Posted on 4:18 PM by Bharathvn

Table of Contents
1. License
2. Document Home and Downloads
2.1. The Bridge Sources And Utilities
2.2. The Mailing-List
2.3. This Document

3. What Is A Bridge?
4. Rules On Bridging
5. Preparing The Bridge
5.1. Get The Files
5.2. Apply The Patches
5.3. Configure The Kernel
5.4. Compile The Kernel
5.5. Compile The Bridge Utilities

6. Set Up The Bridge
6.1. brctl Command Synopsis
6.2. Basic Setup

7. Advanced Bridge Features
7.1. Spanning Tree Protocol
7.2. Bridge And The IP-Chains

8. A Practical Setup Example
8.1. Hardware-setup
8.2. Software-setup
8.3. See It Work
8.4. Bridge Tests
8.4.1. Tear The Patch Wire Test
8.4.2. Kill The Root Bridge Test

A. Network Interface Cards
B. Recommended Reading

About The Linux Modular Bridge And STP

This document describes how to setup a bridge with the recent kernel
patches and brctl utility by Lennert Buytenhek. and tries to explain
about the STP implementation in this code.

With developer kernel 2.3.47 the new bridging code is part of the mainstream.
There are patches for stable kernels 2.2.14 to 2.2.16, where each is also
available as a ipchains-patch.

1. License

Copyright (c) 2000 by Uwe Böhme. This document may be distributed only
subject to the terms and conditions set forth in the LDP License available at

2. Document Home and Downloads

2.1. The Bridge Sources And Utilities

Official url is [] http:// With developer kernel 2.3.47 the new
bridging code is part of the mainstream.

2.2. The Mailing-List

The Bridge-Mailinglist is homed at [

2.3. This Document

This document has it's official homepage at [
/BRIDGE-STP-HOWTO/. It's a part of the Linux Documentation Project located at

Download Types and Locations

Build environment as tar.gziped file
[] http:

HTML-gziped file

PDF-gziped file
[] http:

PS-gziped file
[] http:/

3. What Is A Bridge?

A bridge is a device that separates two or more network segments within one
logical network (e.g. a single IP-subnet).

A bridge is usually placed between two separate groups of computers that talk
with each other, but not that much with the computers in the other group. A
good example of this is to consider a cluster of Macintoshes and a cluster of
Unix machines. Both of these groups of machines tend to be quite chatty
amongst themselves, and the traffic they produce on the network causes
collisions for the other machines who are trying to speak to one another.

The job of the bridge is to examine the destination of the data packets one
at a time and decide whether or not to pass the packets to the other side of
the Ethernet segment. The result is a faster, quieter network with less

The bridging code decides whether to bridge data or to drop it not by looking
at the protocol type (IP, IPX, NetBEUI), but by looking at the MAC-address
unique to each NIC.

Important: It's vital to understand that a bridge is neither a router nor
a fire-wall. Spoken in simple term a bridge behaves like a network switch
(i.e. Layer 2 Switch), making it a transparent network component (which
is not absolutely true, but nearly). Read more about this at Section 4.

In addition, you can overcome hardware incompatibilities with a bridge,
without leaving the address-range of your IP-net or subnet. E.g. it's
possible to bridge between different physical media like 10 Base T and 100
Base TX.

My personal reason for starting to set up a bridge was that in my work I had
to connect Fast Ethernet components to a existing HP Voice Grade network,
which is a proprietary networking standard.

Features Above Pure Bridging

The Spanning Tree Protocol is a nifty method of keeping Ethernet devices
connected in multiple paths working. The participating switches negotiate
the shortest available path by STP. This feature will be discussed in
Section 7.1.

Multiple Bridge Instances
Multiple bridge instances allow you to have more than one bridge on your
box up and running, and to control each instance separately.

There is a patch to the bridging code which allows you to use IP chains
on the interface inside a bridge. More info about this you'll find at
Section 7.2.

4. Rules On Bridging

There is a number of rules you are not allowed to break (otherwise your
bridge will do).

* A port can only be a member of one bridge.

* A bridge knows nothing about routes.

* A bridge knows nothing about higher protocols than ARP. That's the reason
why it can bridge any possible protocol possibly running on your

* No matter how many ports you have in your logical bridge, it's covered by
only one logical interface

* As soon as a port (e.g. a NIC) is added to a bridge you have no more
direct control about it.

| Warning |
|If one of the points mentioned above is not clear to you now, don't |
|continue reading. Read the documents listed in Appendix B first. |

If you ever tried to ping an unmanaged switch, you will know that it doesn't
work, because you don't have a IP-address for it. To switch datagrams it
doesn't need one. The other thing is if you want to manage the switch. It's
too much strain, to take a dumb terminal, walk to the place you installed it
(normally a dark, dusty and warm room, with a lot of green and red Christmas
lights), to connect the terminal and to change the settings.

What you want is remote management, usually by SNMP, telnet, rlogin or (best)
ssh. For all this services you will need a IP. That's the exception to the
transparency. The new code allows you without any problem to assign a IP
address to the virtual interface formed by the bridge-instance you will
create in Section 6.2. All NIC's (or other interfaces) in your bridge will
happily listen and respond to datagrams destined to this IP.

All other data will not interfere with the bridge. The bridge just acts like
a switch.

5. Preparing The Bridge

This section describes what you need and how you do to prepare your bridge.

5.1. Get The Files

Here you can find a list of the files and down-loads you will need for the
setup of the bridge. If you have one of the mentioned files or packages on
your distribution, of course there is no need to create network load.

I'll only mention the files for the 2.2.14 kernel. If you want to try a
different one (e.g. 2.2.15 or the recent development kernel) just replace the
kernel version number and look whether you find it.

Important: You have read the abstract, didn't you? So you know that there
is no need to download any kernel-patch if you're working with a kernel
later than 2.3.47.

File and package list

Unpatched kernel-sources
E.g. linux-2.2.14.tar.bz2 available from your local mirror.
Please check first if you find it in your distribution (take unpatched
kernel-sources). If you don't, please check The Linux Kernel Archive
Mirror System for a close by mirror and down-load it from there.

Bridge patches
Note: If your kernel is later than 2.3.47 you don't need this. The
bridging is part of the mainstream from that version.

Get the bridge kernel patches for your kernel version from [http://]
~buytenh/bridge/. Identify the file by the kernel number.

Note: There are also patches allowing to work with IP chains. I never
tried it, for I don't see the need to fire-wall inside my LAN, and
absolutely no need to bridge against the outer world. Feel free to
contribute about that issue.

Kernel patches for the stable 2.2 kernel.

Available Kernel patches

bridge-0.0.9-against-2.2.18.diff, the main kernel patch against 2.2.18

bridge-ipchains-against-0.0.9-against-2.2.18.diff, an add-on patch for
bridge firewalling against 2.2.18
bridge-ipchains-against-0.0.9-against-2.2.18.diff] http://

bridge-0.0.8-against-2.2.18pre19.diff, the main kernel patch against

bridge-0.0.8-against-2.2.17-0.5.diff, the main kernel patch against

bridge-ipchains-against-0.0.8-against-2.2.18pre19.diff, an add-on patch
for bridge firewalling against 2.2.18pre19
bridge-ipchains-against-0.0.8-against-2.2.18pre19.diff] http://

bridge-ipchains-against-0.0.8-against-2.2.17-0.5.diff, an add-on patch
for bridge firewalling against 2.2.17-0.5
bridge-ipchains-against-0.0.8-against-2.2.17-0.5.diff] http://

bridge-0.0.7-against-2.2.18pre15.diff, the main kernel patch against

bridge-ipchains-against-0.0.7-against-2.2.18pre15.diff, an add-on patch
for bridge firewalling against 2.2.18pre15
bridge-ipchains-against-0.0.7-against-2.2.18pre15.diff] http://

bridge-0.0.7-against-2.2.17.diff, the main kernel patch against 2.2.17

bridge-ipchains-against-0.0.7-against-2.2.17.diff, an add-on patch for
bridge firewalling against 2.2.17
bridge-ipchains-against-0.0.7-against-2.2.17.diff] http://

Bridge configuration utilities
You also will need the bridge configuration utilities to set up the
bridge Section 6. You can also download them from [http://]

5.2. Apply The Patches

Note: If your kernel is later than 2.3.47 you don't need this. The
bridging is part of the mainstream from that version.

