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Automating Unix and Linux Administration

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Common Problems
Bridging mini-HOWTO
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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.

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