Cisco 3845 Software Guide - Page 768

Configuring Advanced PIM Features Chapter 33 Configuring IP Multicast Routing 8. The RP deletes the link to Router C from the outgoing interface of (S,G). The RP triggers a prune message toward the source. Join and prune messages are sent for sources and RPs. They are sent hop-by-hop and are processed by each PIM device along the path to the source or RP. Register and register-stop messages are not sent hop-by-hop. They are sent by the designated router that is directly connected to a source and are received by the RP for the group. Multiple sources sending to groups use the shared tree. You can configure the PIM device to stay on the shared tree. For more information, see the "Delaying the Use of PIM Shortest-Path Tree" section on page 33-34. Delaying the Use of PIM Shortest-Path Tree The change from shared to source tree happens when the first data packet arrives at the last-hop router (Router C in Figure 33-5). This change occurs because the ip pim spt-threshold global configuration command controls that timing. The shortest-path tree requires more memory than the shared tree but reduces delay. You might want to postpone its use. Instead of allowing the leaf router to immediately move to the shortest-path tree, you can specify that the traffic must first reach a threshold. You can configure when a PIM leaf router should join the shortest-path tree for a specified group. If a source sends at a rate greater than or equal to the specified kbps rate, the multilayer switch triggers a PIM join message toward the source to construct a source tree (shortest-path tree). If the traffic rate from the source drops below the threshold value, the leaf router switches back to the shared tree and sends a prune message toward the source. You can specify to which groups the shortest-path tree threshold applies by using a group list (a standard access list). If a value of 0 is specified or if the group list is not used, the threshold applies to all groups. Beginning in privileged EXEC mode, follow these steps to configure a traffic rate threshold that must be reached before multicast routing is switched from the source tree to the shortest-path tree. This procedure is optional. Step 1 Step 2 Command configure terminal access-list access-list-number {deny | permit} source [source-wildcard] Purpose Enter global configuration mode. Create a standard access list. • For access-list-number, the range is 1 to 99. • The deny keyword denies access if the conditions are matched. The permit keyword permits access if the conditions are matched. • For source, specify the multicast group to which the threshold will apply. • (Optional) For source-wildcard, enter the wildcard bits in dotted decimal notation to be applied to the source. Place ones in the bit positions that you want to ignore. Recall that the access list is always terminated by an implicit deny statement for everything. 33-34 Cisco ME 3800X and 3600X Switch Software Configuration Guide OL-23400-01