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Example: Configuring the PIM Assert Timeout

Requirements

Before you begin:

• Configure the router interfaces. See the Junos OS Network Interfaces Library for Routing Devices.
• Configure an interior gateway protocol or static routing. See the Junos OS Routing Protocols Library for Routing Devices.
• Configure PIM Sparse Mode on the interfaces. See Enabling PIM Sparse Mode.

Overview

The role of PIM assert messages is to determine the forwarder on a network with multiple routers. The forwarder is the router that forwards multicast packets to a network with multicast group members. The forwarder is generally the same as the PIM DR.

A router sends an assert message when it receives a multicast packet on an interface that is listed in the outgoing interface list of the matching routing entry. Receiving a message on an outgoing interface is an indication that more than one router forwards the same multicast packets to a network.

In Figure 1, both routing devices R1 and R2 forward multicast packets for the same (S,G) entry on a network. Both devices detect this situation and both devices send assert messages on the Ethernet network. An assert message contains, in addition to a source address and group address, a unicast cost metric for sending packets to the source, and a preference metric for the unicast cost. The preference metric expresses a preference between unicast routing protocols. The routing device with the smallest preference metric becomes the forwarder (also called the assert winner). If the preference metrics are equal, the device that sent the lowest unicast cost metric becomes the forwarder. If the unicast metrics are also equal, the routing device with the highest IP address becomes the forwarder. After the transmission of assert messages, only the forwarder continues to forward messages on the network.

When an assert message is received and the RPF neighbor is changed to the assert winner, the assert timer is set to an assert timeout period. The assert timeout period is restarted every time a subsequent assert message for the route entry is received on the incoming interface. When the assert timer expires, the routing device resets its RPF neighbor according to its unicast routing table. Then, if multiple forwarders still exist, the forwarders reenter the assert message cycle. In effect, the assert timeout period determines how often multicast routing devices enter a PIM assert message cycle.

The range is from 5 through 210 seconds. The default is 180 seconds.

Assert messages are useful for LANs that connect multiple routing devices and no hosts.

Figure 1 shows the topology for this example.

Figure 1: PIM Assert Topology

Configuration

Step-by-Step Procedure

1. Configure the timeout period, in seconds.

   [edit protocols pim]user@host# set assert-timeout 60

2. (Optional) Trace assert messages.

   [edit protocols pim]user@host# set traceoptions file PIM.loguser@host# set traceoptions flag assert detail

3. If you are done configuring the device, commit the configuration.

   user@host# commit
4. To verify the configuration, run the following commands:
   • show pim join
   • show pim statistics