BRCTL(8)                                                              BRCTL(8)


       brctl - ethernet bridge administration


       brctl [command]


       brctl is used to set up, maintain, and inspect the ethernet bridge con-
       figuration in the linux kernel.

       An ethernet bridge is a device commonly used to connect different  net-
       works 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.


       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 eth-
       ernet 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


       Each bridge has a number of ports attached to it. Network traffic  com-
       ing  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  net-
       work (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 considered 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 it’s attached ports.


       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 des-
       tination  ethernet  address (specified in the frame) is located, it can
       ’cheat’ by forwarding the frame to that port only, thus saving 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

       brctl setageingtime <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> sec-


       Multiple ethernet bridges can work together to create even larger  net-
       works  of  ethernets using the IEEE 802.1d spanning tree protocol. This
       protocol is used for finding the shortest path between  two  ethernets,
       and  for  eliminating  loops  from  the topology. As this protocol is a
       standard, linux bridges will interwork properly with other third  party
       bridge  products.  Bridges  communicate  with  eachother by sending and
       receiving BPDUs (Bridge Protocol Data Units). These BPDUs can be recog-
       nised 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  participa-
       tion 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 partici-
       pate 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 priority 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

       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 <priority>. The priority value is an unsigned  8-bit  quan-
       tity (a number between 0 and 255), and has no dimension. This metric is
       used in the designated port and root port selection algorithms.


       brctl(8) replaces the older brcfg tool.


       ipchains(8), iptables(8)


       Lennert Buytenhek <>

                               November 7, 2001                       BRCTL(8)

Man(1) output converted with man2html