Do you know your BASL ?

I was talking to a customer the other day about best practice for optimizing their Active Directory Replication. The conversation headed into Site Configuration and I mentioned whether BASL was configured. The response was a blank  look and a comment about Herbology !  so I thought I might be a good idea to explain what BASL is.

Well is is not a herb !. BASL stands for Bridge All Site Links. By default in Windows 2003 sp1 this is switched on by default.Typically this setting can be left switched on as long as the network is fully routable and all DCs can communicate with each other. Where you will start experiencing problems is in a situation where routing is restricted by firewalls VPN Tunnels. If your replication topology requires you to use this then BASL should then be disabled. If it is not disabled you may start seeing errors in the eventid such as 1311. As the KCC will be trying to connect to DCs in remote sites but will be prevented from doing so because of the routing restrictions. There is an excellent article on the Microsoft's Support Web site which talks through optimizing Active Directory replication . BTW this is a setting configured in Sites and Services, properties of Inter site Transports. This therefore would be reserved for Enterprise Administrator level permissions, because of the implications to your replication topology if this is configured incorrectly.