Q. What is redundant connection mode in Active Directory (AD)? A. Normally, only one replication object is created per namespace between sites, which achieves the most efficient replication. In situations in which branch offices all connect to a hub location, if a domain controller (DC) at the hub goes down, all the remote locations must recalculate replication objects. This results in a huge amount of changes, which when the DC is back, won't fail back. This mode requires two steps: Step one is to enable the redundant connection mode to have two connection objects to the hub location; the second step is to disable detection of failed connection objects because you're assuming a failed DC will be coming back so no need to modify the connection objects. You need to run the commands on all remote locations that will require the redundant connections.

C:\>repadmin /siteoptions /site:Austin +IS_REDUNDANT_SERVER_TOPOLOGY_ENABLED                              Branch1                              Current Site Options: (none)                              New Site Options: IS_REDUNDANT_SERVER_TOPOLOGY_ENABLED                              C:\>repadmin /siteoptions /site:Austin +IS_TOPL_DETECT_STALE_DISABLED                              Branch1                              Current Site Options: IS_REDUNDANT_SERVER_TOPOLOGY_ENABLED                              New Site Options: IS_TOPL_DETECT_STALE_DISABLED IS_REDUNDANT_SERVER_TOPOLOGY_ENABLED                              
— John Savill