Bridging
Definition 1
A network bridge is a computer networking device that creates a single, aggregate network from multiple communication networks or network segments. This function is called network bridging.
Bridging is distinct from routing. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network. In the OSI model, bridging is performed in the data link layer (layer 2). If one or more segments of the bridged network are wireless, the device is known as a wireless bridge.
Definition 2
It is sometimes useful to divide a network, such as an Ethernet segment, into network segments without having to create IP subnets and use a router to connect the segments together. A device that connects two networks together in this fashion is called a “bridge”.
A bridge works by learning the MAC addresses of the devices on each of its network interfaces. It forwards traffic between networks only when the source and destination MAC addresses are on different networks. In many respects, a bridge is like an Ethernet switch with very few ports.
Setup Bridge
The bridge is created using interface cloning. To create the bridge interface:
$ ifconfig bridge create
bridge0
$ ifconfig bridge0
bridge0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
ether 96:3d:4b:f1:79:7a
id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
maxage 20 holdcnt 6 proto rstp maxaddr 100 timeout 1200
root id 00:00:00:00:00:00 priority 0 ifcost 0 port 0
When a bridge interface is created, it is automatically assigned a randomly generated Ethernet address. The maxaddr and timeout parameters control how many MAC addresses the bridge will keep in its forwarding table and how many seconds before each entry is removed after it is last seen. The other parameters control how STP operates.
Next, specify which network interfaces to add as members of the bridge. For the bridge to forward packets, all member interfaces and the bridge need to be up:
$ ifconfig bridge0 addm fxp0 addm fxp1 up
$ ifconfig fxp0 up
$ ifconfig fxp1 up
The bridge can now forward Ethernet frames between fxp0 and fxp1. Add the following lines to /etc/rc.conf so the bridge is created at startup:
cloned_interfaces="bridge0"
ifconfig_bridge0="addm fxp0 addm fxp1 up"
ifconfig_fxp0="up"
ifconfig_fxp1="up"
If the bridge host needs an IP address, set it on the bridge interface, not on the member interfaces. The address can be set statically or via DHCP. This example sets a static IP address:
$ ifconfig bridge0 inet 192.168.0.1/24
It is also possible to assign an IPv6 address to a bridge interface. To make the changes permanent, add the addressing information to /etc/rc.conf.
Reference
- https://en.wikipedia.org/wiki/Bridging_(networking)
- https://docs.freebsd.org/en_US.ISO8859-1/books/handbook/network-bridging.html