IP Multicast in Stub Topologies
IP Multicast Stub Routing in AOS
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Copyright © 2005 ADTRAN, Inc.
61200890L1-29.3A
IP Multicast in Stub Topologies
A stub network is a portion of a network with a single connection to the remainder of the network. The
spokes in a hub-and-spoke network are a common form of stub network, although there are other forms. In
the stub portion of a network, a multicast routing protocol is not necessary for IP multicast operation. Since
there is a single link, there is a single best path and no potential for a loop. All that is needed is a way to
signal to the multicast network when there are active group members within the stub, allowing multicast
streams to be sent to the stub network only when needed.
Referring back to Figure 2 on page 5, the satellite classrooms have a single connection to the
multicast-enabled backbone, making them a stub network. 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 (let’s say address 224.1.1.1).
2.
Satellite Router 1
(the stub router) registers the group membership on that interface.
3. Using its multicast routing protocol,
Satellite Router 1
signals to the backbone router that it needs to
receive packets to that group.
4. This signaling propagates to other routers (depending on the exact multicast routing protocol in use) and
eventually to the
Central Router
.
5. IP packets from the
Media Server
to the group address 224.1.1.1 then flow through the backbone and
ultimately to
PC1
.
While the multicast routing protocol does not consume significantly more bandwidth relative to other
solutions, it does consume CPU and memory resources on the stub router even though it is performing only
a portion of its total job, due to being a stub link.
An alternative would be to configure the satellite routers to proxy the IGMP messages from the classroom
PCs to the multicast network. This alternative setup is described in the next section.
IGMP-Based Multicast Forwarding (IGMP Proxying)
Internet Draft
draft-ietf-magma-igmp-proxy-06.txt
specifies a technique for performing just such a proxy
function.
The following key concepts and terminology need to be understood regarding IGMP Forwarding.
•
The stub router is typically the router at the stub location that connects to the remainder of the
network. In Figure 2 on page 5, the satellite routers are the stub routers.
•
When discussing multicast, it is common for the words upstream and downstream to be used
relative to the distribution tree and the flow of a particular multicast stream. Regarding IGMP
forwarding, it is important to note that these words are used relative to the stub router’s connection
Terminology is tricky in this area. Other vendors support this technique, but some refer to
it as Stub Multicast Routing where the stub router is configured as an IGMP Proxy
Agent. Some vendors have another feature called IGMP Proxy which is an entirely
different capability and not directly related to this application.
