# PIM Virtual Router Settings

Protocol Independent Multicast (PIM) is a multicast routing protocol that runs between routers. Whereas the Internet Group Management Protocol (IGMP) runs between hosts and routers to exchange multicast group membership information, PIM runs between routers to forward multicast traffic to multicast group members throughout the network.

NetScreen devices support the following PIM modes:

- PIM-Sparse Mode (PIM-SM) forwards multicast traffic only to those receivers that request it. Routers running PIM-SM can use the shared path tree or shortest path tree to forward multicast information.

- PIM-Source Specific Multicast Mode (PIM-SSM) is derived from PIM-SM. Like PIM-SM, it forwards multicast traffic to interested receivers only. Unlike PIM-SM, it immediately forms the shortest path tree to the source. It does not use the shared path tree to distribute multicast traffic.

To configure either PIM-SM or PIM-SSM, you must create a PIM routing instance on a specific virtual router.

### To Create or Edit a PIM Routing Instance

1. Enter the necessary information:

Protocol PIM: Select Enable to enable PIM on the virtual router.

Threshold to Switch from RPT to SPT: PIM-SM uses the shared distribution tree with a Rendezvous Point (RP) at the root of the tree. The threshold indicates when receivers can form the shortest path tree ( SPT ) to the source, decreasing the time it takes the receivers to receive the multicast data. By default, NetScreen devices switch to the shortest path tree (SPT) upon receiving the first byte.

Always RPT: Select this option if you want the NetScreen device to always forward multicast traffic to the RP in the shared distribution tree.

Set Threshold at BPS: Enter the rate (in bytes per second) of the multicast traffic that triggers the NetScreen device to switch from the shared distribution tree to the SPT. By default, NetScreen devices switch to the SPT upon receiving the first byte from the RP.

Restore to Default: Select this option if you want the NetScreen device to switch to the SPT at the default threshold, which is one byte.

Access Group: Select the access list that identifies the multicast group(s) for which the virtual router processes PIM messages.

2. Click OK to save your changes and return to the Virtual Router Basic Configuration page. Click Apply to continue configuring the PIM routing instance.