Apply the bridging patch your kernel. If you don`t know how to do that read
the Kernel-HOWTO which can be found in your distribution or at [http://]

Example 1. Applying a kernel patch
root@mbb-1:~ # cd /usr/src/linux-2.2.14
root@mbb-1:/usr/src/linux-2.2.14 # patch -p1 < \


5.3. Configure The Kernel

Now it's time we configure our freshly patched kernel to create the ability
to bridge.

Run make config, make menuconfig or the click-o-rama make xconfig. Select
bridging in the networking option section to be compiled as a module. AFAIK
there is no strong reason why not to compile it as a kernel module, whereas I
heard rumors about problems with compiling the bridging code directly into
the kernel.

root@mbb-1:~ # cd /usr/src/linux-2.2.14
root@mbb-1:/usr/src/linux-2.2.14 # make menuconfig

5.4. Compile The Kernel

Compile your kernel Example 2. Make the new compiled kernel-image to be
loaded. I don't know if the kernel patches only apply to the bridging-module
or also modify some interfaces inside vmlinuz. So it might not be a error to
give a reboot after you updated the kernel-image.

Example 2. Commands To Compile Your Kernel
root@mbb-1:/usr/src/linux-2.2.14 # make dep clean zImage modules modules_install zlilo


5.5. Compile The Bridge Utilities

This is how to compile and install from the scratch. Just unzip the
utilities-tarball, cd into the newly created directory and give a make.

Example 3. Commands To Compile Your Bridge-Utilities
root@mbb-1:/usr/src/linux-2.2.14 # cd /usr/local/src
root@mbb-1:/usr/local/src/ # tar xzvf bridge-utils-0.9.1.tar.gz
root@mbb-1:/usr/local/src # cd bridge
root@mbb-1:/usr/local/src/bridge # make

After the compilation shown in Example 3 have worked properly, you can copy
the executables to let's say /usr/local/sbin/ (at least I did). So the
commands you have to give should be clear, but to be complete see Example 4

Example 4. Copy The Binaries Of The Utilities
root@mbb-1:/usr/local/src/bridge # cd brctl
root@mbb-1:/usr/local/src/bridge/brctl # cp brctl /usr/local/sbin
root@mbb-1:/usr/local/src/bridge/brctl # chmod 700 /usr/local/sbin/brctl
root@mbb-1:/usr/local/src/bridge/brctl # cp brctld /usr/local/sbin
root@mbb-1:/usr/local/src/bridge/brctl # chmod 700 /usr/local/sbin/brctld

Also now you can copy the new man-page to a decent place, as shown in Example

Example 5. Copy The Man-page Of brctl
root@mbb-1:/usr/local/src/bridge # cd doc
root@mbb-1:/usr/local/src/bridge/doc # gzip -c brctl.8 > /usr/local/man/man8/brctl.8.gz


6. Set Up The Bridge

Make sure all your network cards are working nicely and are accessible. If
so, ifconfig will show you the hardware layout of the network-interface. If
you have problems making your cards work please read the Ethernet-HOWTO at
HOWTO/Ethernet-HOWTO.html. Don't mess around with IP-addresses or net-masks.
You will not need it, until you bridge fully operational an up.

After you did the steps mentioned above a modprobe -v bridge should show no
errors. You can check the success by issuing a cat /proc/modules. Also for
each of the network cards you want to use in the bridge the ifconfig
whateverNameYourInterfaceHas should give you some information about the

If your bridge-utilities have been correctly built and your kernel and
bridge-module are OK, then issuing a brctl should show a small command

6.1. brctl Command Synopsis

root@mbb-1:~ # brctl
addbr add bridge (1)
addif add interface to bridge (2)
delbr delete bridge (3)
delif delete interface from bridge (4)
show show a list of bridges (5)
showbr show bridge info (6)
showmacs show a list of mac addrs (7)