3Com CoreBuilder 2500 User Manual - page 66

5-2

C

HAPTER

 5: R

OUTING

WITH

 IP M

ULTICAST

IGMP

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

).

DVMRP

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. 

MBONE

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.

At software revisions earlier than 8.0, CoreBuilder 2500 systems that are 

connected to the MBONE network support up to 16 IP multicast tunnels 

or routing interfaces. CoreBuilder 2500 systems at revision 8.0 or later 

support up to 32 IP multicast tunnels or routing interfaces connected to 

the MBONE.