Manual Reference Pages - IF_BRIDGE (4)
- network bridge device
To compile this driver into the kernel,
place the following line in your
kernel configuration file:
.Cd device if_bridge
Alternatively, to load the driver as a
module at boot time, place the following lines in
driver creates a logical link between two or more IEEE 802 networks
that use the same (or
For example, it is possible to bridge Ethernet and 802.11 networks together,
but it is not possible to bridge Ethernet and Token Ring together.
interface is created at runtime using interface cloning.
most easily done with the
command or using the
interface randomly chooses a link (MAC) address in the range reserved for
locally administered addresses when it is created.
This address is guaranteed to be unique
interfaces on the local machine.
Thus you can theoretically have two bridges on the different machines with
the same link addresses.
The address can be changed by assigning the desired link address using
has non-zero value, newly created bridge will inherit MAC address
from its first member instead of choosing random link-level address.
This will provide more predictable bridge MAC without any
additional configuration, but currently this feature is known
to break some L2 protocols, for example PPPoE that is provided
Now this feature is considered as experimental and is turned off
A bridge can be used to provide several services, such as a simple
802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
A bridge works like a switch, forwarding traffic from one interface
Multicast and broadcast packets are always forwarded to all
interfaces that are part of the bridge.
For unicast traffic, the bridge learns which MAC addresses are associated
with which interfaces and will forward the traffic selectively.
All the bridged member interfaces need to be up in order to pass network traffic.
These can be enabled using
The MTU of the first member interface to be added is used as the bridge MTU.
All additional members are required to have exactly the same value.
The TXCSUM capability is disabled for any interface added to the bridge, and it
is restored when the interface is removed again.
The bridge supports
where the packets are discarded after
processing, and are not processed or forwarded further.
This can be used to multiplex the input of two or more interfaces into a single
This is useful for reconstructing the traffic for network taps
that transmit the RX/TX signals out through two separate interfaces.
address family on bridge interfaces.
variable configures an IPv6 link-local address on
or in a more explicit manner:
address family has a concept of scope zone.
Bridging multiple interfaces change the zone configuration because
multiple links are merged to each other and form a new single link
while the member interfaces still work individually.
This means each member interface still has a separate link-local scope
zone and the
interface has another single,
aggregated link-local scope zone at the same time.
This situation is clearly against the description
"zones of the same scope cannot overlap"
in Section 5,
Although it works in most cases,
it can cause some conterintuitive or undesirable behavior in some
edge cases when both of the
interface and one of the member interface have an IPv6 address
and applications use both of them.
To prevent this situation,
checks whether a link-local scoped IPv6 address is configured on
a member interface to be added and the
interface has IPv6 addresses,
IPv6 addresses on the member interface will be automatically removed
before the interface is added.
This behavior can be disabled by setting
interface flag are not enabled by default on
interface even when
is set to
driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with
backwards compatibility with the legacy Spanning Tree Protocol (STP).
Spanning Tree is used to detect and remove loops in a network topology.
RSTP provides faster spanning tree convergence than legacy STP, the protocol
will exchange information with neighbouring switches to quickly transition to
forwarding without creating loops.
The code will default to RSTP mode but will downgrade any port connected to a
legacy STP network so is fully backward compatible.
A bridge can be forced to operate in STP mode without rapid state transitions
The bridge can log STP port changes to
by enabling the
Packet filtering can be used with any firewall package that hooks in via the
When filtering is enabled, bridged packets will pass through the filter
inbound on the originating interface, on the bridge interface and outbound on
the appropriate interfaces.
Either stage can be disabled.
The filtering behaviour can be controlled using
Controls the handling of non-IP packets which are not passed to
to only allow IP packets to pass (subject to firewall rules), set to
to unconditionally pass all non-IP Ethernet frames.
to enable filtering on the incoming and outgoing member interfaces, set
to disable it.
to enable filtering on the bridge interface, set
to disable it.
to additionally filter on the physical interface for locally destined packets.
to disable this feature.
to enable layer2 filtering with
to disable it.
This needs to be enabled for
will be disabled so that IPFW
is not run twice; these can be re-enabled if desired.
to enable layer2 ARP filtering with
to disable it.
to be enabled.
