# About Domain Name System Service (DNS SRV)

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:

## Transport protocol

Select SRV from the Transport protocol drop-down 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 set to a non-SRV value (for example, UDP or TCP or TLS), and MBG uses Fixed Trunking Mode for the Trunk Service Provider. With Fixed Trunking Mode, you have 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.
  - A value too large could cause users to disconnect and abandon their outgoing calls prematurely, assuming that there is no response.
  - A value too small could cause premature usage of an alternative Trunk Endpoint, although the call would have been successful using the original Trunk Endpoint.
  Note: To determine a value appropriate for your SIP service provider, refer to the Mitel SIP Center of Excellence Interop Guide.
- Default value for this setting is 5 seconds.

## DNS SRV auto-registration

Some SRV TRUNK providers use registration to authenticate through the username/password credentials. When registration-based authentication is used, there are two possible scenarios:

1. Registration is performed on any of the service providers SRV TRUNK endpoints and this registration is shared on all endpoints.
2. Registration is required every time a switch to a different SRV TRUNK endpoint occurs (when doing SRV failovers).

DNS SRV auto-registration check box must be selected if registration is required on failover to an alternate SRV TRUNK endpoint.

- When this option is selected MBG will automatically attempt SRV TRUNK registration upon failover scenarios.
- When this option is cleared MBG will NOT attempt SRV TRUNK registration upon failover scenarios.

Note:

1. This check box is only visible when SRV is selected as the transport protocol.
2. Valid username/password registration credentials must be provided and must match those configured on the PBX for this functionality to be usable.

Refer to the SipCOE Interoperability Guide to determine the correct value for the SRV TRUNK provider that you intend to use.

## 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.

Parent topic: SIP Trunking Proxy Service