Command Section
IF_BRIDGE(4)           FreeBSD Kernel Interfaces Manual           IF_BRIDGE(4)

     if_bridge - network bridge device

     To compile this driver into the kernel, place the following line in your
     kernel configuration file:

           device if_bridge

     Alternatively, to load the driver as a module at boot time, place the
     following lines in loader.conf(5):


     The if_bridge driver creates a logical link between two or more IEEE 802
     networks that use the same (or ``similar enough'') framing format.  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.

     Each if_bridge interface is created at runtime using interface cloning.
     This is most easily done with the ifconfig(8) create command or using the
     cloned_interfaces variable in rc.conf(5).

     The if_bridge 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 only across all if_bridge
     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

     If sysctl(8) node 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 by ng_pppoe(4) and ppp(8).  Now this feature is considered as
     experimental and is turned off by-default.

     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 to
     another.  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 ifconfig(8) or
     ifconfig_<interface>="up" in rc.conf(5).

     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 ``monitor mode'', where the packets are discarded
     after bpf(4) processing, and are not processed or forwarded further.
     This can be used to multiplex the input of two or more interfaces into a
     single bpf(4) stream.  This is useful for reconstructing the traffic for
     network taps that transmit the RX/TX signals out through two separate

     if_bridge supports the AF_INET6 address family on bridge interfaces.  The
     following rc.conf(5) variable configures an IPv6 link-local address on
     bridge0 interface:


     or in a more explicit manner:

           ifconfig_bridge0_ipv6="inet6 auto_linklocal"

     However, the AF_INET6 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 if_bridge
     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, RFC 4007.  Although it works
     in most cases, it can cause some conterintuitive or undesirable behavior
     in some edge cases when both of the if_bridge interface and one of the
     member interface have an IPv6 address and applications use both of them.

     To prevent this situation, if_bridge checks whether a link-local scoped
     IPv6 address is configured on a member interface to be added and the
     if_bridge interface.  When the if_bridge 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 sysctl(8) variable to 1.

     Note that ACCEPT_RTADV and AUTO_LINKLOCAL interface flag are not enabled
     by default on if_bridge interface even when net.inet6.ip6.accept_rtadv
     and/or net.inet6.ip6.auto_linklocal is set to 1.

     The if_bridge 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 via the
     proto command in ifconfig(8).

     The bridge can log STP port changes to syslog(3) by enabling the variable using sysctl(8).

     Packet filtering can be used with any firewall package that hooks in via
     the pfil(9) framework.  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
     sysctl(8):      Controls the handling of non-IP packets
                                      which are not passed to pfil(9).  Set to
                                      1 to only allow IP packets to pass
                                      (subject to firewall rules), set to 0 to
                                      unconditionally pass all non-IP Ethernet
                                      frames.      Set to 1 to enable filtering on the
                                      incoming and outgoing member interfaces,
                                      set to 0 to disable it.      Set to 1 to enable filtering on the
                                      bridge interface, set to 0 to disable
                                      it.  Set to 1 to additionally filter on the
                                      physical interface for locally destined
                                      packets.  Set to 0 to disable this
                                      feature.             Set to 1 to enable layer2 filtering with
                                      ipfirewall(4), set to 0 to disable it.
                                      This needs to be enabled for dummynet(4)
                                      support.  When ipfw is enabled,
                                      pfil_bridge and pfil_member will be
                                      disabled so that IPFW is not run twice;
                                      these can be re-enabled if desired.         Set to 1 to enable layer2 ARP filtering
                                      with ipfirewall(4), set to 0 to disable
                                      it.  Requires ipfw 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 pfil_onlyip is
     enabled.  IPFW can filter Ethernet types using mac-type 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 table.

     The packets destined to the bridging host will be seen by the filter on
     the interface with the MAC address equal to the packet's destination MAC.
     There are situations when some of the bridge members are sharing the same
     MAC address (for example the vlan(4) 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 packet's 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
     interface.  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 if_bridge.  It is not recommended to rely on the order
     chosen by the current if_bridge implementation: it can be changed in the

     The previous paragraph is best illustrated with the following pictures.

        the MAC address of the incoming packet's destination is

        the interface on which packet entered the system is ifX,

        ifX MAC address is xx:xx:xx:xx:xx:xx,

        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 yy:yy:yy:yy:yy:yy; we will call them vlanY1, vlanY2, etc.

     Then if the MAC address nn:nn:nn:nn:nn:nn is equal to the
     xx:xx:xx:xx:xx:xx then the filter will see the packet on the interface
     ifX no matter if there are any other bridge members carrying the same MAC
     address.  But if the MAC address nn:nn:nn:nn:nn:nn is equal to the
     yy:yy:yy:yy:yy:yy then the interface that will be seen by the filter is
     one of the vlanYn.  It is not possible to predict the name of the actual
     interface without the knowledge of the system state and the if_bridge
     implementation details.

     This problem arises for any bridge members that are sharing the same MAC
     address, not only to the vlan(4) 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 if_bridge interface and
     not to the bridge members.  Enabling will
     let you do the additional filtering on the physical interface.

     The following when placed in the file /etc/rc.conf will cause a bridge
     called ``bridge0'' to be created, and will add the interfaces ``wlan0''
     and ``fxp0'' 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:

           create_args_wlan0="wlanmode hostap"
           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
     protocol.  This can be combined with ipsec(4) to provide an encrypted
     connection.  Create a gif(4) 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 up
           ifconfig bridge0 create
           ifconfig bridge0 addm fxp0 addm gif0 up

     Note that FreeBSD 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 gif(4) manual

     gif(4), ipf(4), ipfw(4), pf(4), ifconfig(8)

     The if_bridge driver first appeared in FreeBSD 6.0.

     The bridge driver was originally written by Jason L. Wright
     <[email protected]> as part of an undergraduate independent study at the
     University of North Carolina at Greensboro.

     This version of the if_bridge driver has been heavily modified from the
     original version by Jason R. Thorpe <[email protected]>.

     Rapid Spanning Tree Protocol (RSTP) support was added by Andrew Thompson
     <[email protected]>.

     The if_bridge 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.

FreeBSD 11.1-RELEASE-p4          July 27, 2013         FreeBSD 11.1-RELEASE-p4
Command Section