Understanding Trunk Precedence and Trunk Group Preference for SRX Series Integrated Convergence Services

This topic explains the order in which trunks are selected to route a call. It covers the relationship between trunk group preference and trunk precedence.

Trunk groups are discrete, reusable entities. You create them independent of route patterns, just as you create trunks independent of trunk groups. You can add multiple trunk groups to a route pattern in your dial plan, each of which can contain multiple trunks.

If a route pattern contains multiple trunk groups, you can assign preference numbers to them. The trunk group with the lowest preference number is tried first to route a call that matches the route pattern. You can change the order in which trunk groups are tried by changing their preference numbers. For example, you might add a trunk group a few weeks after you created the route pattern and want it tried first. For that to occur, you could give the new trunk group a preference of 1.

The trunks of a route pattern are tried to route a call in order that includes preference and precedence:

• The trunks of the trunk group with the lowest (preference) number are tried first. If you do not use preference numbers, the trunk groups are tried in the order in which you added them to the dial plan.

  Note: Zero (0) has no meaning within the context of trunk group preference. A trunk group that you assign it to is the last one used.

• The trunks of a trunk group are tried in the order in which you added them to the trunk group. The first trunk that you added to the trunk group takes precedence, and so on through the list of trunks.
• If all trunks in the trunk group with the lowest preference number are busy, the trunk group with the next lowest preference number is tried. Its trunks are tried in the order in which you added them to the trunk group, and so on until an available trunk is identified and the call is routed. If all trunks of all trunk groups are unavailable to route the call, the following message is played:

  _{“We are sorry. All circuits are busy now. Please try again later.”}

Consider this simple example. A route pattern for long-distance calls includes the following two trunk groups. The trunks of these trunk groups are used to route called numbers that match a long-distance digit pattern. Trunk Group A is preferred because it has the lower preference number (1).

							Trunk Group A (preference 1) contains two trunks:
						

trunk1

trunk2

							Trunk Group B (preference 2 ) contains two trunks:
						

trunk9

trunk4

The system attempts to route the first call through trunk1 of Trunk Group A. The trunk is busy, so it tries trunk2. Trunk2 is available, and the call is routed.

The system attempts to route the second call through trunk1 of Trunk Group A. The trunk is busy, so it tries trunk2. Trunk2 is busy. Next it tries trunk9 of Trunk Group B because trunk9 is the first trunk specified. Trunk9 is available and the call is routed.

Related Topics

• Understanding Trunk Reusability for SRX Series Integrated Convergence Services
• Understanding Integrated Convergence Services Media Gateway Trunks and Dial Plans
• Understanding SRX Series Integrated Convergence Services Media Gateway Trunks