HP Survivable Branch Communication zl Module powered by Microsoft Lync Planning and Design Guide 2011-02

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Design Considerations
Planning for High Availability
This configuration ensures that branch users have access to Lync services in
case they cannot reach the SBM.
It is recommended that you use least cost routing with the SBM. That is, you
create routes that direct calls through the SBM or PSTN gateway for which
those calls are local, when possible. However, you should always create a
route with a low priority that directs all calls through the local SBM, which
allows the SBM to continue to forward non-local calls in failover situations.
DNS Requirements
The SBM requires a DNS server to resolve its Media Gateway fully qualified
domain name (FQDN) to an IP address. If the DNS server is at the data center,
and the WAN connection fails, the SBM can continue to route calls through
the PSTN connection only until its DNS entries time out (about 15 minutes).
Therefore, you must make DNS services available at the branch site. Prefera-
bly, you should deploy a DNS server at the branch. The SBM can specify this
DNS server as its primary or secondary server.
At very small branches (fewer than 100 users) without a DNS server, you can
enable DNS services on the SBM itself. Log in to the SBM’s remote desktop
(the HP Survivable Branch Communications zl Module powered by Micro-
soft Lync
TM
Administrator’s Guide explains how). Then follow standard
Microsoft guidelines to add the DNS service to the Windows Server 2008
running on the SBM. When you configure the DNS server, make it a secondary
server in the zone. You might need to adjust the domain DNS server settings
to allow entries to be replicated to the SBM’s server.