My understanding of bridging is that it ties together two interfaces at layer 2. I am looking at a Ubiquiti Nanostation2 running OpenWRT that has an ethernet port 'eth0' and a wifi port 'ath0'. The ethernet port (the 'wan' port) is not part of the bridge and the bridge is just a single interface. Can anyone clarify this? - seems very different to Ubuntu.

/etc/config/network:

config 'interface' 'loopback'
    option 'ifname' 'lo'
    option 'proto' 'static'
    option 'ipaddr' '127.0.0.1'
    option 'netmask' '255.0.0.0'

config 'interface' 'wan'
    option 'ifname' 'eth0'
    option 'proto' 'dhcp'

config 'interface' 'wifi'
    option 'ipaddr' '192.168.13.1'
    option 'type' 'bridge'
    option 'proto' 'static'
    option 'netmask' '255.255.255.0'
    option 'ifname' 'wifi0'