PIM-DM Version Protocol Operation

PIM-DM Version 1, Protocol Operation

The source based trees that are constructed in a PIM-DM environment are created in the same manner as DVMRP as shown in Figure 6-4.

In Figure 6-4, router A receives a multicast packet from the source and examines the source IP address of the packet to see if the packet was received on the Reverse Path Forwarding (RPF) interface. The RPF interface is used to send a unicast packet back to the source. Becasuse the source is directly attached to router A, the interface is the RPF.

Figure 6-4: Dynamically created source-based trees

Router A then floods the packet on all interfaces except for the interface on which the packet was received. When router B receives the packet from router A, router B will determine if the packet was received on the RPF interface for the particular source. The packet passes the RPF test and so the packet is forwarded to router C and receiver 1. Router C performs the same RPF on the packet and forwards the packet to router B and receiver 2. When B receives the packet from C and C receives the packet from B, the RPF test fails since the packet was not received on the interface that is on the shortest path back to the source. The packet is then discarded. If we take a close look at Figure 6-4, we can see that we have a source tree for each receiver that connects each receiver to the source.

The RPF interface is selected by examining the IP routing table, an example of which is given in Listing 6-1. From the sample routing table, we can determine the RPF interface for any multicast source. Remember that the multicast source is a unicast class A, B, or C address and not a multicast class D address. For example, if the router receives a multicast packet on the serial 1 interface from the source 130.10.9.1, should the packet be forwarded? By examining the routing table in Listing 6-1 we find that the unicast route back to 130.10.9.1 is through interface serial 0 so the packet did not arrive on the RPF interface. For this case, the multicast packet would be dropped and no further processing would occur. We can determine the RPF interface for each known source network by examining the routing table. Each route listed contains a forwarding interface, which is also the RPF interface. How would the router handle multicast traffic from sources not in the routing table? For this situation the default route would be used.

Listing 6-1: Example Cisco router IP routing table

Codes: C_—_connected, S_—_static, I_—_IGRP, R_—_RIP, M_—_mobile, B_—_BGP

D_—_EIGRP, EX_—_EIGRP external, O_—_OSPF, IA_—_OSPF inter area

E1_—_OSPF external type 1, E2_—_OSPF external type 2, E_—_EGP

i_—_IS-IS, L1_—_IS-IS level21, L2_—_IS-IS level22, *_—_candidate default

Gateway of last resort is not set

I130.10.128.0 255.255.255.0 [100/1115174 ] via 130.10.11.3, 00:00:40, Serial1

C130.10.252.0 255.255.255.0 is directly connected, Loopback0

I130.10.253.0 255.255.255.0 [100/265657 ] via 130.10.11.3, 00:00:40, Serial1

I130.10.246.0 255.255.255.0 [100/1115611 ] via 130.10.11.3, 00:00:40, Serial1

O130.10.8.0 255.255.255.0 [110/2641 ] via 130.10.5.5, 00:12:29, Serial0

O IA 130.10.9.0 255.255.255.0 [110/5268 ] via 130.10.5.5, 00:12:29, Serial0

C130.10.10.0 255.255.255.0 is directly connected, Ethernet0

C130.10.11.0 255.255.255.0 is directly connected, Serial1

I130.10.12.0 255.255.255.0 [100/1115111 ] via 130.10.11.3, 00:00:41, Serial1

I130.10.13.0 255.255.255.0 [100/265257 ] via 130.10.11.3, 00:00:41, Serial1

O IA 130.10.251.251 255.255.255.255 [110/5263 ] via 130.10.5.5, 00:12:33, Serial0

O IA 130.10.250.250 255.255.255.255 [110/2632 ] via 130.10.5.5, 00:12:33, Serial0

O 130.10.5.5 255.255.255.255 [110/2631 ] via 130.10.5.5, 00:12:33, Serial0

O 130.10.5.1 255.255.255.255 [110/5262 ] via 130.10.5.5, 00:12:33, Serial0

C 130.10.5.0 255.255.255.0 is directly connected, Serial0

O IA 130.10.100.0 255.255.255.192 [110/2632 ] via 130.10.5.5, 00:00:13, Serial0

I 193.10.10.0 [100/1115174 ] via 130.10.11.3, 00:00:45, Serial1