3com 2500 User Guide

C

HAPTER

 5: R

OUTING

 

WITH

 IP M

ULTICAST

The CoreBuilder 2500 system is capable of dynamic multicast filtering 
based on the Internet Group Management Protocol (IGMP). This protocol 
ensures that multicast packets are flooded only to appropriate ports in a 
routing interface.

IGMP tracks end station group membership within a multicast group. 
Membership in a group is dynamic, and hosts can be a member of more 
than one group at a time. The system avoids propagating multicast 
broadcasts to the entire subnetwork by confining them within the 
multicast group (a process called IGMP snooping).

The Distance Vector Multicast Routing Protocol (DVMRP) establishes 
multicast delivery paths over a series of routing devices. DVMRP is a 
distance-vector-routing protocol, similar to the IP Routing Information 
Protocol (RIP). Multicast routers exchange distance vector updates that 
contain lists of destinations and the distance in router hops to each 
destination. They maintain this information in a routing table. 

The Internet Multicast Backbone (MBONE) uses DVMRP. Because of 
DVMRP, the CoreBuilder 2500 system can establish delivery paths without 
direct connections to multicast routers. 

The MBONE is the Internet’s experimental multicast backbone network. 
Users can test multicast applications and technology on the MBONE 
without waiting for Internet multicast standards to be set. You can 
connect to the MBONE through any Internet service provider (ISP).

MBONE routers forward multicast packets over an interface or over a 
multicast tunnel only if the Time-To-Live (TTL) value in the packet is larger 
than the tunnel’s threshold. See “Multicast Tunnels” on page 5-5 for 
more information about tunnels.