1BRCTL(8)                                                              BRCTL(8)
2
3
4

NAME

6       brctl - ethernet bridge administration
7

SYNOPSIS

9       brctl [command]
10

DESCRIPTION

12       brctl is used to set up, maintain, and inspect the ethernet bridge con‐
13       figuration in the linux kernel.
14
15       An ethernet bridge is a device commonly used to connect different  net‐
16       works of ethernets together, so that these ethernets will appear as one
17       ethernet to the participants.
18
19       Each of the ethernets  being  connected  corresponds  to  one  physical
20       interface  in  the  bridge. These individual ethernets are bundled into
21       one bigger ('logical') ethernet, this bigger  ethernet  corresponds  to
22       the bridge network interface.
23
24
25

INSTANCES

27       The  command  brctl addbr <name> creates a new instance of the ethernet
28       bridge. The network interface  corresponding  to  the  bridge  will  be
29       called <name>.
30
31       The  command brctl delbr <name> deletes the instance <name> of the eth‐
32       ernet bridge. The network interface corresponding to the bridge must be
33       down before it can be deleted!
34
35       The  command  brctl  show  shows  all current instances of the ethernet
36       bridge.
37
38
39

PORTS

41       Each bridge has a number of ports attached to it. Network traffic  com‐
42       ing  in  on  any  of  these  ports will be forwarded to the other ports
43       transparently, so that the bridge is invisible to the rest of the  net‐
44       work (i.e. it will not show up in traceroute(8) ).
45
46       The  command  brctl  addif  <brname>  <ifname>  will make the interface
47       <ifname> a port of the bridge <brname>.  This  means  that  all  frames
48       received  on  <ifname> will be processed as if destined for the bridge.
49       Also, when sending frames on <brname>, <ifname> will be considered as a
50       potential output interface.
51
52       The  command  brctl  delif  <brname> <ifname> will detach the interface
53       <ifname> from the bridge <brname>.
54
55       The command brctl show <brname>  will  show  some  information  on  the
56       bridge and its attached ports.
57
58
59

AGEING

61       The bridge keeps track of ethernet addresses seen on each port. When it
62       needs to forward a frame, and it happens to know on which port the des‐
63       tination  ethernet  address (specified in the frame) is located, it can
64       'cheat' by forwarding the frame to that port only, thus saving a lot of
65       redundant copies and transmits.
66
67       However,  the  ethernet  address  location  data  is  not  static data.
68       Machines can move to other ports, network cards can be replaced  (which
69       changes the machine's ethernet address), etc.
70
71       brctl  showmacs <brname> shows a list of learned MAC addresses for this
72       bridge.
73
74       brctl setageing <brname> <time> sets the ethernet (MAC) address  ageing
75       time,  in seconds. After <time> seconds of not having seen a frame com‐
76       ing from a certain address, the bridge  will  time  out  (delete)  that
77       address from the Forwarding DataBase (fdb).
78
79       brctl setgcint <brname> <time> sets the garbage collection interval for
80       the bridge <brname> to <time> seconds. This means that the bridge  will
81       check  the  forwarding database for timed out entries every <time> sec‐
82       onds.
83
84
85

SPANNING TREE PROTOCOL

87       Multiple ethernet bridges can work together to create even larger  net‐
88       works  of  ethernets using the IEEE 802.1d spanning tree protocol. This
89       protocol is used for finding the shortest path between  two  ethernets,
90       and  for  eliminating  loops  from  the topology. As this protocol is a
91       standard, linux bridges will interwork properly with other third  party
92       bridge  products.  Bridges  communicate  with  eachother by sending and
93       receiving BPDUs (Bridge Protocol Data Units). These BPDUs can be recog‐
94       nised by an ethernet destination address of 01:80:c2:00:00:00.
95
96       The spanning tree protocol can also be turned off (for those situations
97       where it just doesn't make sense, for example when this  linux  box  is
98       the only bridge on the LAN, or when you know that there are no loops in
99       the topology.)
100
101       brctl(8) can be used for configuring  certain  spanning  tree  protocol
102       parameters. For an explanation of these parameters, see the IEEE 802.1d
103       specification (or send me an email). The default values should be  just
104       fine.  If you don't know what these parameters mean, you probably won't
105       feel the desire to tweak them.
106
107       brctl stp <bridge> <state> controls this bridge  instance's  participa‐
108       tion in the spanning tree protocol. If <state> is "on" or "yes" the STP
109       will be turned on, otherwise it will be turned off.  When  turned  off,
110       the  bridge  will not send or receive BPDUs, and will thus not partici‐
111       pate in the spanning tree protocol.  If  your  bridge  isn't  the  only
112       bridge  on the LAN, or if there are loops in the LAN's topology, DO NOT
113       turn this option off. If you turn this option off, please know what you
114       are doing.
115
116
117       brctl  setbridgeprio  <bridge> <priority> sets the bridge's priority to
118       <priority>. The priority value is an unsigned 16-bit quantity (a number
119       between  0  and 65535), and has no dimension. Lower priority values are
120       'better'. The bridge with the lowest priority  will  be  elected  'root
121       bridge'.
122
123       brctl setfd <bridge> <time> sets the bridge's 'bridge forward delay' to
124       <time> seconds.
125
126       brctl sethello <bridge> <time> sets the bridge's 'bridge hello time' to
127       <time> seconds.
128
129       brctl setmaxage <bridge> <time> sets the bridge's 'maximum message age'
130       to <time> seconds.
131
132       brctl setpathcost <bridge> <port> <cost> sets the port cost of the port
133       <port> to <cost>. This is a dimensionless metric.
134
135       brctl  setportprio  <bridge>  <port>  <priority> sets the port <port>'s
136       priority to <priority>. The priority value is an unsigned  8-bit  quan‐
137       tity (a number between 0 and 255), and has no dimension. This metric is
138       used in the designated port and root port selection algorithms.
139
140
141

NOTES

143       brctl(8) replaces the older brcfg tool.
144
145

SEE ALSO

147       ipchains(8), iptables(8)
148
149

AUTHOR

151       Lennert Buytenhek <buytenh@gnu.org>
152
153
154
155                               November 7, 2001                       BRCTL(8)
Impressum