A bridge is a way to connect two Ethernet segments together in a protocol independent way. Packets are forwarded based on Ethernet address, rather than IP address (like a router). Since forwarding is done at Layer 2, all protocols can go transparently through a bridge. (reference).
You can create a bridge in cobbler in the following way:
You can specify any bridging options you would like, so please read the brctl manpage for details if you are unfamiliar with bridging.
NOTE You must install the bridge-utils package during the build process for this to work in the %post section of your build.