📄 configuration transparent bridging.htm
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<title>Cisco - Configuring Transparent Bridging</title>
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<h1>Configuring Transparent Bridging</h1>
<hr>
<h2>Introduction</h2>
<p>This document will help you configure transparent bridging.
We'll start with a general description of bridging, and then give
you more detailed information about transparent bridging, as well
as several configuration examples. </p>
<h2>Bridging</h2>
<p>Bridges connect and transfer data between local area networks
(LANs). There are four kinds of bridging: </p>
<ul>
<li><b>Transparent bridging</b> - 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. </li>
<li><b>Source-route bridging</b> - 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. </li>
<li><b>Translational bridging</b> - used to bridge data
between different media types. This is typically used to
go between Ethernet and FDDI or Token Ring to Ethernet. </li>
<li><b>Source-route translational bridging</b> - 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.) </li>
</ul>
<p>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. </p>
<p>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. </p>
<h2>Transparent Bridging</h2>
<p>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. </p>
<p>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. </p>
<p>Let's look at an example to see how this works: </p>
<pre>
| | |
------------------------------------- LAN1
| |
------ ------
| B1 | | B2 |
------ ------
| |
------------------------------------- LAN2
| | |
</pre>
<p>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. </p>
<p>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. </p>
<p>Cisco's transparent bridging software has the following
features: </p>
<ul>
<li>Complies with the IEEE 802.1d standard. </li>
<li>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. </li>
<li>Filters based on Mac address, protocol type, and vendor
code. </li>
<li>Groups serial lines into circuit groups for load
balancing and redundancy. </li>
<li>Provides the ability to bridge over X.25, Frame Relay,
SMDS, and PPP networks. </li>
<li>Provides for compression of LAT frames. </li>
<li>Allows for interfaces to be treated as a single logical
network for IP, IPX, etc., so that bridge domains can
communicate with routed domains. </li>
</ul>
<h2>Configuration Examples</h2>
<p>These configurations show only the commands required for
transparent bridging, not for IP or other protocol support. </p>
<h3>Example # 1 : Simple Transparent Bridging</h3>
<pre>
| | | |
-------------------------------------- LAN1
|
----------
| ROUTER |
----------
|
-------------------------------------- LAN2
| | | |
</pre>
<p>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. </p>
<ul>
<li>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. </li>
</ul>
<pre>
Interface ethernet 0
bridge group 1
</pre>
<pre> Interface ethernet 1
bridge group 1 </pre>
<pre> bridge group 1 protocol ieee </pre>
<p>In this example, the IEEE 802.1d standard is the spanning-tree
protocol. If every bridge in the network is Cisco, use the
command <b>bridge group 1 protocol ieee</b> 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 <b>bridge group 1 protocol dec</b> to assure backward
compatibility. Since the IEEE and DEC spanning trees are not
compatible, mixing these protocols in the network will give
unpredictable results. </p>
<h3>Example # 2 : Transparent Bridging with Multiple Bridge
Groups</h3>
<pre>
LAN3
|----------|
|
- | -
| ---------- |
LAN1|---| ROUTER |---|LAN2
| ---------- |
- | -
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