April 22, 2019

New HOWTO: Bridging Mini-HOWTO

Bridging Mini-HOWTO

  • March 13, 2001
 Bridging mini-Howto
  Christopher Cole, cole@coledd.com
  v1.21, March 7, 2001

  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 traf�
  fic. A bridge is usually placed between two separate groups of comput�
  ers 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 colli�

  Table of Contents

  1. Setup

  2. Common problems


  1.  Setup

  1. Get ``Bridge Config'':


  BRCFG may also be found at:


  2. Enable multiple ethernet devices on your machine by adding this 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

  3. Recompile the kernel with BRIDGING enabled.

  4. 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 2 eth files, change
     the line containing ``DEVICE=lo'' to ``DEVICE=eth0'' and
     ``DEVICE=eth1''.  Other distributions may deviate from this, do
     what you need to do!  If there are more than 2 interfaces to this
     bridge, be sure to make the corresponding configurations to those,
     as well.

  5. 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

  6. Once the system is back up, put the ethernet cards into promiscuous
     mode, so they will look at every packet that passes by its

       ifconfig eth0 promisc ; ifconfig eth1 promisc

  All interfaces which are connected to network segments to be bridged
  are to be put into promiscuous mode.

  7. Turn bridging ON using the brcfg program:

       brcfg -ena

  8. Verify that there is different traffic on each interface:

       tcpdump -i eth0      (in one window)
       tcpdump -i eth1      (in another window)

  9. Run a sniffer or tcpdump on another machine to verify the bridge is
     separating the segment correctly.

  2.  Common problems


        I get the message

          ioctl(SIOCGIFBR) failed: Package not installed

     What does this mean?

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


        Machines on one side cannot ping the other side!


     �  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.


        I cannot telnet/ftp from the bridge! Why?

        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.


        What do I need to set up in the way of routing?

        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


        The bridge appears to work, but why doesn't ``traceroute'' show
        the bridge as a part of the path?

        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.


        Is it necessary to compile IP_FORWARD into the kernel?

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


        Why are the physical ethernet addresses for port 1 and port 2
        the same according to the ``brcfg'' program?  Shouldn't they be

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


        Bridging does not appear to be an option when performing a make
        config on the kernel.  How does one enable it?

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


        Too many hubs (4 or more) 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?

        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.


        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?

        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.

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