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I have been Google'd and searched for the answer to these questions, but I cannot find a simple answer.

So my questions are about BVI interfaces. What are they? Why do you use them? In what scenario are they useful?

I've searched through CCNA books, but couldn't find an answer. I've also seen the (long) descriptions on Cisco's site, but frankly, I couldn't understand.

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up vote 11 down vote accepted

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.

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So if I understand, packets that are destined / coming out of a network in the bridged group go through the BVI first, which then decides what to do with it, based on the destination MAC. Also, the bridged networks can communicate with each other as if they're on a "switch", and the BVI regulates access to/from this "switch"? – Inf.S Mar 31 '11 at 21:53
@Inf.S You are basically right, only the BVI is a routed interface so the forwarding decision it takes depends not on the destination mac address but on the destination IP address of the packet. It is the same concept as a "VLAN interface", that is the routed interface that L3 represents all the L2 ports on the VLAN to which it is attached. – jliendo Mar 31 '11 at 21:59
Ok, one last question: Can you give me a real-life scenario where a BVI would be useful or required? – Inf.S Mar 31 '11 at 22:18
@Inf.S Real life scenario (so real that it is the one I have configured in my router) is the one described in the answer, that is, I want to have my wireless users and my wired users to be on the same subnet. I do this by first bridging the wireless interface with the VLAN interface that "represents" the physical ports where my wired users are connected and then I create a BVI interface "attached" to this "bonding" (of wireless and VLAN interface). It is this BVI interface that is going to be the default router for my wireless and wired hosts. – jliendo Mar 31 '11 at 23:46
Another example would be two offices of the same company connected via a T1. You might use a BVI to "bridge" the two offices in order to have all of the hosts on the same subnet and using a common DNS infrastructure and having access to the same NetBIOS namespace (Network Neighborhood) facilitating a common "view" of network resources and infrastructure without having to route the required protocols from one physical network to another (NetBIOS, RPC, LDAP, etc). – joeqwerty Apr 1 '11 at 0:37

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.

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here is my real time scenario,

I'm trying to implement redundancy between core and distribution level. I was thinking to Portchannel [using 2 fastethernet/gigabit Ethernet] to connect back to distribution switch from Primary router and backup router. Then implement VRRP on Port channel that too with 802.1q [for sub interfaces]. But in order to do this I need to have switches in stack. Portchannel won;t work between two independent switches. Then implemented BVI between two physical ports [Fast0/1 & F0/2] on Primary and secondary router. Using BVI we can have redundant link between 2 independent switches.

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StackOverflow is a question/answer format. Please read the FAQ: – tristan Aug 19 '13 at 15:00

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