By choosing Device > Fallback Bridging, you can configure a fallback bridging group in which the switch bridges together two or more VLANs, routed ports, or both, essentially connecting multiple interfaces into one bridge domain. Use the Fallback Bridging window to create, modify, delete, or view the details of a fallback bridging group.
A VLAN bridge domain is represented by using the switch virtual interface (SVI). A set of SVIs and routed ports (which do not have any VLANs associated with them) can be configured to form a bridge group. Fallback bridging does not allow the spanning-tree instances from the VLANs being bridged to collapse; each VLAN has its own STP instance and a separate spanning-tree instance, called the VLAN-bridge spanning-tree instance, which runs on top of the bridge group to prevent loops.
Bridge groups within the same switch function as distinct bridges; that is, bridged traffic and bridge protocol data units (BPDUs) cannot be exchanged between different bridge groups on a switch. An interface can be a member of only one bridge group. Use a bridge group for each separately bridged (topologically distinct) network connected to the switch.
There are two reasons for defining network interfaces as a bridge group:
"Configuring Fallback Bridging," Catalyst 3550 Multilayer Switch Software Configuration Guide