BRIDGE(4) BSD Kernel Interfaces Manual BRIDGE(4)
bridge -- bridging support
FreeBSD supports bridging on Ethernet-type interfaces, including VLANs.
Bridging support can be either compiled into the kernel, or loaded at
runtime as a kernel module.
A single FreeBSD host can do bridging on independent sets of interfaces,
which are called ``clusters''. Each cluster connects a set of inter-
faces, and is identified by a ``cluster-ID'' which is a number in the
range 1..65535. A cluster in fact is very similar to what commercial
switches call a ``VLAN''. Note however that there is no relation whatso-
ever between the cluster-ID and the IEEE 802.1q VLAN-ID which appears in
the header of packets transmitted on the wire. In fact, in most cases
there is no relation between the so-called ``VLAN identifier'' used in
most commercial switches, and the IEEE 802.1q VLAN-ID.
By putting both physical and logical (vlan(4)) interfaces in the same
cluster, a FreeBSD box can also implement what in commercial terms is
called a ``trunk'' interface. This means that packets coming from one of
the interfaces in a cluster will appear on the wire of the ``parent''
interface of any VLAN interface in a cluster, with the proper VLAN tag.
Similarly, packets coming from a parent interface of any VLAN interface
in a cluster will have the VLAN tag stripped, and will be forwarded to
other interfaces in a cluster. See the EXAMPLES section for more
Runtime operation of the bridge is controlled by several sysctl(8) vari-
ables, as follows.
Set to 1 to enable bridging, set to 0 to disable it.
Set to 1 to enable ipfw(8) processing of bridged packets. Note
that ipfw(8) rules only apply to IP packets. Non-IP packets are
accepted by default. See the BUGS section and the ipfw(8) man-
page for more details on the interaction of bridging and the
Set to 1 to enable ipf(8) processing of bridged packets. Note
that ipf(8) rules only apply to IP packets. Non-IP packets are
accepted by default.
Set to the list of interfaces to bridge. Interfaces are sepa-
rated by spaces, commas or tabs. Each interface can be option-
ally followed by a colon and an integer indicating the cluster it
belongs to (defaults to 1 if the cluster-ID is missing), e.g.
``dc0:1,dc1,vlan0:3 dc2:3'' will put dc0 and dc1 in cluster num-
ber 1, and vlan0 and dc2 in cluster number 3. See the EXAMPLES
section for more examples.
The list of interfaces is rescanned every time the list is modi-
fied, bridging is enabled, or new interfaces are created or
destroyed. An explicit request to refresh the bridge configura-
tion can also be done by writing any value to
net.link.ether.bridge.refresh. Interfaces that are in the list
but cannot be used for bridging (because they are non-existing,
or not Ethernet or VLAN) are not used and a warning message is
Bridging requires interfaces to be put in promiscuous mode, and transmit
packets with Ethernet source addresses different than their own. Some
interfaces (e.g. wi(4)) do not support this functionality. Also, bridg-
ing is not compatible with interfaces which use hardware loopback,
because there is no way to tell locally generated packets from externally
A simple bridge configuration with three interfaces in the same cluster
can be set as follows. No cluster-ID is specified here, which will cause
the interfaces to appear as part of cluster #1.
If you do not know what actual interfaces will be present on your system,
you can just put all existing interfaces in the configuration, as fol-
sysctl net.link.ether.bridge.config="`ifconfig -l`"
This will result in a space-separated list of interfaces. Out of the
list, only Ethernet and VLAN interfaces will be used for bridging,
whereas for others the kernel will produce a warning message.
More complex configurations can be used to create multiple clusters, e.g.
will create two completely independent clusters.
Finally, interesting configurations involve VLANs and parent interfaces.
As an example, the following configuration will use interface dc0 as a
``trunk'' interface, and pass packets for 802.1q VLANs 10 and 20 to phys-
ical interfaces dc1 and dc2, respectively:
ifconfig vlan0 vlan 10 vlandev dc0
ifconfig vlan1 vlan 20 vlandev dc0
Note how there is no relation between the 802.1q VLAN identifiers (10 and
20) and the cluster-ID's (34 and 56) used in the bridge.config variable.
Note also that the trunk interface does not even appear in the
bridge.config, as VLAN tag insertion/removal is performed by the vlan(4)
devices. When using VLAN devices, care must be taken by not creating
loops between these devices and their parent interfaces.
Care must be taken not to construct loops in the bridge topology. The
kernel supports only a primitive form of loop detection, by disabling
some interfaces when a loop is detected. No support for a daemon running
the spanning tree algorithm is currently provided.
With bridging active, interfaces are in promiscuous mode, thus causing
some load on the system to receive and filter out undesired traffic.
When passing bridged packets to ipfw(8), remember that only IP packets
are passed to the firewall, while other packets are silently accepted.
Also remember that bridged packets are accepted after the first pass
through the firewall irrespective of the setting of the sysctl variable
net.inet.ip.fw.one_pass, and that some ipfw(8) actions such as divert do
not apply to bridged packets. It might be useful to have a rule of the
skipto 20000 ip from any to any bridged
near the beginning of your ruleset to implement specific rulesets for
/boot/kernel/bridge.ko bridge loadable module.
ip(4), ng_bridge(4), vlan(4), ipf(8), ipfw(8), sysctl(8)
Bridging was introduced in FreeBSD 2.2.8 by Luigi Rizzo
BSD September 20, 2003 BSD