BVI - What is it and what are its uses?

[grapevine]

BVI - What is that and what are its uses?

[Peter Paluch]

Ranji,

A BVI is in fact quite similar to an SVI (interface Vlan). You can define a software bridging between various ports of a router, similar to switching between various ports on a switch. If the ports on a switch belong to the same VLAN and the switch is capable of multilayer switching, you can create an interface Vlan for that VLAN and allow the hosts in that VLAN to use the IP address of the interface Vlan as their default gateway.

The same goes for interface BVI - Bridged Virtual Interface. When configuring software bridging, you define a group of interfaces that are bridged - the router performs bridging (i.e. software-based switching) of frames between all member ports of a bridge group, in essence forming a single broadcast domain - an IP subnet. If the devices in the common bridge group want to access other IP networks, they need a gateway, so you create an associated interface BVI that is also a part of the bridge group, and devices in the bridge group then use the IP address of the BVI interface as their gateway.

For exampe, imagine a router with two FastEthernet interfaces:

bridge irb
!
interface FastEthernet0/0
 no ip address
 no shutdown
 bridge-group 1
!
interface FastEthernet0/1
 no ip address
 no shutdown
 bridge-group 1
!
interface BVI1
 ip address 10.0.0.1 255.255.255.0
 no shutdown
!
bridge 1 route ip

This configuration would make your router to basically behave as a 2-port "switch" on its Fa0/0 and Fa0/1 interfaces, and devices connected to these ports would use the 10.0.0.1 as their default gateway to other networks.

You rarely configure bridging exactly this way these days, as switches are orders of magnitude faster and have way more ports. Still, there are situations where you need to bridge two interfaces, taking packets out of frames of one technology and putting them into frames of a different technology, without routing them, just repackaging but still carrying them between interfaces. This is often done in, say, DSL if the router is configured to act in bridge mode - take IP packets coming to Ethernet interface and simply repackage them into PPP or ATM+AAL5 cells on the DSL WAN port (and vice versa).

Best regards,

Peter

[Karsten Iwen · Accepted solution]

Best BVI-explaination I've ever read!

-- 
Don't stop after you've improved your network! Improve the world by lending money to the working poor:
http://www.kiva.org/invitedby/karsteni

[Rolitto]

Neat, thanks!

[codingfreak]

Thanks for the explanation

[i_banglawala]

just the right amount of information . very helpful indeed !

[arunec539]

Hello Peter,

I have two firewalls where hsrp is configured and each firewall is connected to an access switch. For now one switch is connected to a router and I want to make use of another port on the router to connect the other access switch.For redundancy purpose, I am thinking to use the other port. Will the BVI helps to achieve it.

[vivek-srivastava]

How BVI differs from Port-channel bundling ? Can we say that BVI supports load sharing in anyway like Etherchannel ?

[Joseph W. Doherty]

"Can we say that BVI supports load sharing in anyway like Etherchannel ?"

Not that I'm aware of.

"How BVI differs from Port-channel bundling ?"

PO interface provides a virtual interface that provides the aggregate bandwidth of all its active member ports, also provides redundancy when using multiple ports 

BVI, on a router, is much like a SVI on a L3 switch. It provides a virtual interface not tied to just one physical port.

On a L3 switch a SVI might be used with L2 PO interfaces.  I've never tried it, but on a router, possibly, a BVI might be used with L2 PO interfaces too.

BTW, on later IOS versions, the BVI has been renamed, but I forget what's its new name.

[paul driver]

Hello
An BVI used in bridging
Please review the below link it will explain nicely how a bridging works

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_tech_note09186a0080094663.shtml

Res
Paul


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[Leo Laohoo]

The most common use of Bridged Virtual Interface is when you configure wireless access points (APs).

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[Peter Paluch]

Hi Leo,

I agree - but it should be said that on APs, the BVIs are used just for management and control plane traffic, and they do not provide the gateway function for which they have originally been invented when IRB became supported on routers. In other words, BVIs on autonomous APs are there to assign IP addresses to APs so they can be remotely managed, talk to RADIUS/TACACS servers etc., but not for routing purposes.

Best regards,

Peter

[Leo Laohoo]

Hi Peter,

That's correct. However, I've never encountered BVI until I started playing with APs. And this is the point I'm trying to imply.

PS: Happy New Year to all! :)

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[Peter Paluch]

Hi Leo,

Yes - of course. I did not mean to belittle your answer and experiences - I am sorry and I apologize if it sounded that way.

Happy New Year to you, too! Although here in Slovakia, we still have 2013 as of writing this response. Yeah, we're quite behind

Best regards,

Peter

[DominicMcIntire90023]

I realize I'm a few years behind on commenting, but just found this and it was an incredibly helpful explanation. Thank you.