Shared Distribution Tree
In Figure 20-15, the group traffic that flows over the path from the RP (R3) to R5 to R4 is called a shared distribution tree. It is also called a root-path tree (RPT) because it is rooted at the RP. If the network has multiple sources for the same group, traffic from all the sources would first travel to the RP (as shown with the traffic from host S1 in Figure 20-14), and then travel down this shared RPT to all the receivers. Because all sources in the multicast group use a common shared tree, a wildcard notation of (*,G) is used to identify an RPT, where * represents all sources and G represents the multicast group address. The RPT for the group 228.8.8.8 shown in Figure 20-14 would be written as (*,228.8.8.8). Example 20-7 shows the multicast route table entry for R4 in Figure 20-15. On a Cisco router, the show ip mroute command displays the multicast route table entries.
Example 20-7 Multicast Route Table Entry for the Group 228.8.8.8 for R4
The interpretation of the information shown in Example 20-7 is as follows:
■ The first line shows that the (*,G) entry for the group 228.8.8.8 was created 8 seconds ago, and if R4 does not forward group packets using this entry in 2 minutes and 58 seconds, it will expire. Every time R4 forwards a packet, the timer is reset to 3 minutes. This entry was created because R4 received an IGMP Join message from H1.
■ The RP for this group is 10.1.10.3 (R3). The S flag indicates that this group is using the sparse-mode (PIM-SM) routing protocol. The C flag indicates that R4 has a directly connected group member for 228.8.8.8.
■ The incoming interface for this (*,228.8.8.8) entry is s0 and the RPF neighbor is 10.1.6.5. Note that for the SPT, the RPF interface is chosen based on the route to reach the RP, not the route used to reach a particular source.
■ Group traffic is forwarded out on the Ethernet0 interface. In this example, Ethernet0 was added to the outgoing interface list because an IGMP Report message was received on this interface from H1. This interface has been in the forwarding state for 8 seconds. The Prune timer indicates that if an IGMP Join is not received again on this interface within the next 2 minutes and 52 seconds, it will be removed from the outgoing interface list.
Example 20-7 Multicast Route Table Entry for the Group 228.8.8.8 for R4
The interpretation of the information shown in Example 20-7 is as follows:
■ The first line shows that the (*,G) entry for the group 228.8.8.8 was created 8 seconds ago, and if R4 does not forward group packets using this entry in 2 minutes and 58 seconds, it will expire. Every time R4 forwards a packet, the timer is reset to 3 minutes. This entry was created because R4 received an IGMP Join message from H1.
■ The RP for this group is 10.1.10.3 (R3). The S flag indicates that this group is using the sparse-mode (PIM-SM) routing protocol. The C flag indicates that R4 has a directly connected group member for 228.8.8.8.
■ The incoming interface for this (*,228.8.8.8) entry is s0 and the RPF neighbor is 10.1.6.5. Note that for the SPT, the RPF interface is chosen based on the route to reach the RP, not the route used to reach a particular source.
■ Group traffic is forwarded out on the Ethernet0 interface. In this example, Ethernet0 was added to the outgoing interface list because an IGMP Report message was received on this interface from H1. This interface has been in the forwarding state for 8 seconds. The Prune timer indicates that if an IGMP Join is not received again on this interface within the next 2 minutes and 52 seconds, it will be removed from the outgoing interface list.
