To bring up the second BRI (Bearer Resource Identifier) at 50 percent load, you would use the isdn command in the context of a router configuration. The specific command typically looks like this: isdn outgoing call-setup delay 0. Additionally, you may need to configure the load balancing settings to ensure that the second BRI is activated when the load reaches 50 percent. The exact command can vary depending on the router's operating system, so consult the appropriate documentation for your device.