A BVI interface is used when you want to bridge between some interfaces but route from this group of interfaces to other interfaces. This is the same concept of VLAN interfaces on the 3550 (or above):
bridge 10 protocol ieee
bridge 10 route IP
!
interface ethernet0
bridge-group 10
!
interface ethernet1
bridge-group 10
!
interface ethernet2
ip address 10.1.1.1 255.255.255.0
!
interface bvi 10
ip address 10.2.1.1 255.255.255.0
!
In the example above interfaces ethernet0 & 1 are bridged so all connected devices share the same broadcast domain. To allow routing from this interface a BVI is created for the Bridge-group (in this case 10), this is then the Layer-3 interface for devices connected to Ethernet0 & 1. If a device on ethernet0 wants to talk to a device on ethernet1 it is bridged (the 2 devices are in the same IP network). If a device on Ethernet0 want to talk to a device on Ethernet2 it is routed by the BVI interface.
There is some documentatoin on CCO but I couldn't be bothered searching for it... Have a search for IRB
HTH
Andy
