Bridging is one of the most flexible methods of interconnecting network segments. Bridging is necessary when non-routable protocols like Network Basic Input/Output System (NetBIOS) and Local-Area Transport (LAT) are used to communicate between devices connected across different segments. Bridging can also be configured for routable protocols instead of being routed.
These are the steps involved in configuring bridging across ISDN:
Define the protocols to bridge. IP packets are routed by default unless they are explicitly bridged. All others are bridged by default unless they are explicitly routed. To bridge IP packets, disable IP routing by issuing the no ip routing global configuration command.
Issue the bridge-group interface sub-command to assign an interface to a bridge group. Frames are bridged only among interfaces in the same group.
To specify the type of spanning tree bridging protocol and also to identify a bridge group, issue the bridge protocol global configuration.
To identify all transparent bridge packets as interesting, issue the dialer-list protocol global configuration.
The final step is to configure the destination by issuing the dialer string interface sub-command for unauthenticated calls, or by issuing the dialer mapbridge interface sub-command when authentication is used.
Note: For the bridge across ISDN to function properly, the underlying Dial-on-Demand Routing (DDR) connection must be functional and be aware that bridging on an ISDN connection tends to keep the connection active for very long periods, if not permanently.
For sample configurations, refer to these documents: