Introduction Before You Begin
Conventions
Prerequisites
Components Used Bridging Transparent Bridging Configuration Examples
Example 1: Simple Transparent Bridging
Example 2: Transparent Bridging with Multiple Bridge Groups
Example 3: Bridging Over a Wide Area Network
Example 4: Remote Transparent Bridging Over X.25
Example 5: Remote Transparent Bridging Over Frame Relay With No Multicast
Example 6: Remote Transparent Bridging Over Frame Relay With Multicast
Example 7: Remote Transparent Bridging Over Frame Relay With Multi Sub-Interfaces
Example 8: Remote Transparent Bridging Over Switched Multimegabit Data Service (SMDS)
Example 9: Remote Transparent Bridging With Circuit Group Related Information
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Design 1: Simple, but to be avoided...
Design 2: More complex, but safer and more efficient... Guidelines
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Introduction
The purpose of this document is to help you configure transparent bridging. This document starts with a
general description of bridging, and provides more detailed information about transparent bridging, as well as
several configuration examples.
Before You Begin
Conventions
For more information on document conventions, see the Cisco Technical Tips Conventions.
Prerequisites
There are no specific prerequisites for this document.
Components Used
This document is not restricted to specific software and hardware versions.
The information presented in this document was created from devices in a specific lab environment. All of the
devices used in this document started with a cleared (default) configuration. If you are working in a live
network, ensure that you understand the potential impact of any command before using it.
Bridging
Bridges connect and transfer data between LANs. The following are four kinds of bridging:
Transparent bridging - found primarily in Ethernet environments, and is mostly used to bridge
networks which have the same media types. Bridges keep a table of destination addresses and
outbound interfaces.
Source-Route Bridging (SRB) - found primarily in Token Ring environments. Bridges only
forward frames based on the routing indicator contained in the frame. Endstations are responsible for
determining and maintaining the table of destination addresses and routing indicators. For more
information, refer to Understanding and Troubleshooting Local Source-Route Bridging.
Translational bridging - used to bridge data between different media types. This is typically used to
go between Ethernet and FDDI or Token Ring to Ethernet.
Source-Route Translational Bridging (SR/TLB) - a combination of source-route bridging and
transparent bridging that allows communication in mixed Ethernet and Token Ring environments.
Translational bridging without routing indicators between Token Ring and Ethernet is also called
SR/TLB. For more information, refer to Understanding and Troubleshooting Source-Route
Translational Bridging.
Bridging happens at the data-link layer, which controls data flow, handles transmission errors, provides
physical addressing, and manages access to the physical medium. Bridges analyze incoming frames, make
forwarding decisions based on those frames, and forward the frames to their destinations. Sometimes, such as
in SRB, the frame contains the entire path to the destination. In other cases, such as in transparent bridging,
frames are forwarded one hop at a time toward the destination.
Bridges can be either remote or local. Local bridges provide direct connections between many LAN segments
in the same area. Remote bridges connect LAN segments in different areas, usually over telecommunication
lines.
Transparent Bridging
The Spanning Tree Algorithm (STA) is a vital part of transparent bridging. The STA is used to discover a
loop-free subset of the network's topology dynamically. To do this, the STA places bridge ports that create
loops, when active, into a standby, or blocking, condition. Blocking ports can be activated if the primary port
fails, so they provide redundant support. For more information, refer to the IEEE 802.1d specification.
The Spanning Tree calculation occurs when the bridge is powered up and whenever a topology change is
detected. Configuration messages called Bridge Protocol Data Units (BPDUs) trigger the calculation. These
messages are exchanged at regular intervals, usually one to four seconds.
The example below shows how this works.
If B1 were the only bridge, things would work fine, but with B2, there are two ways to communicate between
the two segments. This is called a bridging loop network. Without the STA, a broadcast from a host from
LAN1 is learned by both bridges, and then B1 and B2 send the same broadcast message to LAN2. Then, both
B1 and B2 think that that host is connected to LAN2. In addition to this basic connectivity problem, broadcast
messages in networks with loops can cause problems with the bandwidth of the network.
With the STA, however, when B1 and B2 come up, they both send out BPDU messages which contain
information that determines which one is the root bridge. If B1 is the root bridge, it becomes the designated
bridge to both LAN1 and LAN2. B2 will not bridge any packets from LAN1 to LAN2, since one of its ports
will be in blocking status.
If B1 fails, B2 does not receive the BPDU that it expects from B1, so B2 sends out a new BPDU that starts the
STA calculation again. B2 becomes the root bridge, and traffic is bridged by B2.
