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 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 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 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 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 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 each other by sending and
       receiving BPDUs (Bridge Protocol 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 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 num-
       ber  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 <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.

NOTES

       brctl(8) replaces the older brcfg tool.

SEE ALSO

       ipchains(8), iptables(8)

AUTHOR

       Lennert Buytenhek <buytenh@gnu.org>

								 November 7, 2001							  BRCTL(8)