3Com 5000 ATM manual Overview

Overview 5-5

The result is that multicast traffic is flooded through a multicast group-specific subset of the switched fabric, namely that subset that contains either end stations that have joined the given multicast group, or multicast routers.

The location of multicast group members is determined by snooping on the IGMP/IP report packets sent from the hosts. This information is aged out and refreshed by hearing new reports. the location of the multicast routers can either be configured by management or be determined by snooping on protocols such as:

■Protocol-Independent Multicast (PIM)

■Multicast Open Shortest Path First (MOSPF)

■Distance Vector Multicast Routing Protocol (DVMRP)

The CoreBuilder 5000 hub uses the following two types of IGMP packets to determine the flow of multicast data:

■Queries — Query packets (IGMP Type = 1) are flooded to all ports in the vbridge.

■Reports — Report packets (IGMP Type = 2 and 6) are only sent to multicast router ports. They are not forwarded to other ports because they inhibit hosts on those ports from reporting their own membership in those groups. This results in overly restrictive filtering by the hub.

IGMP is a host-to-router protocol that assumes there is shared medium between both types of IGMP packets. Because of this assumption, when a host hears a report for a host group IP that wants to join, that host assumes that it does not need to send a report to receive this host group traffic. This occurs when the IGMP protocol is optimized to minimize the number of reports sent. This is not true for filtering switches that interconnect the multicast routers to hosts, however.