Cisco Group Membership Protocol CGMP

WAN connection

Server room 48 ports

Server room 48 ports

já Building A W 96 ports

Building B, 1st floor 96 ports

Building B, 1st floor 96 ports

Building C, 1st floor 24 ports

Building C, 1st floor 24 ports

Building B, 2nd floo 72 ports

Building B, 2nd floo 72 ports

Just as broadcast frames are forwarded to every port within a broadcast domain, so too are frames carrying IP multicast packets After all, a broadcast domain is nothing more than a multicast group to which all hosts belong Figure 5-14 illustrates the problem Three group members are attached to a 24-port switch An IGMP Membership Report is sent to the router, and the router begins forwarding the appropriate multicast session onto the subnet Because IGMP is a Layer 3 protocol, the Ethernet switch has no easy way to determine what ports the group members are on As a result, the multicast traffic is forwarded to all 23 ports (discounting the source port)

Figure 5-14 One of the Three Group Members Sends an IGMP Membership Report, Joining Multicast Group A (a) When the Router Forwards the Multicast Session, the Switch Replicates the Frames to All Ports Except the Source Port (b)

Figure 5-14 One of the Three Group Members Sends an IGMP Membership Report, Joining Multicast Group A (a) When the Router Forwards the Multicast Session, the Switch Replicates the Frames to All Ports Except the Source Port (b)

Obviously, the preferable behavior is for the switch to forward the multicast session only out of those ports to which the group members are attached If this can be accomplished, switching is not only more efficient, but also is the preferable way to implement LANs that carry multicast sessions For example, a videoconferencing multicast stream uses approximately 1 Mbps of bandwidth, and an MPEG II video stream can use approximately 4 Mbps If these sessions can be limited to the group members' ports, network and host resources can be conserved

Cisco Group Membership Protocol (CGMP) is designed to do exactly that—distribute multicast sessions only to those switch ports on which group members are located Before examining the operation of CGMP, the next section takes a brief look at some other solutions for regulating switched multicast traffic

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