connection to the backbone is the
stub router’s upstream connection. The connection to the classroom LAN is the stub router’s
downstream connection.
• A downstream connection faces multicast clients (PCs in Figure 2) and performs the IGMP router
function as would a typical multicast router. The upstream connection faces the multicast network
and would normally run a multicast routing protocol. With IGMP Forwarding, the stub router
instead runs an IGMP client process on the upstream interface, appearing to the multicast network
as a single multicast client. Even though the link to the backbone router is typically a WAN
interface, the backbone router still performs the IGMP router process on that interface and will use
IGMP for communicating group membership activity with the stub router.
• IGMP forwarding is best used when there is a single active link to the multicast network. This
includes redundancy solutions such as dial backup where either the primary link or the backup link
is active at any given time. In situations where there is more than one simultaneously active link to
the multicast network (used for redundancy, load sharing, meshing, etc.), it may be possible through
extremely careful network design to use IGMP forwarding. However, a multicast routing protocol
is strongly recommended to protect against loops and to ensure that multicast packets are
transmitted and received on the proper path.
Using IGMP forwarding, when a student logs into PC1 and selects a class from the Media Server web
interface, the following actions occur:
1. PC1 sends an IGMP message on its LAN indicating that it wants to join the multicast group for that
class (again, let’s say address 224.1.1.1).
2. Satellite Router 1 (stub router) registers the group membership on that interface.
3. Using its IGMP client on its upstream interface, the Satellite Router 1 sends an IGMP message to the
backbone router indicating that it wants to join the multicast group.
4. The backbone router registers the group membership on that interface and then uses its multicast routing
protocol to signal the backbone in the usual manner.
5. IP packets from the media server to the group address 224.1.1.1 then flow through the backbone and
ultimately to PC1 .