BVI interfaces on cisco AP

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Mar 27th, 2007
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Hi all, I have come across a configuration whereas on a cisco AP the ethernet interface does not have an ip address, it looks like instead there as

a bridge virtual interface on there. Can anyone tell me what the BVI is being used for and why its on there ?


thanks

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answanso Tue, 03/27/2007 - 10:15
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Its used to bridge the radio traffic onto the fastethernet. Its done this way because typically an AP is not doing routing, so there needs to be a way for it to get off to the default gateway. If you have two IP addresses (1 for the radio and 1 for the FastEthernet) some sort of routing needs to be done. The BVI allows this traffic to be bridged between both the fastethernet and radio with 1 ip address.

carl_townshend Tue, 03/27/2007 - 13:10
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can you explain this a little please? We have a setup on the AP where the fast ethernet is configured with no ip address and the bvi has an ip address, why is it like this, we are only using them as an AP so why not just put an ip address on the fast ethernet interface ?

Edison Ortiz Wed, 03/28/2007 - 12:53
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Because the fast ethernet is a layer 2 interface on this case. The BVI is cloning its layer3 functionalities.


Think of a switchport being able to run L3 or L2 but using the 'switchport' or 'no switchport' command on its interface.



carl_townshend Wed, 03/28/2007 - 14:29
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thanks for the reply, so what benifit is this having on my AP ? we are using them as an AP only so surely using the fastethernet interface would of done, why use a BVI in this case, can anyone explain a little more ?

answanso Wed, 03/28/2007 - 14:53
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Basically the radio is an interface. Given that, you can assign an IP address to it. But if you did that you would have to use two different IP ranges, 1 for the radio interface and 1 for the ethernet interface. If you have two different connected networks, then you would need routing to get from one to the other. Something that a AP doesn't do as its a wireless bridge. So to fix this issue, we place both interfaces into a bridge-group. This bridge-group says all traffic from the radio out the ethernet, and all traffic in the ethernet out the radio. This also allows you to keep both interfaces in the same subnet, which removes the need for routing between networks.


example:


Radio 10.12.200.1

Ethernet 10.12.201.1


A packet arrives on the radio destined to google lets say. Our default gateway is 10.12.200.1 (the wireless interface) when it arrives on the AP (since we can not do routing) we have two routes 10.12.201.0 via ethernet and 10.12.200.0 via the radio. The packet for google fails because we have no idea how to get there...


example:


Radio bridge-group 1

Ethernet bridge-group 1

BVI1 10.12.200.2


But if you use a BVI, your address for both the radio is 10.12.200.2 or something like that, and your default gateway becomes that of the router. This bridges the traffic directly onto the network and the routing is done on the router with the address of (lets say) 10.12.200.1 . Now the router looks in its routing table and sees we have a route and the packet can be switched to google.


You can see the first option is not ideal and that is why BVI is used here.


HTH

Anthony

carl_townshend Thu, 03/29/2007 - 01:19
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Does this need to be done on all cisco AP's ? as at my old job, we just configured the ip address on the ethernet port for management only, the clients worked fine without using any kind of BVI, So I dont understand why you need a BVI ?

carl_townshend Thu, 03/29/2007 - 04:45
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so is a bvi the same as an svi on a switch, where you have multiple layer 2 interfaces that can see a layer 3 interface? is a bvi a layer 3 interface then ?

answanso Thu, 03/29/2007 - 06:59
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Not the same, but similar if you want to think about it that way.

answanso Thu, 03/29/2007 - 06:56
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So what Brand of APs were at your previous job and were they managed through a web interface? Older APs were managed via the web interface and ran a different OS. Plus if it was a different brand AP, then you wouldnt see a BVI as that is a Cisco thing.


I explained to you why we need a BVI, I will retouch on this for you. There are two interfaces on an AP. APs can not route therefore you can not assign the radio an IP and have the ethernet be in a different IP. So you need to bridge them together so both the radio and the ethernet are on the same segment. Please refer to my previous example.


carl_townshend Thu, 03/29/2007 - 08:32
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in my old company we were also using cisco AP's, why does the radio interface need an ip address? when you setup an AP from scratch, if I put an ip address on the fast ethernet interface, will it automatically create a BVI interface and put the radio and fastethernet into the same group ? it did look that way when I did it!!

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