Bridging mini-HOWTO

Christopher Cole

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March 2001

Revision History
Revision 1.22 2002-05-20 Revised by: tab
Converted to Docbook 4.1 SGML and added GFDL per Christopher Cole
Revision 1.21 2001-03-07 Revised by: cc

This document describes how to setup an ethernet bridge. What is an ethernet bridge? An ethernet bridge is a device that controls data packets within a subnet in an attempt to cut down the amount of traffic. A bridge is usually placed between two separate groups of computers that talk within themselves, but not so 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. A bridge would be placed between these groups of computers. The job of the bridge is then 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 collisions.

1. Setup

  • Get Bridge Config: BRCFG.tgz

  • BRCFG may also be found at:

  • Enable multiple ethernet devices on your machine by adding this line to your /etc/lilo.conf, and re-run lilo:

    append = "ether=0,0,eth1"

    If you have three interfaces on your bridge, use this line instead:

    append = "ether=0,0,eth1 ether=0,0,eth2"

    More interfaces can be found by adding more ether statements. By default a stock Linux kernel probes for a single ethercard, and once one is found the probe ceases. The above append statement tells the kernel to keep probing for more ethernet devices after the first one is found. Alternatively, the boot parameter can be used instead:

    linux ether=0,0,eth1

    Or, with 3 interfaces, use:

    linux ether=0,0,eth1 ether=0,0,eth2
  • Recompile the kernel with BRIDGING enabled.

  • A bridge should not have an IP address. It CAN, but a plain bridge doesn't need one. To remove the IP address from your bridge, go to /etc/sysconfig/network-scripts/ (for a RedHat system) and copy ifcfg-lo0 to ifcfg-eth0 & ifcfg-eth1. In these two new files, change the line containing DEVICE=lo to DEVICE=eth0 and DEVICE=eth1. Since other distributions may deviate from this, you may need to refer to additional documentation. If there are more than 2 interfaces to this bridge, be sure to make the corresponding configurations to those, as well.

  • Reboot so you are running the new kernel with BRIDGING in it, and also to make sure that an IP addresses are not bound to the network interfaces.

  • Once the system is backed up, put the ethernet cards into promiscuous mode, so they will look at every packet that passes by its interface:

    ifconfig eth0 promisc ; ifconfig eth1 promisc
    All interfaces which are connected to network segments to be bridged are to be put into promiscuous mode.
  • Turn bridging ON using the brcfg program:

    brcfg -ena
  • Verify that there is different traffic on each interface:

    tcpdump -i eth0     (in one window)
    tcpdump -i eth1     (in another window)
  • Run a sniffer or tcpdump on another machine to verify the bridge is separating the segment correctly.

2. Common Problems

Q: I get the message ioctl(SIOCGIFBR) failed: Package not installed . What does this mean?
Q: Machines on one side cannot ping the other side!
Q: I cannot telnet/ftp from the bridge! Why?
Q: What do I need to set up in the way of routing?
Q: The bridge appears to work, but why doesn't traceroute show the bridge as a part of the path?
Q: Is it necessary to compile IP_FORWARD into the kernel?
Q: Why are the physical ethernet addresses for port 1 and port 2 the same according to the brcfg program? Shouldn't they be different?
Q: Bridging does not appear to be an option when performing a make config on the kernel. How does one enable it?
Q: Too many hubs (4 or more) are chained one after another in series, cause timing problems on an ethernet. What effect does a bridge have in a subnet that is layered with hubs?
Q: Can a bridge interface to both 10Mb and 100Mb ethernet segments? Will such a configuration slow down the rest of the traffic on the high speed side?

A: You don't have bridging capability in your kernel. Get a 2.0 or greater kernel, and recompile with the BRIDGING option enabled.


  • Did you enable bridging using brcfg -ena? (brcfg should say bridging is ENABLED)
  • Did you put the interfaces into promiscuous mode? (issue the ifconfig command. The PROMISC flag should be on for both interfaces.)
  • If using multiple-media interface adapters, make sure that the correct one is enabled. You may need to use the config/setup program that came with the network interface card.

A: This is because there is no IP address bound to any of bridge interfaces. A bridge is to be a transparent part of a network.

A: Nothing! All routing intelligence is handled by the bridging code in the kernel. To see the ethernet addresses as they are learned by the bridge, use the brcfg program in debug mode:

brcfg -deb

A: Due to the nature of a bridge, a traceroute should NOT show the bridge as a part of the path. A bridge is to be a transparent component of the network.

A: No. The bridging code in the kernel takes care of the packet transport. IP_FORWARD is for a gateway that has IP addresses bound to its interfaces.

A: No. Every port on a bridge intentionally is assigned the same physical ethernet address by the bridging code.

A: During the kernel config, answer "Y" to the question, Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL) [Y/n/?].

A: A bridge resets the 3/4/5 hubs rule. A bridge does not deal with packets the way a hub does, and is therefore not a contributor to timing problems on a network.

A: Yes, a bridge can tie together a 10Mb segment with a 100Mb segment. As long as the network card on the fast network is 100Mb capable, TCP takes care of the rest. While it's true that the packets from a host in the 100Mb network communicating to a host in the 10Mb network are moving at only 10Mb/s, the rest of the traffic on the fast ethernet is not slowed down.

3. Copyright

Copyright © 2002 Christopher Cole

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

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Version 1.1, March 2000

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