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The configurations for a Trunking application and Access application are very similar. Both are based on the "Trunk Group" concept. It may seem strange that a set of phones that register with unique IP addresses are considered a trunk group, but it is actually helpful in configuring and debugging the SBC Core.

As for how a set of phones can be considered a trunk group, this is accomplished by the following mechanism:

  1. Phones Register to a SIP Port on the SBC
  2. The SIP Port is in a Zone
  3. The Trunk Group is in the same Zone
  4. The Trunk Group is configured to Require/Support Registration (see below)

The key differences between an Access Configuration and a Trunking Configuration are shown below:

Table : Key Differences between Access and Trunking Configuration


Access Details

Trunking Details

Trunk Group: Registration
Required parameter

On the Trunk group facing the phones (PHONES in our example), the "Registration Required" parameter must be set. You should also set the Expires timer which controls how often the phones will Register. And the Inside Expires timer, which is how often the SBC will Register with the Feature Server (PBX or Class 5 Softswitch) on behalf of the phone. The Inside timer is typically 1 hour, while the phones Registration timer is typically 60 seconds.

Registration is not required, so this parameter is not set.

IP Peer: IP address

The IP Peer for messaging TO the phones is not explicitly configured (indicating to the SBC that it should look in its local stored Registration table to determine the IP
address to use to route the INVITE to the phone). The Routing label in the 'Standard Route' for this Zone should be configured with only the TrunkGroup and no ipPeer.

IP address of the far end.

Ingress IP Prefix

The Ingress IP Prefix (Network Selector Table in GSX 9000 terminology) for messaging FROM the phones is This indicates that no fixed IP address should be required in receiving messages from the phones (the Registration table is checked instead). The Ingress IP Prefix is located under the Trunk Group (which is under the Zone).

IP address of the far end.

Trunk Group: NAT/NAPT Settings

If there is a NAT/NAPT device (Network Address Translation) between the phones and the SBC (common if phones are on the public internet), the NAPT for Signaling flag and the NAPT for Media flag should be turned on. These flags are part of the SIP Trunk group configuration.

Generally there is no NAT on
Trunking configurations.

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