I reviewed the AD configuration at a customer’s lately and had to deal the option “Bridge all site links”.
You can find the option in the Active Directory Sites and Services configuration.
Here is my brief summary for you:
Site Link Bridges
In the default configuration the option Bridge all site links is activated.
To check the option go to:
Active Directory Sites and Services \Inter-Site Transports\IP > Properties
This option allows the ISTG (inter site topology generator) to built automatic replication-connections among all sites. All site links are transitive for the ISTG. It means that connections between sites which do not have a direct site link can be build. This Active Directory Sites and Services configuration is valid for the whole forest and thus binding for all domains of the forest.
Example for a transitive site link with the option [Bridge all site links] activated:
Should the above mentioned option be deactivated, a replication-connection between Site A and C is impossible. Only after having created a Site Link between them the ISTG can build a connection.
When is it useful or necessary to deactivate the option “Bridge all site links”?
The ISTG assumes that all nets with domain controllers are fully routed. Furthermore, the communication between these must be limited by a firewall. Every domain controller can reach any other.
As soon as this rule is not valid anymore, the option “Bridge all site links” must to be deactivated to prevent replication problems.
Let’s expand the example by a Site D which stands behind a firewall and can only communicate with Site B. Under certain circumstances ISTG would create a connection between Site D and C which would not work because of the firewall. Deactivating the option “Bridge all site links” would solve this problem but create a new one because a connection between A and C would not be possible anymore.
So that people do not have to worry about whether there are Site Links between all necessary sites or not, so-called ‘Site Link Bridges’ were introduced. These work similar to the option “Bridge all site links”, except that certain sites have to be connected transitively. In our case we would create a ‘Site Link Bridge’ between A, B and C.
Like this, the ISTG can build a connection between A and C without an existing site link as the connections between A-B and B-C are transitive for it.