BRCTL(8)                                                                                 BRCTL(8)



NAME
       brctl - ethernet bridge administration

SYNOPSIS
       brctl [command]

DESCRIPTION
       brctl  is  used  to set up, maintain, and inspect the ethernet bridge configuration in the
       linux kernel.

       An ethernet bridge is a device commonly used to connect different  networks  of  ethernets
       together, so that these ethernets will appear as one ethernet to the participants.

       Each of the ethernets being connected corresponds to one physical interface in the bridge.
       These individual ethernets are bundled into one bigger ('logical') ethernet,  this  bigger
       ethernet corresponds to the bridge network interface.



INSTANCES
       The  command brctl addbr <name> creates a new instance of the ethernet bridge. The network
       interface corresponding to the bridge will be called <name>.

       The command brctl delbr <name> deletes the instance <name> of  the  ethernet  bridge.  The
       network interface corresponding to the bridge must be down before it can be deleted!

       The command brctl show shows all current instances of the ethernet bridge.



PORTS
       Each  bridge  has  a  number  of ports attached to it. Network traffic coming in on any of
       these ports will be forwarded to the other ports transparently,  so  that  the  bridge  is
       invisible to the rest of the network (i.e. it will not show up in traceroute(8) ).

       The  command  brctl addif <brname> <ifname> will make the interface <ifname> a port of the
       bridge <brname>. This means that all frames received on <ifname> will be processed  as  if
       destined  for  the bridge. Also, when sending frames on <brname>, <ifname> will be consid‐
       ered as a potential output interface.

       The command brctl delif <brname> <ifname> will detach  the  interface  <ifname>  from  the
       bridge <brname>.

       The  command brctl show <brname> will show some information on the bridge and its attached
       ports.



AGEING
       The bridge keeps track of ethernet addresses seen on each port. When it needs to forward a
       frame, and it happens to know on which port the destination ethernet address (specified in
       the frame) is located, it can 'cheat' by forwarding the frame to that port only, thus sav‐
       ing a lot of redundant copies and transmits.

       However, the ethernet address location data is not static data. Machines can move to other
       ports, network cards can be replaced (which changes the machine's ethernet address), etc.

       brctl showmacs <brname> shows a list of learned MAC addresses for this bridge.

       brctl setageing <brname> <time> sets the ethernet (MAC) address ageing time,  in  seconds.
       After  <time> seconds of not having seen a frame coming from a certain address, the bridge
       will time out (delete) that address from the Forwarding DataBase (fdb).

       brctl setgcint <brname> <time>  sets  the  garbage  collection  interval  for  the  bridge
       <brname>  to <time> seconds. This means that the bridge will check the forwarding database
       for timed out entries every <time> seconds.



SPANNING TREE PROTOCOL
       Multiple ethernet bridges can work together to create even larger  networks  of  ethernets
       using the IEEE 802.1d spanning tree protocol. This protocol is used for finding the short‐
       est path between two ethernets, and for eliminating loops from the topology. As this  pro‐
       tocol  is  a standard, linux bridges will interwork properly with other third party bridge
       products. Bridges communicate with each other by sending and receiving BPDUs (Bridge  Pro‐
       tocol  Data  Units).  These  BPDUs can be recognised by an ethernet destination address of
       01:80:c2:00:00:00.

       The spanning tree protocol can also be turned off (for  those  situations  where  it  just
       doesn't make sense, for example when this linux box is the only bridge on the LAN, or when
       you know that there are no loops in the topology.)

       brctl(8) can be used for configuring certain spanning tree  protocol  parameters.  For  an
       explanation  of these parameters, see the IEEE 802.1d specification (or send me an email).
       The default values should be just fine. If you don't know what these parameters mean,  you
       probably won't feel the desire to tweak them.

       brctl  stp  <bridge> <state> controls this bridge instance's participation in the spanning
       tree protocol. If <state> is "on" or "yes" the STP will be turned on, otherwise it will be
       turned off.  When turned off, the bridge will not send or receive BPDUs, and will thus not
       participate in the spanning tree protocol. If your bridge isn't the  only  bridge  on  the
       LAN, or if there are loops in the LAN's topology, DO NOT turn this option off. If you turn
       this option off, please know what you are doing.


       brctl setbridgeprio <bridge> <priority> sets the bridge's priority to <priority>. The pri‐
       ority  value  is  an  unsigned  16-bit quantity (a number between 0 and 65535), and has no
       dimension. Lower priority values are 'better'. The bridge with the lowest priority will be
       elected 'root bridge'.

       brctl setfd <bridge> <time> sets the bridge's 'bridge forward delay' to <time> seconds.

       brctl sethello <bridge> <time> sets the bridge's 'bridge hello time' to <time> seconds.

       brctl setmaxage <bridge> <time> sets the bridge's 'maximum message age' to <time> seconds.

       brctl  setpathcost <bridge> <port> <cost> sets the port cost of the port <port> to <cost>.
       This is a dimensionless metric.

       brctl setportprio <bridge> <port> <priority> sets the port <port>'s  priority  to  <prior‐
       ity>.  The  priority value is an unsigned 8-bit quantity (a number between 0 and 255), and
       has no dimension. This metric is used in the designated port and root port selection algo‐
       rithms.



NOTES
       brctl(8) replaces the older brcfg tool.


SEE ALSO
       ipchains(8), iptables(8)


AUTHOR
       Lennert Buytenhek <buytenh AT gnu.org>



                                         November 7, 2001                                BRCTL(8)