3com 2500 User Guide

The CoreBuilder 2500 system uses three multicast routing algorithms:

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Flooding

■

Spanning Tree

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Reverse Path Forwarding

In most flooding algorithms, a network node receives a packet that was 
sent to a multicast destination. The node determines whether the packet 
is an original that it has not received before or a duplicate of a packet 
that it has received before. If the packet is original, the node forwards the 
packet on all interfaces except the incoming interface. If the packet is a 
duplicate, the node discards it.

This flooding algorithm is useful when network robustness is important. 
The algorithm does not depend on routing tables. Multicast destinations 
receive packets as long as at least one path to the destinations exists and 
no errors occur during transmission.

The Spanning Tree algorithm detects loops and logically blocks redundant 
paths within the network. The paths form a loopless graph, or tree, 
spanning all the nodes in the network. A port in the Spanning Tree 
blocking state does not forward or receive data packets.

After the algorithm eliminates redundant paths, the network 
configuration stabilizes. When one or more of the paths in the stable 
topology fail, the protocol automatically recognizes the changed 
configuration and activates redundant links. This strategy ensures that all 
nodes remain connected.

Figure 5-1   Simple Network Implemented Without Using Spanning Tree

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D

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C

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E

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