IP Multicast Stub Routing in AOS
IP Multicast Addressing and Protocols
61200890L1-29.3A
Copyright © 2005 ADTRAN, Inc.
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routers (ultimately reaching the router connected to the source of the multicast stream). This allows a
distribution tree to be built from the source to all receivers, thus ensuring the following:
•
There is a path to all receivers.
•
No stream is sent where there are no receivers.
•
There are no loops in the distribution tree.
•
The proper path is used between a source and a receiver.
These are critical factors in building a distribution tree.
When a router receives a multicast packet, the source IP address in the packet is examined and tested to
make sure the packet was received on the proper interface for the routing protocol in use. If not, the packet
is dropped. This is referred to as reverse path forwarding (RPF).
If the packet is not dropped, the router will forward multicast packets through an interface only if one of
the following is true:
•
The router has detected that members of the specific multicast group are present on the segment
attached to that interface (learned through IGMP or statically configured).
•
The router has detected another multicast router using a multicast routing protocol, and that router
is on the correct path to current group members.
Of the various multicast routing protocols, PIM (Protocol Independent Multicast) is the most common.
The following sections contain short descriptions of several multicast routing protocols. Some important
concepts for multicast routing protocols include:
•
Source-based or shared trees
•
Dense or sparse mode
•
Reverse path flooding
•
Pruning
The methods used by each protocol are listed below, but a detailed definition is beyond the scope of this
document.
Distance Vector Multicast Routing Protocol (DVMRP)
DVMRP (RFC1075) is a dense mode, source-based tree protocol. It provides it own unicast routing
protocol that is very similar to RIP. It is possible that DVMRP will resolve a different path than the
network’s own unicast protocol. DVMRP is used in the Internet Multicast Backbone (MBONE), an
experimental multicast network.
Multicast Open Shortest Path First (MOSPF)
MOSPF (RFC1584) is a dense mode, source-based tree protocol. As with OSPF, MOSPF is for use
within one administrative or routing domain. MOSPF uses OSPF as the unicast routing protocol and
works by adding multicast information to OSPF link state advertisements. This protocol is limited when
there are many sources and when links are not stable. It is not widely used today.
Protocol Independent Multicast (PIM)
PIM has a sparse mode (shared tree) and dense mode (source-based tree) variation. PIM relies on
information in the router’s unicast routing table without concern over how the routes are installed in the
table (thus the name protocol independent). PIM is the most widely-used protocol.
