The second parameter for determining the active route in Juniper Networks' three aggregate routes with the same route preference.

B

In Juniper Networks routing, when multiple routes are available for a destination, the router uses a set of criteria to determine which route to use as the active or preferred route. The first parameter used is the route preference, which is a value assigned to each route that indicates its relative preference over other routes. If multiple routes have the same preference value, the router then uses the second parameter to determine the active route.

In the case of three aggregate routes with the same preference, the second parameter used to determine the active route is the highest metric value. The metric value is a measurement of the cost or distance to the destination network, and routes with lower metric values are preferred over routes with higher metric values.

Therefore, in this scenario, the route with the lowest metric value will be selected as the active route. If two or more routes have the same lowest metric value, the router will use the third parameter (i.e., the route with the lowest IP address) to break the tie.

The other answer options listed in the question, i.e., secondary route preference and lowest community value, are not used as parameters to determine the active route when multiple routes have the same preference value. Route preference and metric values are the primary factors used in Juniper Networks routing to determine the best path to a destination network.