Changing the Preference for a Domain Controller

Feel free to disregard this post since it is just a reminder for me because in two occasions now I needed to change the preference for a Domain Controller and I did not have this information handy.

When clients authenticate, the DNS server returns a list of servers. In DNS, service (SRV) records have three values associated with them. For example, in DNS Manager, you would see an SRV record that looks similar to the following:

_ldap._tcp._dc._msdcs.nwtraders.local [0] [100] [389] dc1.nwtraders.local

The numeric values of the middle portion of this display are defined as follows:

  • [0] represents the priority of the record. A client must attempt to contact the target host with the lowest-numbered priority it can reach. Target hosts with the same priority should be tried in an order defined by the weight of the record. The range is 0–65535.
  • [100] represents the weight of the record. Weight determines how records of the same priority will be load balanced. The higher this value, the more likely the client will choose the domain controller identified in this SRV record against which to perform queries. Typically, weights are adjusted depending on the hardware platform of the domain controller. If there are two domain controllers, one with more processing power than the other, the more powerful domain controller can handle more query requests and thus should be given a greater percentage of the total workload.
  • [389] represents the network port on which the service record will listen for activity.

Notes:

  • Exchange Directory Access uses only the weight value to determine which server the client should prefer. Therefore, administrators can use the priority value to control Active Directory load generated by logons, and the weight value to control Active Directory load generated by Exchange. A higher weight results in a higher probability that Directory Access will choose a server. Directory Access treats a weight of 0 the same as it treats a weight of 1. If Directory Access cannot read the weight, it uses a default weight of 100. 
  • The LDAP weight value determines the percentage of clients (not queries) which will discover a domain controller. This percentage is equal to the LDAPSrvWeight of the server, divided by the combined LDAPSrvWeight of all domain controllers in the site with the same priority.

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Posted by Gabriel Maciel

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