Bridging mini-HOWTO

Christopher Cole

            cole@coledd.com
          

March 2001

Revision History
Revision 1.222002-05-20Revised by: tab
Converted to Docbook 4.1 SGML and added GFDL per Christopher Cole
Revision 1.212001-03-07Revised 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.

Table of Contents
1. Setup
2. Common Problems
3. Copyright
3.1. GNU Free Documentation License
3.2. PREAMBLE
3.3. APPLICABILITY AND DEFINITIONS
3.4. VERBATIM COPYING
3.5. COPYING IN QUANTITY
3.6. MODIFICATIONS
3.7. COMBINING DOCUMENTS
3.8. COLLECTIONS OF DOCUMENTS
3.9. AGGREGATION WITH INDEPENDENT WORKS
3.10. TRANSLATION
3.11. TERMINATION
3.12. FUTURE REVISIONS OF THIS LICENSE
3.13. How to use this License for your documents

1. Setup

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?

Q: I get the message ioctl(SIOCGIFBR) failed: Package not installed . What does this mean?

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

Q: Machines on one side cannot ping the other side!

A:

  • 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.

    • Q: I cannot telnet/ftp from the bridge! Why?

    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.

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

    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

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

    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.

    Q: Is it necessary to compile IP_FORWARD into the kernel?

    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.

    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?

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

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

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

    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?

    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.

    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: 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".

    3.1. GNU Free Documentation License

    Version 1.1, March 2000

    Copyright (C) 2000 Free Software Foundation, Inc. 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

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