A site link bridge represents a set of site links which are capable of communicating with one another. Usually the site link bridge is a Router 
. A dedicated device or a multihomed computer configured for IP forwarding, which passes data packets from one network to another. or group of routers. Site link bridges maximize replication.
If your network is
fully routed, you do not need to create site link bridges because all site links for a specific protocol are bridged by default.
1) Open Active Directory Sites and Services. Expand Sites
2) Expand inter-site transports. Right-click either IP or SMPT, depending on the protocol for which you want to disable the default bridging of site links.
3) Click Properties.
4) In Properties, on the General tab, clear the Bridge all site links check box. Then click OK.
5) You have returned to Active Directory Sites and Services. Again, expand Sites, then Inter-site Transports.
6) Right-click either IP or SMTP, depending on the protocol for which you want to create a site link bridge. Click New Site Link Bridge.
7) In the New Object-Site Link Bridge dialog, type a name for this new site link bridge in the name box.
8) Click Add. Then click OK. Your sites will be linked.