MAN page from RedHat EL 8 bridge-utils-1.7.1-2.el8.x86_64.rpm
Updated: November 7, 2001Index
brctl - ethernet bridge administration
is used to set up, maintain, and inspect the ethernet bridgeconfiguration in the Linux kernel.
An ethernet bridge is a device commonly used to connect differentnetworks of ethernets together, so that these ethernets will appear asone ethernet to the participants.
Each of the ethernets being connected corresponds to one physicalinterface in the bridge. These individual ethernets are bundled intoone bigger ('logical') ethernet, this bigger ethernet corresponds tothe bridge network interface.
The commandbrctl addbr <name>
creates a new instance of the ethernet bridge. The network interfacecorresponding to the bridge will be called <name>.
The commandbrctl delbr <name>deletes the instance <name> of the ethernet bridge. The networkinterface corresponding to the bridge must be down before it can bedeleted!
The commandbrctl showshows all current instances of the ethernet bridge.
Each bridge has a number of ports attached to it. Network trafficcoming in on any of these ports will be forwarded to the other portstransparently, so that the bridge is invisible to the rest of thenetwork (i.e. it will not show up intraceroute(8)
The commandbrctl addif <brname> <ifname>will make the interface <ifname> a port of the bridge <brname>. Thismeans that all frames received on <ifname> will be processed as ifdestined for the bridge. Also, when sending frames on <brname>,<ifname> will be considered as a potential output interface.
The commandbrctl delif <brname> <ifname>will detach the interface <ifname> from the bridge <brname>.
The commandbrctl show <brname>will show some information on the bridge and its attached ports.
The bridge keeps track of ethernet addresses seen on each port. Whenit needs to forward a frame, and it happens to know on which port thedestination ethernet address (specified in the frame) is located, itcan 'cheat' by forwarding the frame to that port only, thus saving alot of redundant copies and transmits.
However, the ethernet address location data is not staticdata. 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, thebridge will time out (delete) that address from the ForwardingDataBase (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 databasefor timed out entries every <time> seconds.
SPANNING TREE PROTOCOL
Multiple ethernet bridges can work together to create even largernetworks of ethernets using the IEEE 802.1d spanning treeprotocol. This protocol is used for finding the shortest path betweentwo ethernets, and for eliminating loops from the topology. As thisprotocol is a standard, Linux bridges will interwork properly withother third party bridge products. Bridges communicate with each otherby sending and receiving BPDUs (Bridge Protocol Data Units). TheseBPDUs can be recognized by an ethernet destination address of01:80:c2:00:00:00.
The spanning tree protocol can also be turned off (for thosesituations where it just doesn't make sense, for example when thisLinux box is the only bridge on the LAN, or when you know that thereare no loops in the topology.)
brctl(8)can be used for configuring certain spanning tree protocolparameters. For an explanation of these parameters, see the IEEE802.1d specification (or send me an email). The default values shouldbe just fine. If you don't know what these parameters mean, youprobably won't feel the desire to tweak them.
brctl stp <bridge> <state>controls this bridge instance's participation in the spanning treeprotocol. If <state> is "on" or "yes" the STP will be turned on,otherwise it will be turned off. When turned off, the bridge will notsend or receive BPDUs, and will thus not participate in the spanningtree protocol. If your bridge isn't the only bridge on the LAN, or ifthere are loops in the LAN's topology, DO NOT turn this option off. Ifyou 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 anunsigned 16-bit quantity (a number between 0 and 65535), and has nodimension. Lower priority values are 'better'. The bridge with thelowest 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 adimensionless metric.
brctl setportprio <bridge> <port> <priority>sets the port <port>'s priority to <priority>. The priority value isan unsigned 8-bit quantity (a number between 0 and 255), and has nodimension. This metric is used in the designated port and root portselection algorithms.
is obsolete. Some features such as STP guard, hairpin mode, fast leave and rootblock are intentionally not implemented in this command.Instead usebridge
package for a more full set of features.
Lennert Buytenhek <buytenhAATTgnu.org>Stephen Hemminger <stephenAATTnetworkplumber.org>
- SPANNING TREE PROTOCOL
- SEE ALSO
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