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if_bridge (Network Bridge Device)

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 create command.

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 ifconfig.

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 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.

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 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 stream. This is useful for reconstructing the traffic for network taps that transmit the RX/TX signals out through two separate interfaces.

Spanning Tree

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.

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

Packet Filtering

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:
Set to 1 to enable layer2 filtering with ipfw, set to 0 to disable it. This needs to be enabled for dummynet support.
Set to 1 to enable layer2 ARP filtering with ipfw, 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 interfaces: they are currenly 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 future.

The previous paragraph is best illustrated with the following pictures. Let

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 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 "%intimecfg%\{nodename\etc\netuser.cfg" will cause a bridge called "bridge0" to be created, and will add the interfaces 'ven0' and 'ie1g0' 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.rta -q bridge create 
ifconfig.rta bridge0 addm ven0 addm ie1g0 up
See Also