[This is pre-release documentation and subject to change in future releases. This topic's current status is: Milestone-Ready]

Topic Last Modified: 2010-07-16

This topic provides information about preparing users for branch-site resiliency, preparing for voicemail survivability, and the relevant hardware and software requirements.

Preparing for Branch Users for Branch-Site Resiliency

Prepare users for branch-site resiliency by assigning them to a primary and backup Registrar and configuring voice routing policies and plans.

Registrar Assignments for Branch Users

Whichever branch-site resiliency solution you choose, you will need to assign a primary Microsoft Communications Server 2010 Registrar to each user. Branch site users should always register with the Registrar at the branch office, regardless whether that Registrar resides in the Survivable Branch Appliance, Survivable Branch Server, or stand-alone Communications Server 2010 Standard or Enterprise Edition Server. It is recommended that each user also be assigned a backup Registrar. A DNS SRV record is required to direct users from the primary registrar to the backup registrar. If the Survivable Branch Appliance goes down, branch site users are thereby able to register seamlessly with the backup Registrar.

Voice Routing for Branch Users

Creating a separate user-level VoIP policy for users in a branch site is recommended. This policy should have routes to egress from the Survivable Branch Appliance or Server gateway and backup routes to egress from a gateway in the central site. Regardless of where the user is registered, either on the Survivable Branch Appliance or Server Registrar or the backup Registrar cluster in the central site, the user’s VoIP policy is always in effect.

To ensure that inbound calls to the branch user will work when the Survivable Branch Appliance or Server is not available (for example, if it is down for maintenance), set a backup route on the Survivable Branch Appliance or Server Gateway to point to the backup Registrar pool (running collocated Mediation Server) in the central site. So if the Survivable Branch Appliance or Server Registrar and Mediation Server are not available, and the user is registered with the backup Registrar pool in the data center, inbound calls can still be routed to the user.

We recommend creating a user-level dial plan and assigning it to branch users. For more information, see Create the VoIP Routing Policy for Branch Users, in the Deployment Guide.

Preparing for Voicemail Survivability

Exchange UM is usually installed only in the data center and not in branch sites. A caller should be able to deposit a voicemail message for the callee, even if the WAN from the branch to the data center is unavailable and the Exchange UM server in the data center is therefore not reachable over IP.

Survivable Branch Appliances and Servers provide voicemail survivability for branch users during a WAN outage. Specifically, if you are using a Survivable Branch Appliance or Server, when the WAN is unavailable the Survivable Branch Appliance or Server will re-route unanswered calls over the PSTN to Exchange UM in the data center. The Survivable Branch Appliance or Server will also provide the ability for a user to retrieve voicemail messages through the PSTN during a WAN outage. Lastly, during a WAN outage the Survivable Branch Appliance or Server will queue missed call notifications and then uploads them to the Exchange UM server when the WAN is restored.

To enable this scenario, it is recommended that an Exchange administrator provision an Exchange UM Message Taking Mode Auto Attendant (AA). This is a special type of AA that disables all other generic functionality, such as transfer to a user or transfer to an operator, and limits the AA to only accept messages. Alternatively, the Exchange administrator may choose to use a generic AA, or an AA customized to route the call to an operator.

The Communications Server administrator should set the AA phone number described above as the Exchange UM Auto Attendant phone number in the Survivable Branch Appliance or Server voicemail rerouting configuration.

Similarly, to enable branch users to get to their voicemail messages when the WAN is unavailable, the Communications Server administrator should set the Exchange UM subscriber access number that corresponds to the branch user’s Exchange UM dial plan as the Exchange UM subscriber access number in the Survivable Branch Appliance voicemail rerouting configuration.

The Exchange UM users dial plan should be configured so there is only one dial plan associated with all branch users who need access to the Get Voicemail functionality during a WAN outage.

Hardware and Software Requirements for Branch-Site Resiliency

The hardware and software requirements vary depending on our resiliency solution.

Requirements for Survivable Branch Appliances

Required hardware and software is built into the Survivable Branch Appliance. However, it is also recommended that each branch site deploy a DHCP server to obtain client IP addresses. Otherwise, when the DHCP lease expires, clients will not have IP connectivity.

If the enterprise DNS servers are only located in central sites, they will not be accessible to branch users during a WAN outage and therefore Communications Server discovery using DNS SRV will fail.

Communications Server 2010 clients can discover the Communications Server using DHCP Option 120 (SIP Registrar Option). This can be configured in one of two ways:

  • Configure the DHCP server to reply to DHCP 120 queries, which return the FQDN of the Registrar, OR

  • Enable Communications Server DHCP. When this is enabled the Communications Server Registrar responds to DHCP Option 120 queries. Note that the Communications Server Registrar does not respond to any DHCP queries other than DHCP Options 120, so it is safe to enable Communications Server DHCP.

Additionally, for larger Branch sites that have multiple subnets, DHCP relay agents should be enabled to forward DHCP Option 120 queries to the DHCP Server (configuration 1 above) or the Communications Server Registrar (configuration 2 above).

Finally, branch site users must be enabled for Enterprise Voice and provisioned with an appropriate Unified Communications endpoint.

Requirements for Survivable Branch Servers

The requirements for Survivable Branch Servers are the same as the requirements for any Communications Server server role. For more information, see Determining Your Infrastructure Requirements, in this guide.

Requirements for Full-Scale Communications Server Branch-Site Deployments