Configuring the SPT Switchover

A high volume of multicast data traffic increases the load of a Rendezvous Point (RP), and may result in a fault. To solve this problem, PIM-SM allows the RP or the DR at the group member side to trigger the shortest path tree (SPT) switchover when the rate of multicast packets is high.

Prerequisites

Before configuring the SPT switchover, complete the following tasks:

Configuring a unicast routing protocol
Configuring Basic PIM-SM Functions

Context

In a PIM-SM network, each multicast group corresponds to a rendezvous point tree (RPT). At first, all multicast sources encapsulate data in Register messages, and send them to the Rendezvous Point (RP) in the unicast mode. The RP decapsulates the messages and forwards them along the RPT.

Forwarding multicast data by using the RPT has the following defects:

The DR at the source side and the RP need to encapsulate and decapsulate packets.
Forwarding path may not be the shortest path from the source to receivers.
Large-volume data flow increases the load of the RP, and may cause a fault.

The solution to the preceding defects is that:

SPT switchover triggered by the RP: The RP sends a Join message to the source, and establishes a multicast route along the shortest path from the source to the RP. The subsequent packets are forwarded along the path.
SPT switchover triggered by the DR at the member side: The DR at the member side checks the forwarding rate of multicast data. If the DR finds that the rate exceeds the threshold, the DR triggers the SPT switchover immediately. The DR sends a Join message to the source, and establishes a multicast route along the shortest path from the source to the DR. The subsequent packets are forwarded along the path.

Procedure

Access the system view.
system-view
Configure a basic ACL.
1. Run the acl [ number ] acl-number [ vpn-instance vpn-instance-name ] command to create a basic ACL and access its view.
2. Run the rule [ rule-id ] { deny | permit } source { source-ip-address { 0 | source-wildcard } | address-set address-set-name | any } command to configure rules for the basic ACL.
  
  Use parameter source to define the multicast group range to which the specified rate threshold applies.
Return to the system view.
quit
Access the PIM view.
pim [ vpn-instance vpn-instance-name ]
Set the SPT switchover condition.
spt-switch-threshold { traffic-rate | infinity } [ group-policy basic-acl-number [ order order-value ] ]
- traffic-rate: specifies the rate threshold of the SPT switchover.
- infinity: indicates that the SPT switchover is not triggered forever.
- group-policy basic-acl-number [ order order-value]: specifies the range of the multicast groups that use the threshold. By default, the threshold is applicable to all multicast groups.
- If no ACL is configured, the device applies the specified rate threshold to all multicast groups.
- If an ACL is configured, the device uses configured ACL rules to determine whether to apply the specified rate threshold to a multicast group.
  - If a multicast group matches an ACL rule and the action is permit, the device applies the specified rate threshold to this group.
  - If a multicast group matches an ACL rule and the action is deny, the device performs an SPT switchover immediately after the SPT switchover condition is met.
  - If a multicast group does not match any ACL rule or if the specified ACL does not exist or does not contain rules, the device performs an SPT switchover immediately after the SPT switchover condition is met.
Set the interval for checking the forwarding rate of multicast data.
timer spt-switch interval