brctl - ethernet bridge administration
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
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-
SPANNING TREE PROTOCOL
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
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
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
brctl sethello <bridge> <time> sets the bridge’s ’bridge hello time’ to
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.
Lennert Buytenhek <firstname.lastname@example.org>
November 7, 2001 BRCTL(8)
Man(1) output converted with