PIM-SM Assert Message

PIM-SM Assert Message

To avoid duplicate multicast packets from traversing multi-access networks, PIM-SM uses the Assert message to determine a designated forwarding router for a multi-access network. Figure 7-14 demonstrates the situation that would warrant the Assert mechanism. The steps taken are as follows:

1.   Router A receives multicast traffic.

2.   Routers B and C are PIM-SM neighbors, so the multicast traffic is forwarded to routers B and C.

3.   Router D is a PIM-SM neighbor, so routers B and C forward the traffic onto the ethernet LAN. Assume router B transmits first. Router C receives the multicast packet on an interface that has this group in the oilist. This alerts router C to the fact that a PIM-SM neighbor on the ethernet LAN has forwarded traffic for the group.

4.   Router C forwards the multicast packet to routers B and D. B notices that the packet has arrived on an output interface for the group. Router D really doesn’t care because this router is not forwarding traffic for the group onto the ethernet LAN. Router D has received the same multicast packet twice, a situation that needs to be eliminated.

If a router receives a multicast packet for which it has a state, either (S,G) or (*,G) on an outgoing interface, then the router knows that there is another router forwarding packets onto the network. For example, the serial interfaces for both routers B and C are the RPF interfaces back to the multicast source. When router A receives a packet from the source, the packet is forwarded to both routers B and C. With no other mechanism in place, both routers B and C will forward the traffic to router D, creating duplicate packets on the network. Assert messages are used to avoid this situation.

An Assert message also contains the group address and mask for the multicast source and the router’s metric back to the source (see Figure 7-15). If both routers have an equal metric back to the source, then the router with the highest IP address becomes the forwarder for the network. The router that is not the forwarder prunes the interface.

Figure 7-15: PIM Assert packet format

Back in Figure 7-14, even though router D does not send Assert messages, it must listen to the Assert messages and determine which router is the designated router for the LAN. This information is necessary so that router D knows where to send Prune and Join messages for the group.

The Assert process is straightforward if both routers are running the same IP routing protocol. Recall that PIM-SM uses whatever protocol has been configured on the router to determine the RPF interface and the metric for the RPF interface. For the configuration in Figure 7-16, both routers on the multi-access network are running OSPF and the metrics back to the source are comparable. The OSPF metric is calculated by dividing 100,000,000 by the bandwidth of the link. The metric for the T1 link is approximately 67 and for the 28.8K link the metric is 3472. By comparing the metrics of the two links back to the source, we can easily choose the T1 link because it has a smaller metric than the 28.8K link. If different routing protocols are being utilized, then the metrics cannot be compared.

Figure 7-16: Routers B and C have comparable metrics to the source, so they can be used in an Assert message to elect the designated forwarder.

In Figure 7-17, router B is running OSPF and router C is running RIP. Comparing the metric back to the source for the two routers is like comparing apples and oranges. OSPF uses the speed of the interface to determine the metric and RIP uses a simple hop count. In this case, the metric preference value in the assert packet is used to determine which router will forward traffic and which router will prune the interface. Metric preference is analogous to an administrative distance for a unicast routing protocol. For example, the default administrative distance for RIP is 120 and for OSPF it is 110. Using the defaults always causes an OSPF route to be preferred over a RIP route.

Figure 7-17: Routers B and C have metrics that cannot be compared. The Assert mechanism would use the metric preference to determine the designated forwarder.

Metric preferences can also be configured for each unicast-routing protocol. When PIM-SM receives an Assert message for a group, the metric preference is compared to its own metric preference. If they are equal, then the metrics can be compared to determine which router will forward traffic. If the metric preference values are different, then the router with the lowest metric preference is selected as the forwarder on the network. If we assign a lower metric value for OSPF than for RIP, then the routers on the multi-access network in Figure 7-17 will select the OSPF router to forward traffic, and the RIP router will prune its interface for the group.