# About Domain Name System Service

A basic architecture for communication between a service provider and a client involves MiVoice Border Gateway (MBG) with SIP Trunks, normally a single Trunk Endpoint. This works well until the single trunk endpoint experiences an issue, such as being unreachable because of network issues, or has hardware or maintenance issues. During this downtime, the SIP Trunk Provider cannot provide service. Thus the architecture does not provide resiliency. The SIP Trunk Provider can implement resiliency by providing more than one trunk endpoints. When one trunk endpoint encounters an outage, services can be provided by switching to another trunk endpoint. However, in the absence of the DNS SRV feature, the MBG administrator has to manually create a Trunk entry for every Trunk Endpoint and enter the address and port of every Trunk Endpoint.

The function of Domain Name Server Service (DNS SRV) is to automate the provisioning for resiliency. The DNS SRV feature enables you to have multiple trunk endpoints automatically configured to support resiliency. During an outage of one trunk endpoint, the MBG searches for and engages the next available functioning Trunk Endpoint ensuring uninterrupted service.

The following fields have been added to enable this feature:

## DNS SRV support

Select this checkbox to enable MBG to use SRV Trunking Mode for the Trunk Service Provider. When this option is enabled, the Remote Trunk endpoint port and Remote Trunk endpoint address fields are automatically configured and cannot be edited.

By default, this option is not enabled, and MBG uses Fixed Trunking Mode for the Trunk Service Provider. With Fixed Trunking Mode, you will need to manually configure the Remote Trunk endpoint port and Remote Trunk endpoint address fields.

## SRV query domain

Enter the SIP Trunk provider's DNS domain for which the SRV queries are to be performed when using the SIP Trunking Mode. For example, abccompany.com. This field is alphanumeric.

## DNS SRV resiliency timeout

The timer controls the switching between trunk end points when the timer expires. The MBG continues to switch endpoints until the call is abandoned after 32 seconds.

- Valid values for this setting range from 1 second to 32 seconds.

- Default value for this setting is 5 seconds.

## Limitations

Following are limitations the DNS SRV feature has on other functionality of MBG:

- RE-INVITE CONVERSION functionality is not supported when SRV TRUNKs are in use.

- RE-INVITE FILTERING functionality is not supported when SRV TRUNKs are in use.

- Any PBX-side ring response timer, if used, must be set to a higher value than the DNS SRV resiliency timeout value. Because the PBX relies on its ring response timer to determine when to terminate a call attempt, and the value needs to provide a time window sufficiently large to allow MBG to perform resiliency attempts in case of failure or unavailability on the TRUNK side.