.\"	$NetBSD: bridge.4,v 1.12 2018/08/14 06:21:36 maxv Exp $
.\"
.\" Copyright 2001 Wasabi Systems, Inc.
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.\" Written by Jason R. Thorpe for Wasabi Systems, Inc.
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.Dd August 14, 2018
.Dt BRIDGE 4
.Os
.Sh NAME
.Nm bridge
.Nd network bridge device
.Sh SYNOPSIS
.Cd "pseudo-device bridge"
.Sh DESCRIPTION
The
.Nm
driver creates a logical link between two or more IEEE 802 networks
that use the same (or
.Dq 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.
.Pp
To use
.Nm bridge ,
the administrator must first create the interface and configure
the bridge parameters.
The bridge is created using the
.Xr ifconfig 8
.Cm create
subcommand.
The learning and forwarding behavior and other parameters of a
bridge are configured by the
.Xr brconfig 8
utility.
.Pp
A bridge can be used to provide several services, such as a simple
802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
.Pp
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.
.Pp
The
.Nm
driver implements the IEEE 802.1D Spanning Tree protocol (STP).
Spanning Tree is used to detect and remove loops in a network topology.
.Pp
Transparent filtering for IP and IPv6 packets can be added with the kernel
configuration option
.Cd options BRIDGE_IPF .
.Pp
When filtering is enabled, bridged packets will pass through the filter
inbound on the originating interface and outbound on the appropriate
interfaces.
ARP and REVARP packets are forwarded without being filtered and others
that are not IP nor IPv6 packets are not forwarded when filtering is
enabled.
.Pp
Note that packets to and from the bridging host will be seen by the
filter on the interface with the appropriate address configured as well
as on the interface on which the packet arrives or departs.
.Pp
The
.Nm
driver will enable passing of VLAN tagged packets automatically if the underlying
interfaces support it.
This is to facilitate XEN network configurations with
.Xr xennet 4 .
.Sh SEE ALSO
.Xr l2tp 4 ,
.Xr options 4 ,
.Xr xennet 4 ,
.Xr brconfig 8 ,
.Xr ipf 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Nx 1.6 .
.Sh AUTHORS
.An -nosplit
The
.Nm bridge
driver was originally written by
.An Jason L. Wright
.Aq jason@thought.net
as part of an undergraduate independent study at the University of
North Carolina at Greensboro.
.Pp
This version of the
.Nm
driver has been heavily modified from the original version by
.An Jason R. Thorpe
.Aq thorpej@wasabisystems.com .
.Sh BUGS
The
.Nm
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.
.Pp
The
.Nm
driver currently does not support snooping via
.Xr bpf 4 .