From Adam Gordon:
"I hope all is well. The easiest way to answer your question is to briefly define the function and differences between a Site Link and a Site Link Bridge, as noted below:
Site Links: Site links are logical connections that are established between sites is Active Directory that define a path between these sites. A site link defines the direction of Active Directory replication between sites. You can use either RPC over IP or SMTP as the transport protocol for moving replication data over a site link.
Site link bridge: In Active Directory, you can use a site link bridge to link sites that share common Active Directory data but who do not have a site link. The data typically shared by these sites is the Application directory partition. By default, site links for the same IP transport that have sites in common are bridged, which enables the KCC to treat the set of associated site links as a single route. If you categorically do not want the KCC to consider some routes, or if your network is not fully routed, you can disable automatic bridging of all site links. When this bridging is disabled, you can create site link bridge objects and manually add site links to a bridge.
Bridging Site Links Manually
If your IP network is composed of IP segments that are not fully routed, you can disable Bridge all site links for the IP transport. In this case, all IP site links are considered nontransitive, and you can create and configure site link bridge objects to model the actual routing behavior of your network. A site link bridge has the effect of providing routing for a disjoint network (networks that are separate and unaware of each other). When you add site links to a site link bridge, all site links within the bridge can route transitively.
A site link bridge object represents a set of site links, all of whose sites can communicate through some transport. Site link bridges are necessary if both of the following conditions are true:
A site contains a domain controller that hosts a domain directory partition that is not hosted by a domain controller in an adjacent site (a site that is in the same site link).
That domain directory partition is hosted on a domain controller in at least one other site in the forest.
Site link bridge objects are used by the KCC only when the Bridge all site links setting is disabled. Otherwise, site link bridge objects are ignored.
You create a site link bridge object for a specific transport by specifying two or more site links for the specified transport.
Requirements for manual site link bridges
Each site link in a manual site link bridge must have at least one site in common with another site link in the bridge. Otherwise, the bridge cannot compute the cost from sites in one link to the sites in other links of the bridge. If bridgehead servers that are capable of the transport that is used by the site link bridge are not available in two linked sites, a route is not available.
Hopefully that helps.
Please let me know if you need more clarification, or if you have any additional questions. "
**if the post above has answered the question, please mark the topic as solved.