(Optional) Configuring SPT Switchover Conditions

PIM-SM allows a Rendezvous Point (RP) or a receiver's Designated router (DR) to trigger a shortest path tree (SPT) switchover when the rate of multicast packets is high. You can configure SPT switchover conditions and set the interval for checking the forwarding rate of multicast packets on the receiver's DR.

Context

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

Forwarding multicast data along an RPT has the following disadvantages:

The source's DR and the RP need to frequently encapsulate and decapsulate messages.
The forwarding path may not be the shortest path from the source to receivers.
Heavy data traffic increases the load of the RP, which may cause a fault.

The solutions to this problem are as follows:

SPT switchover triggered by the RP

The RP sends a Join message to the source to create a multicast route along the shortest path from the source's DR to the RP. Then, subsequent multicast packets are directly transmitted along this path.
SPT switchover triggered by the receiver's DR

The receiver's DR checks the forwarding rate of multicast data. If the receiver's DR finds that the rate exceeds the threshold, the receiver's DR triggers the SPT switchover immediately. The receiver's DR then sends a Join message to the source to set up a multicast route along the shortest path from the source's DR to the receiver's DR. Subsequent packets are forwarded along this path.

By default, the RP triggers the SPT switchover immediately after receiving the first Register message and the receiver's DR triggers the SPT switchover immediately after receiving the first multicast data packet. The router can work normally with default control parameters. You are allowed to adjust SPT switchover parameters based on the specific networking environment.

If there is no special requirement, default parameter values are recommended.

Procedure

Run system-view
The system view is displayed.
Configure a basic numbered ACL or a named ACL as needed.
- Configure a basic numbered ACL.
  1. Run acl [ number ] basic-acl-number [ match-order { auto | config } ]
    A basic numbered ACL is created, and the basic numbered ACL view is displayed.
  2. Run rule [ rule-id ] [ name rule-name ] { deny | permit } source { source-ip-address { source-wildcard | 0 } | any }
    Rules are configured for the basic numbered ACL.
- Configure a named ACL.
  1. Run acl name acl-name basic [ match-order { auto | config } ]
    A named ACL is created, and the named ACL view is displayed.
  2. Run rule [ rule-id ] [ name rule-name ] { deny | permit } source { source-ip-address { source-wildcard | 0 } | any }
    Rules are configured for the named ACL.
If a basic numbered ACL is used, run the rule command and set the source parameter to a multicast group range to which the specified rate threshold applies.

If a named ACL is used, run the rule command and set the destination parameter to a multicast group range to which the specified rate threshold applies.
Run quit
Return to the system view.
Run pim [ vpn-instance vpn-instance-name ]
The PIM view is displayed.
Run spt-switch-threshold { traffic-rate | infinity } [ group-policy { basic-acl-number | acl-name acl-name } [ order order-value ] ]
SPT switchover conditions are configured.

This command takes effect on all routers that may function as a receiver's DRs, but does not take effect on the RP.
- traffic-rate: specifies the rate threshold of the multicast data at which the receiver's DR triggers the SPT switchover.
  
  If traffic-rate is specified, Step 4 is required.
- infinity: indicates that the receiver's DR never triggers the SPT switchover. Thus, multicast data can be transmitted to the receiver only along the RPT.
- group-policy basic-acl-number: specifies the range of the multicast groups to which the set rate threshold applies.
- order order-value: adjusts the order of the ACLs in the group-policy list. If a group matches multiple ACLs, the threshold is selected in the order specified by order-value.
- 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.
(Optional) Run timer spt-switch interval
The interval for checking the forwarding rate of multicast packets is set.
Run commit
The configuration is committed.