3com 2500 User Guide

This section describes the characteristics of the CoreBuilder 2500 
multicast interface.

The DVMRP metric value determines the cost of a multicast interface. 
A higher cost results in a slower link. The default value is 1. 

The TTL threshold determines whether the interface forwards multicast 
packets to other switches and routers in the subnetwork. If the interface 
TTL is greater than the packet TTL, then the interface does not forward 
the packet. The default value of 1 means that the interface forwards 
most packets.

The rate limit determines how many multicast packets can travel over the 
interface per second. The CoreBuilder 2500 system drops multicast traffic 
that travels faster than this rate. The default value of 0 means that no 
rate limit is in effect. A lower rate limit results in fewer multicast packets 
traveling over the interface. 

Multicast tunnels logically connect two multicast routers through one or 
more unicast routers. The multicast router at the local endpoint of the 
tunnel puts multicast packets in a format that unicast routers can 
interpret and forward. The multicast router at the receiving endpoint 
reformats the packets into their multicast format. Tunnels are virtual links 
through the unicast IP network.

Multicast tunnels have characteristics similar to those of a multicast 
interface: a DVMRP metric value, a TTL threshold, and a rate limit. When 
you define a multicast tunnel, you also specify the destination address of 
the multicast router at the tunnel’s remote endpoint.