Cisco's transparent bridging software has the following features:
Complies with the IEEE 802.1d standard.
Provides two STPs, the IEEE standard BPDU format and the old format known as DEC, which is
compatible with digital and other LAN bridges for backward compatibility.
Filters based on Media Access Control (MAC) address, protocol type, and vendor code. ·
Groups serial lines into circuit groups for load balancing and redundancy. ·
Provides the ability to bridge over X.25, Frame Relay, Switched Multimegabit Data Service (SMDS),
and Point-to-Point Protocol (PPP) networks.
Provides for compression of Local Area Transport (LAT) frames.
Allows for interfaces to be treated as a single logical network for IP, IPX, and so on, so that bridge
domains can communicate with routed domains.
Configuration Examples
These configurations show only the commands required for transparent bridging, not for IP or other protocol
support.
Example 1: Simple Transparent Bridging
In this example, there are several PCs on LAN1, which is located on one floor. LAN2 also has many PCs and
some servers, but it is on a different floor. The systems on each LAN use either IP, IPX, or DECNET. Most of
the traffic can be routed, but there are some application systems that were developed with proprietary
protocols and can't be routed. This traffic (such as NetBIOS and LAT) must be bridged.
Note: Prior to Cisco IOS Software version 11.0, a protocol could not be both bridged and routed in the same
router. As of Cisco IOS Software version 11.0, a protocol may be bridged on some interfaces and routed on
others. This is called Concurrent Routing and Bridging (CRB). However, the bridged and routed interfaces
cannot pass traffic to each other. As of Cisco IOS Software version 11.2, you may bridge and route protocols
simultaneously and pass traffic from the bridged interfaces to the routed interfaces and vice versa. This is
known as Integrated Routing and Bridging (IRB).
In this example, the IEEE 802.1d standard is the STP. If every bridge in the network is Cisco, issue the
command bridge 1 protocol ieee on all of the routers. If there are different bridges in the network and these
bridges are using the old bridging format that was first developed at DEC, issue the bridge 1 protocol dec
command to assure backward compatibility. Since the IEEE and DEC Spanning Trees are not compatible,
mixing these protocols in the network give unpredictable results.
Example 2: Transparent Bridging with Multiple Bridge Groups
In this example, the router acts as two different bridges, one between LAN1 and LAN2, and one between
LAN3 and LAN4. Frames from LAN1 are bridged to LAN2, however, not to LAN3 or LAN4, and vice versa.
In other words, frames are bridged only among interfaces in the same group. This grouping feature is
commonly used to separate networks or users.
This example uses the same topology as Example 3, however, instead of the lease line that connects the two
routers, RouterA and RouterB are connected through an X.25 cloud.
Example 5: Remote Transparent Bridging Over Frame Relay With NoMulticast
This example uses the same topology as Example 3, however, instead of the lease line that connects the two
routers, RouterA and RouterB are connected through a Public Frame Relay network. The Frame Relay
bridging software uses the same spanning-tree algorithm as the other bridging functions, but it allows packets
to be encapsulated for transmission across a Frame Relay network. The commands specify Internet to
Data-Link Connection Identifier (DLCI) address mapping and maintain a table of both the Ethernet and
DLCIs.
RouterA RouterB
-------- --------
Interface ethernet 0 Interface ethernet 0
bridge-group 1 bridge-group 1
Interface serial 0 Interface serial 0
encapsulation frame-relay encapsulation frame-relay
frame-relay map bridge 25 broadcast frame-relay map bridge 30 broadcast
bridge-group 1 bridge-group 1
group 1 protocol dec bridge 1 protocol dec
Example 6: Remote Transparent Bridging Over Frame Relay With
Multicast
This example uses the same topology as Example 5, however, the Frame Relay network supports a multicast
facility in this example. The multicast facility learns about the other bridges on the network, eliminating the
need for the frame-relay map command to be issued.
RouterA RouterB
-------- --------
Interface ethernet 0 Interface ethernet 0
bridge-group 2 bridge-group 2
Interface serial 0 Interface serial 0
encapsulation frame-relay encapsulation frame-relay
bridge-group 2 bridge-group 2
bridge 2 protocol dec bridge 2 protocol dec
Example 7: Remote Transparent Bridging Over Frame Relay With Multi
Sub-Interfaces
Example 9: Remote Transparent Bridging With Circuit Group
In normal operation, parallel network segments cannot all be carrying traffic at the same time. This is
necessary to prevent frame looping. In the case of serial lines, however, you may want to increase the
available bandwidth by using multiple parallel serial lines. Use the circuit- group option to do this.
