CANOGA PERKINS 9171 Configuration Guide - Manual (Page 147)

CanogaOS Configuration Guide
24-5
Downstream
Away from the root of the tree. The root of tree might be either the Source or the RP.
Source-Based Trees
In the Source-Based Trees concept, the forwarding paths are based on the shortest
unicast path to the source. If the unicast routing metric is hop counts, the branches of the
multicast Source-Based Trees are minimum hop. If the metric is delay, the branches are
minimum delay.
For every multicast source, there is a corresponding multicast tree that directly connects
the source to all receivers. All traffic to the members of an associated group passes
along the tree made for their source. Source-Based Trees have two entries with a list of
outgoing interfaces-- the source address and the multicast group.
Shared Trees
Shared trees or RP trees (RPT) rely on a central router called the Rendezvous Point (RP)
that receives all traffic from the sources, and forwards that traffic to the receivers. All
hosts might not be receivers. There is a single tree for each multicast group, regardless
of the number of sources. Only the routers on the tree know about the group, and
information is sent only to interested receivers. With an RP, receivers have a place to
join, even if no source exists.
The shared tree is unidirectional, and information flows only from the RP to the receivers.
If a host other than the RP has to send data on the tree, the data must first be tunneled to
the RP, and then multicast to the members. This means that even if a receiver is also a
source, it can only use the tree to receive packets from the RP, and not to send packets
to the RP (unless the source is located between the RP and the receivers).
Bootstrap Router (BSR)
When a new multicast sender starts sending data packets, or a new receiver starts
sending the Join message towards the RP for that multicast group, it needs to know the
next-hop router towards the RP. The BSR provides group-to-RP mapping information to
all the PIM routers in a domain, allowing them to map to the correct RP address.
24.3.3 Data Flow from Source to Receivers
Sending out Hello Messages
PIM routers periodically send Hello messages to discover neighboring PIM routers. Hello
messages are multicast using the address 224.0.0.13 (ALL-PIM-ROUTERS group).
Routers do not send any acknowledgement that a Hello message was received. A
holdtime value determines the length of time for which the information is valid. In
PIM-SM, a downstream receiver must join a group before traffic is forwarded on the
interface.
Electing a Designated Router
In a multi-access network with multiple routers connected, one of them is selected to act
as a designated router (DR) for a given period of time. The DR is responsible for sending
Join/Prune messages to the RP for local members.