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Introduction

This document will help you configure transparent bridging. We'll start with a general 888 of bridging, and then give you more detailed information about transparent bridging, as well as several configuration examples.

Bridging

Bridges connect and transfer data between local area networks (LANs). There 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 - found primarily in Token Ring environments. Bridges only forward frames based on the routing indicator contained in the frame. End stations are responsible for determining and maintaining the table of destination addresses and routing indicators.
• 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 - 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 source-route transparent 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, like in source-route bridging, the frame contains the entire path to the destination. In other cases, like 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 would 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, see 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.

Let's look at an example to see how this works:

|          |          |
------------------------------------- LAN1
|           |
------       ------
| B1 |       | B2 |
------       ------
|           |
------------------------------------- LAN2
|          |          |

If B1 was 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 will be learned by both bridges, and then B1 and B2 will send the same broadcast message to Lan2. Then, both B1 and B2 will 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 will become 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 failed, B2 would not receive the BPDU that it expects from B1, so B2 would send out a new BPDU that would start the STA calculation again. B2 would become the root bridge, and traffic would be bridged by B2.

Cisco's transparent bridging software has the following features:

• Complies with the IEEE 802.1d standard.
• Provides two spanning tree protocols, the IEEE standard BPDU format and the old format called DEC that is compatible with Digital and other LAN bridges for backward compatibility.
• Filters based on 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, SMDS, and PPP networks.
• Provides for compression of LAT frames.
• Allows for interfaces to be treated as a single logical network for IP, IPX, etc., 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

|          |          |          |
-------------------------------------- LAN1
|
----------
| ROUTER |
----------
|
-------------------------------------- LAN2
|          |          |          |

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 (like 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 version 11.0, a protocol may be bridged on some interfaces and routed on others. 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.

Interface ethernet 0
bridge group 1

  Interface ethernet 1
bridge group 1 

  bridge group 1 protocol ieee 

In this example, the IEEE 802.1d standard is the spanning-tree protocol. If every bridge in the network is Cisco, use the command bridge group 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 Digital Equipment Corporation (DEC), then use the command bridge group 1 protocol dec to assure backward compatibility. Since the IEEE and DEC spanning trees are not compatible, mixing these protocols in the network will give unpredictable results.

Example # 2 : Transparent Bridging with Multiple Bridge Groups

LAN3
|----------|
|
-       |        -
|   ----------   |
LAN1|---| ROUTER |---|LAN2
|   ----------   |
-       |        -
|
|----------|
LAN4

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, but 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.

interface ethernet 0
bridge group 1

  interface ethernet 1
bridge group 1 

  interface ethernet 2
bridge group 2 

  interface ethernet 3
bridge group 2 

  bridge group 1 protocol ieee
bridge group 2 protocol dec 

Example # 3 : Bridging Over a Wide Area Network

-                                -
|   ----------S0    ----------   |
| E0| ROUTER |---   | ROUTER |E0 |
LAN1|---|   A    |  / S0|   B    |---|LAN2
|   |        |  ----|        |   |
|   ----------      ----------   |
-                                -

In this example, the two LANs are connected by a T1 link.

Router A                        Router B
--------                        --------
Interface ethernet 0            Interface ethernet 0
bridge-group 1                  bridge-group 1

  Interface serial 0              Interface serial 0
bridge-group 1 		  bridge-group 1 

  bridge group 1 protocol ieee    bridge group 1 protocol ieee 

Example # 4 : Remote Transparent Bridging Over X.25

This example uses the same topology as Example 3, but instead of the lease line that connects the two routers, let's say that Router A and Router B are connected through an X.25 cloud.

Router A                             Router B
--------                             --------
Interface ethernet 0                 Interface ethernet 0
bridge-group 1                       bridge-group 1

  Interface serial 0                   Interface serial 0
encapsulation x25                    encapsulation x25
x25 address 31370019027              x25 address 31370019134
x25 map bridge 31370019134broadcast  x25 map bridge 31370019027 broadcast
bridge-group 1 		       bridge-group 1 

  bridge group 1 protocol ieee         bridge group 1 protocol ieee 

Example # 5 : Remote Transparent Bridging Over Frame Relay With No Multicast

This example uses the same topology as Example 3, but instead of the lease line that connects the two routers, let's say that Router A and Router B 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 DLCI address mapping and maintain a table of both the Ethernet and DLCIs.

Router A                              Router B
--------                              --------
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 

  bridge group 1 protocol dec           bridge group 1 protocol dec 

Example # 6 : Remote Transparent Bridging Over Frame Relay With Multicast

This example uses the same topology as Example 5, but here the Frame Relay network supports a multicast facility. The multicast facility learns about the other bridges on the network, eliminating the need for the frame-relay map command.

Router A                        Router B
--------                        --------
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 group 2 protocol dec     bridge group 2 protocol dec 

Example # 7 : Remote Transparent Bridging Over Frame Relay With multi- sub-interface

To use a sub-interface, you must define bridge-group statements on both the sub-interface and the main interface.

Router A                             Router B
--------                             --------
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
! 				       !
interface Serial0.1 point-to-point   interface Serial0.1 point-to-point
frame-relay interface-dlci 101       frame-relay interface-dlci 100
bridge-group 2        	       bridge-group 2
! 				       !
interface Serial0.2 point-to-point   interface Serial0.2 point-to-point
frame-relay interface-dlci 103       frame-relay interface-dlci 103
bridge-group 2 		       bridge-group 2 

  bridge group 2 protocol dec          bridge group 2 protocol dec 

Example # 8 : Remote Transparent Bridging Over SMDS

Router A                             Router B
--------                             --------
Interface ethernet 0                 Interface ethernet 0
bridge-group 2                       bridge-group 2

  Interface Hssi0                      Interface Hssi0
encapsulation smds                   encapsulation smds smds
address c449.1812.0013               smds address c448.1812.0014 s
mds multicast BRIDGE 		       smds multicast BRIDGE
e449.1810.0040                       e449.1810.0040
bridge-group 2 		       bridge-group 2 

  bridge group 2 protocol dec          bridge group 2 protocol dec 

Example # 9 : Remote Transparent Bridging With Circuit Group

In normal operation, parallel network segments can't 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.

-                                 -
|   ----------S0   S0----------   |
|   |        |---/---|        |   |
|   | ROUTER |S1   S1| ROUTER |   |
| E0|        |---/---|        |E0 |
LAN1|---|   A    |S2   S2|   B    |---|LAN2
|   |        |---/---|        |   |
|   ----------       ----------   |
-                                 -

Router A 			Router B
-------- 			--------
Interface ethernet 0 		Interface ethernet 0
bridge-group 2 			bridge-group 2 

Interface serial0 		Interface serial0
bridge-group 2			bridge-group 2
bridge-group 2 circuit-group 1 	bridge-group 2 circuit-group 1 

Interface serial1 		Interface serial1
bridge-group 2 			bridge-group 2
bridge-group 2 circuit-group 1 	bridge-group 2 circuit-group 1 

Interface serial2 		Interface serial2
bridge-group 2 			bridge-group 2
bridge-group 2 			circuit-group 1 bridge-group 2 circuit-group 1 

bridge group 2 protocol dec 	bridge group 2 protocol dec 

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