Bridging mini-Howto
 Christopher Cole  [email protected]
 v1.11, 7 September 1998

 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 com
 puters 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 traf
 fic 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.
 ______________________________________________________________________

 Table of Contents


 1. Setup

 2. Common problems

 ______________________________________________________________________

 1.  Setup


 1. Get ``Bridge Config'':

      <ftp://shadow.cabi.net/pub/Linux/BRCFG.tgz>

 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
    interfaces.

 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
    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.

 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


 1.

    Question
       I get the message


         ioctl(SIOCGIFBR) failed: Package not installed

    What does this mean?

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


 2.

    Question
       Machines on one side cannot ping the other side!

    Answer

      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.


 3.

    Question
       I cannot telnet/ftp from the bridge! Why?

    Answer
       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.


 4.

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

    Answer
       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

 5.

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

    Answer
       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.


 6.

    Question
       Is it necessary to compile IP_FORWARD into the kernel?

    Answer
       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.


 7.

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

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


 8.

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

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


 9.

    Question
       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?

    Answer
       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.

 10.

    Question
       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?

    Answer
       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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