Configuring Multicast Forwarding Cache Properties for Flow Maps

The forwarding-cache statement enables you to configure the forwarding cache properties of entries defined by a flow map. You can specify a timeout of never to make the forwarding entries permanent, or you can specify a timeout in the range from 1 through 720 minutes. If you set the value to never, you can specify the non-discard-entry-only option to make an exception for entries that are in the pruned state. The never non-discard-entry-only statement allows entries in the pruned state to time out while entries in the forwarding state never time out.

You can use longer timeouts or permanent forwarding cache entries in source redundancy scenarios to decrease the time delay inherent in switching from one source to another. With source redundancy, only one source (for example, s1) actively sends traffic at any given time, but the sources can switch. This means that the forwarding state for another source (for example, s2) can time out.

Even though the Routing Engine has the corresponding PIM states set up, when the first (s2,g) packet arrives on each routing device after a switchover from source s1 to source s2, the routing device must reinstall the forwarding path (s2,g) if the path has timed out. If many data streams exist, the switchover can take considerable time. Using longer timeouts or permanent cache entries helps reduce the inherent switchover delay.

To configure the multicast forwarding cache timeout for a specified flow map, include the forwarding-cache statement:

You can include this statement at the following hierarchy levels:

• [edit logical-systems logical-system-name routing-instances routing-instance-name routing-options multicast flow-map flow-map-name]
• [edit logical-systems logical-system-name routing-options multicast flow-map flow-map-name]
• [edit routing-instances routing-instance-name routing-options multicast flow-map flow-map-name]
• [edit routing-options multicast flow-map flow-map-name]

For an overview of logical systems and a detailed example of logical system configuration, see the Junos Logical Systems Feature Guide.

Note: The permanent forwarding state must exist on all routing devices in the path for fast source switchover to function properly.

For information about configuring multicast flow maps, see Creating a Multicast Flow Map.