Member and Nonmember Sources

You might have noticed that so far nothing has been said about how sources deliver their traffic to the core In many multicast applications, a sender also is a group member CBT takes advantage of this fact, so a sender that is also a group member—a member source—can reach the core by virtue of the fact that its directly connected router is on-tree Figure 5-40 illustrates this concept Here, the host labeled SGI is a member source of group 1 Because the host is a group member, its local router has already joined the CBT tree for group 1 Therefore, when SGI sources packets for group 1, the local router can forward the packets up the tree

Figure 5-40 SGI Is a Member Source for Group 1 Its Local Router Has Joined the Group I Tree and Forwards Packets up the Tree Toward the Source

CBT tree

Source traffic

Core

-ipp

A fundamental characteristic of CBT is described in this behavior Namely, CBT uses bidirectional trees In other words, multicast traffic can not only travel downstream on the tree from the core to group members, but it also can travel upstream on the tree from a member source to the core This is in contrast to the other shared-tree protocol, PIM-SM, which uses unidirectional trees

Of course, not all sources are group members Therefore, CBT also must have a mechanism for accommodating these nonmember sources The mechanism is a simple IP-in-IP tunnel, as shown in Figure 5-41 Here, the same host is originating multicast traffic for group 1, but the host itself is not a member of the group When its local router receives the traffic, it creates a tunnel to the core (assuming the router is running CBT and therefore knows the address of the core) The multicast traffic is then unicast to the core, which passes the traffic onto the group tree

Figure 5-41 If the Source Host Is Not a Group Member, Its Local CBT Router Encapsulates the Source Traffic in an IP-in-IP Tunnel and Unicasts the Traffic to the Core

Figure 5-41 If the Source Host Is Not a Group Member, Its Local CBT Router Encapsulates the Source Traffic in an IP-in-IP Tunnel and Unicasts the Traffic to the Core

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