ARP and REVARP packets are forwarded without being filtered and others
that are not IP nor IPv6 packets are not forwarded when
IPFW can filter Ethernet types using
so all packets are passed to
the filter for processing.
The packets originating from the bridging host will be seen by
the filter on the interface that is looked up in the routing
The packets destined to the bridging host will be seen by the filter
on the interface with the MAC address equal to the packets destination
There are situations when some of the bridge members are sharing
the same MAC address (for example the
interfaces: they are currently sharing the
MAC address of the parent physical interface).
It is not possible to distinguish between these interfaces using
their MAC address, excluding the case when the packets destination
MAC address is equal to the MAC address of the interface on which
the packet was entered to the system.
In this case the filter will see the incoming packet on this
In all other cases the interface seen by the packet filter is chosen
from the list of bridge members with the same MAC address and the
result strongly depends on the member addition sequence and the
actual implementation of
It is not recommended to rely on the order chosen by the current
implementation: it can be changed in the future.
The previous paragraph is best illustrated with the following
- the MAC address of the incoming packets destination is
- the interface on which packet entered the system is
MAC address is
- there are possibly other bridge members with the same MAC address
- the bridge has more than one interface that are sharing the
same MAC address
we will call them
Then if the MAC address
is equal to the
then the filter will see the packet on the interface
no matter if there are any other bridge members carrying the same
But if the MAC address
is equal to the
then the interface that will be seen by the filter is one of the
It is not possible to predict the name of the actual interface
without the knowledge of the system state and the
This problem arises for any bridge members that are sharing the same
MAC address, not only to the
ones: they we taken just as the example of such situation.
So if one wants the filter the locally destined packets based on
their interface name, one should be aware of this implication.
The described situation will appear at least on the filtering bridges
that are doing IP-forwarding; in some of such cases it is better
to assign the IP address only to the
interface and not to the bridge members.
will let you do the additional filtering on the physical interface.
The following when placed in the file
will cause a bridge called
to be created, and will add the interfaces
to the bridge, and then enable packet forwarding.
Such a configuration could be used to implement a simple
802.11-to-Ethernet bridge (assuming the 802.11 interface is
in ad-hoc mode).
ifconfig_bridge0="addm wlan0 addm fxp0 up"
For the bridge to forward packets all member interfaces and the bridge need
to be up.
The above example would also require:
ifconfig_wlan0="up ssid my_ap mode 11g"
Consider a system with two 4-port Ethernet boards.
The following will cause a bridge consisting of all 8 ports with Rapid Spanning
Tree enabled to be created:
ifconfig bridge0 create
ifconfig bridge0 \
addm fxp0 stp fxp0 \
addm fxp1 stp fxp1 \
addm fxp2 stp fxp2 \
addm fxp3 stp fxp3 \
addm fxp4 stp fxp4 \
addm fxp5 stp fxp5 \
addm fxp6 stp fxp6 \
addm fxp7 stp fxp7 \
The bridge can be used as a regular host interface at the same time as bridging
between its member ports.
In this example, the bridge connects em0 and em1, and will receive its IP
address through DHCP:
ifconfig_bridge0="addm em0 addm em1 DHCP"
The bridge can tunnel Ethernet across an IP internet using the EtherIP
This can be combined with
to provide an encrypted connection.
interface and set the local and remote IP addresses for the
tunnel, these are reversed on the remote bridge.
ifconfig gif0 create
ifconfig gif0 tunnel 188.8.131.52 184.108.40.206 up
ifconfig bridge0 create
ifconfig bridge0 addm fxp0 addm gif0 up
6.1, 6.2, 6.3, 7.0, 7.1, and 7.2 have a bug in the EtherIP protocol.
For more details and workaround, see
driver first appeared in
.Fx 6.0 .
driver was originally written by
.An Jason L. Wright
as part of an undergraduate independent study at the University of
North Carolina at Greensboro.
This version of the
driver has been heavily modified from the original version by
.An Jason R. Thorpe
Rapid Spanning Tree Protocol (RSTP) support was added by
.An Andrew Thompson
driver currently supports only Ethernet and Ethernet-like (e.g., 802.11)
network devices, with exactly the same interface MTU size as the bridge device.
Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.