Wireless Access

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Access network design for branch, remote, outdoor and campus locations with Aruba access points, and mobility controllers.
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Subnet Sizing

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  • 1.  Subnet Sizing

    Posted Jan 30, 2012 09:15 AM

    I have a scenario with a controller and a separate gateway router connected to the controller's uplink. Configured on this controller is a User VLAN pool. My question relates to the subnet size per User VLAN. I believe that the documentation recommends class C sized subnets. I believe that one key reason for this subnet size is to minimize ARP broadcasts. For example, If my gateway router sends an ARP, the controller can act as an ARP Proxy if it has the IP/MAC information, otherwise it broadcasts the ARP out all its interfaces including the wireless interfaces. Reducing the subnet size reduces the number of ARPs a wireless station will see.


    With my 8 co-located controllers, this subnet size leads to hundreds of VLANs! I believe that, in my case, I can increase the subnet size to 512 or even 1024 addresses because of the following:


    a) Each controller uses a separate VLAN pool (ie every controller uses different VLAN IDs). Therefore, a controller will not see an ARP that was intended for another controller.  This prevents a controller from ARPing while the destination station is associated to another controller.  I don't need mobility across controllers.


    b) There are no wired stations on the controller for which the controller may not have IP/MAC information.


    c) The controller will have IP/MAC information for all associated stations. I don't expect much if any traffic to non associated stations because 99% of the traffic is client/server,  initiated by the wireless station.


    Based on the above, I would like to increase my subnet size per User VLAN to 512 or even 1024 addresses. I would appreciate any feedback from the forum on this.



  • 2.  RE: Subnet Sizing

    Posted Jan 30, 2012 10:33 AM

    So let me clarify, you want to have a single vlan ie. - = VLAN 1; - = VLAN 2, etc. 


    You technically could do this, but I would not suggest, it. There are benefits to keeping the subnet's in the Class C area (breaking up broadcast domains, security reasons, etc.) far out weigh the ability to use a Class B.  I'm sure there are also other ways of getting the desired affect you wish to do without creating such a large subnet.


    Also, once you get past a /24 you enter into the Class B subnets.


    Anyway, just my 2 cents, how ever accurate that truly is.



  • 3.  RE: Subnet Sizing

    Posted Jan 30, 2012 11:47 AM

    If you would like to increase the subnet size and keep the ARP broadcasts to minimum, I would suggest turning on the following two features under the Virtual AP profile 


    broadcast-filter all      - - - - Drop all broadcast or multicast traffic in the air
    broadcast-filter arp    - - - - Convert broadcast ARP requests in the air to unicast


    config terminal 

    wlan virtual-ap <vap-name>




  • 4.  RE: Subnet Sizing

    Posted Feb 02, 2012 06:20 PM

    Does it make sense to use bcmc-optimization with, broadcast-filter-all & broadcast-filter-arp?



  • 5.  RE: Subnet Sizing
    Best Answer

    Posted Feb 06, 2012 11:51 AM

    Yes, bc-mc optimization makes perfect sense.  I have pretty large VLANs for my RAPs (/21s) and bc-mc optimization is a must.  I have no site-to-site communications and I certainly don't want any unnecessary broadcasts going between sites.


    If you use any kind of multicast application, don't use broadcast-filter all, as it will break multicast.  I've also played with broadcast-filter arp and had mixed results with that, so be sure to test before putting that in full production. 


    Keep in mind, at least last I checked, the broadcast-filter commands are only for wifi clients and don't work for wired clients on the RAP.  bc-mc optimization works for all clients.