Configuring Transparent Bridging
Technology Overview
BC-24
Cisco IOS Bridging and IBM Networking Configuration Guide
Transparent Bridging Features
Cisco’s transparent bridging software implementation has the following features:
•Complies with the IEEE 802.1D standard.
•Provides the ability to logically segment a transparently bridged network into virtual LANs.
•Provides two Spanning-Tree Protocols—an older bridge protocol data unit (BPDU) format that is
compatible with Digital Equipment Corporation (DEC) and other LAN bridges for backward
compatibility and the IEEE standard BPDU format. In addition to features standard with these
spanning-tree protocols, Cisco’s proprietary software provides for multiple domains for spanning
trees. The spanning-tree parameters are configurable.
•Allows frame filtering based on Media Access Control (MAC) address, protocol type, or the vendor
code. Additionally, the bridging software can be configured to selectively filter local-area transport
(LAT) multicast service announcements.
•Provides deterministic load distribution while maintaining a loop-free spanning tree.
•Provides the ability to bridge over Asynchronous Transfer Mode (ATM), dial-on-demand routing
(DDR), FDDI, Frame Relay, multiprotocol Link Access Procedure, Balanced (LAPB), Switched
Multimegabit Data Service (SMDS), and X.25 networks.
•Provides concurrent routing and bridging, which is the ability to bridge a given protocol on some
interfaces in a router and concurrently route that protocol on other interfaces in the same router.
•Provides integrated routing and bridging, which is the ability to route a given protocol between
routed interfaces and bridge groups, or to route a given protocol between bridge groups.
•Provides fast-switched transparent bridging for Frame Relay encapsulated serial and High-Speed
Serial Interface (HSSI) interfaces, according to the format specified in RFC 1490.
•Provides fas t-switched tra nsparent brid ging for the ATM inter face on the Cis co 7000, Cisco 4500,
and Cisco4000 series routers, according to the format specified in RFC 1483.
•Provides for compression of LAT frames to reduce LAT traffic through the network.
•Provides both bridging and routing of VLANs.
Cisco access servers and routers can be configured to serve as both multiprotocol routers and MAC-level
bridges, bridging any traffic that cannot otherwise be routed. For example, a router routing the Internet
Protocol (IP) can also bridge DEC’s LAT protocol or NetBIOS traffic.
Cisco routers also support remote bridging over synchronous serial lines. As with frames received on all
other media types, dynamic learning and configurable filtering applies to frames received on serial lines.
Transit bridging of Ethernet frames across FDDI media is also supported. The term transit refers to the
fact that the source or destination of the frame cannot be on the FDDI media itself. This allows FDDI to
act as a highly efficient backbone for the interconnection of many bridged networks. The configuration
of FDDI transit bridging is identical to the configuration of transparent bridging on all other media types.
Integrated Routing and Bridging
While concurrent routing and bridging makes it possible to both route and bridge a specific protocol on
separate interfaces within a router, the protocol is not switched between bridged and routed interfaces.
Routed traffic is confined to the routed interfaces; bridged traffic is confined to bridged interfaces. A
specified protocol may be either routed or bridged on a given interface, but not both.