Reverse Path Forwarding; Pruning - 3Com CoreBuilder 2500 User Manual

5-4 C 5: R
HAPTER OUTING WITH
Reverse Path
Forwarding

Pruning

IP M
ULTICAST
Figure 5-2 shows a spanning tree for the network in Figure 5-1. The
spanning tree consists of links 1, 2, 3, and 4.
1
B
A
3 4
D
E
6
Figure 5-2 Spanning Tree Algorithm Implemented to Block Redundant Paths
The MBONE network uses the reverse path forwarding (RPF) multicast
algorithm. RPF avoids duplicate paths on multiaccess links by using a
routing table to compute a logical spanning tree for each network
source. The algorithm consists of three steps:
1 When the system receives a multicast packet, the algorithm notes the
packet's source network and the interface that received the packet.
2 If the receiving interface is on the shortest path toward the source
network, the system forwards the packet to all interfaces except the
interface where the packet was received.
3 If the receiving interface is not on the shortest path toward the source
network, the system drops the packet.
Pruning is a method used in the RPF algorithm to forward packets to a
spanning tree only if group members exist in the tree. This method results
in fewer spanning trees, but it requires dynamic updates to the routing
table.
Nodes that are at the border of the network and have no point beyond
them in the RPF spanning tree are called leaf nodes. Leaf nodes all receive
the first multicast packet. If a group member is attached to a leaf node,
the node continues to accept packets. If no group member is attached to
the leaf node, the node sends back a prune message to the router that
sent the packet. The message notifies the router not to send any further
packets to this group. In the CoreBuilder 2500 system, the Administration
Console IP multicast
pruning occurs on the Spanning Tree.
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C
5
includes information about when
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