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IF_BRIDGE(4)		 BSD Kernel Interfaces Manual		  IF_BRIDGE(4)

     if_bridge -- network bridge device

     device if_bridge

     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 ex-
     ample, 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).

     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 hub,	forwarding traffic from	one interface to an-
     other.  Multicast and broadcast packets are always	forwarded to all in-
     terfaces 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.

     The if_bridge driver implements the IEEE 802.1D Spanning Tree protocol
     (STP).  Spanning Tree is used to detect and remove	loops in a network

     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, this behaviour can be controlled using sysctl(8):  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 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

     ARP and REVARP packets are	forwarded without being	filtered and others
     that are not IP nor IPv6 packets are not forwarded	when pfil(9) filtering
     is	enabled.  IPFW can filter Ethernet types using mac-type	so all packets
     are passed	to the filter for processing.

     Note that packets to and from the bridging	host will be seen by the fil-
     ter on the	interface with the appropriate address configured as well as
     on	the interface on which the packet arrives or departs.

     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 following when	placed in the file /etc/rc.conf	will cause a bridge
     called "bridge0" to be created, and will add the interfaces "wi0" and
     "fxp0" to the bridge, and then enable packet forwarding.  Such a configu-
     ration could be used to implement a simple	802.11-to-Ethernet bridge (as-
     suming the	802.11 interface is in ad-hoc mode).

	   ifconfig_bridge0="addm wi0 addm fxp0	up"

     Consider a	system with two	4-port Ethernet	boards.	 The following will
     cause a bridge consisting of all 8	ports with Spanning Tree enabled to be

	   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 \

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

     The if_bridge driver first	appeared in FreeBSD 6.0.

     The bridge	driver was originally written by Jason L. Wright
     <> 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 <>.

     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.

BSD				 June 10, 2005				   BSD


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