What is a Cisco BVI interface? What is it used for?

Solution 1:

A BVI (Bridge Group Virtual Interface) is a routed interface that represents a set of interfaces that gets bridged.

So lets say for example that in your router you have two interfaces that needs to get bridged (one wireless and one wired for example) and you want them to behave as if they were part of the same layer two broadcast domain (because you want your hosts in either the wired or wireless interfaces behave as if they were in the same subnet). In this scenario, a BVI would act as the routed interface for that "merged" (really bridged) interfaces.

Any packets that wants to "escape" this merged layer two domain (wired + wireless) will have to go through this BVI.

To this BVI you could assign a layer three address, QoS policies, access-lists or any other "service" you may assign to any physical interface.

From this document:

When you configure and enable routing on the BVI, packets that come in on a routed interface, which are destined for a host on a segment in a bridge group, are routed to the BVI. From the BVI, the packet is forwarded to the bridging engine, which forwards it through a bridged interface. This is forwarded based on the destination MAC address. Similarly, packets that come in on a bridged interface, but are destined for a host on a routed network, first go to the BVI. Next, the BVI forwards the packets to the routing engine before it sends them out of the routed interface.

I really hopes this helps you understand BVIs.

Solution 2:

BVI can replace Vlan interfaces. So instead of having a Vlan interface that routes packets (coming from acces port attached to that vlan number) that needs to be routed outside the vlan, the BVI does the same thing, routing packets outside de layer 2 domain from L2 interfaces that are bridged to that BVI.

In another words, if you understand how Vlan interfaces works for routing packets inter-vlan, you understand BVI as they are the exact same thing, just another way of doing things. Why choose one over the other , I don't know.