Your company has a main office and four branch offices. The main office is located in London.
The network contains an Active Directory domain named contoso.com. Each office contains one domain
controller that runs Windows Server 2012. The Active Directory site topology is configured as shown in
the exhibit. (Click the Exhibit button.)You discover that when a domain controller in a branch office is offline for maintenance, users in that
branch office are authenticated by using the domain controllers in any of the sites.
You need to recommend changes to Active Directory to ensure that when a domain controller in a
branch office is offline, the users in that branch office are authenticated by the domain controllers in
London.
What should you include in the recommendation?
Exhibit

A.
Modify the DC Locator DNS Records settings.

B.
Disable site link bridging.

C.
Modify the site link costs.

D.
Modify the service location (SRV) records in DNS.

Explanation:
If local DC (domain controller) is not available, DC Locator service will look for another DC in a different
site.
Note: The following sequence describes how the Locator is able to find a domain controller (see step 3
below) :
1. On the client (the computer locating the domain controller), the Locator is initiated as an RPC to the
local Net Logon service. The Locator application programming interface (API) (DsGetDcName) is
implemented by the Net Logon service.
2. The client collects the information that is needed to select a domain controller and passes the
information to the Net Logon service by using the DsGetDcName API.3. The Net Logon service on the client uses the collected information to look up a domain controller for
the specified domain in one of two ways:
For a DNS name, Net Logon queries DNS by using the IP/DNS-compatible Locator — that is,
DsGetDcName calls the DnsQuery API to read the Service Resource (SRV) records and A records from
DNS, after it appends an appropriate string to the front of the domain name that specifies the SRV
record.
Etc.
Domain Controller Locator
https://technet.microsoft.com/en-us/library/cc961830.aspx