Given my understanding of the way arp works, not sure I understand how it is possible for a large subnet to cause a client arp table to become exhausted unless that client for some reason is directly communicating with all of the other endpoints on the large subnet.
My understanding is that the table is only populated in response to arp queries that the client has initiated, even though it can “hear” responses from other clients that are sent as a broadcast. It is easy enough to verify this on Windows with an arp –a. I also don’t believe that broadcast traffic can have a material impact on clients these days due to increases in CPU power at the magnitude of Moore’s Law. Pete Morrissey From: The EDUCAUSE Wireless Issues Constituent Group Listserv [mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU] On Behalf Of Osborne, Bruce W (Network Services) Sent: Thursday, July 28, 2016 7:21 AM To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU Subject: Re: [WIRELESS-LAN] How big are your wireless segments? Tim, Another issue mentioned in the Aruba document is that clients had limited arp table size. Large subnets can exhaust them, causing service issues. Layer 2 apps like Chromecast & Apple TV are handled by vendor-specific solutions. Aruba Networks’ solution is called AirGroup. It is basically a software defined network solution that works quite well for us with Apple TV. We are planning, but have not yet implemented this for Roku & Chromecast. Bruce Osborne Wireless Engineer IT Network Services - Wireless (434) 592-4229 LIBERTY UNIVERSITY Training Champions for Christ since 1971 From: Tim Tyler [mailto:ty...@beloit.edu] Sent: Wednesday, July 27, 2016 12:26 PM Subject: Re: How big are your wireless segments? So I am guessing from this conversation that the reason the bandwidth consumption remains the same regardless of one or multiple vlans is because the frequency still sees the broadcast even if most vlans do not. And the frequency is what counts. {please correct me if I am wrong}. Hence an arp from a client uses the same amount of bandwidth regardless of the number of total clients that see it because vlans share the same bandwidth (frequency) with one another given any AP. Even if bandwidth is not an issue, wouldn’t performance still remain an issue if end devices have to process and drop/ignore higher volumes of broadcast traffic on a regular basis? And if one resolves that issue by blocking all broadcast traffic, does that affect layer 2 apps like Chromecast? Tim From: The EDUCAUSE Wireless Issues Constituent Group Listserv [mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>] On Behalf Of Jake Snyder Sent: Tuesday, July 26, 2016 11:25 AM To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU> Subject: Re: [WIRELESS-LAN] How big are your wireless segments? Actually, they don't have to "respond." They have to process the incoming frame. If they aren't listening for that port, they will ignore or drop the packet. If you are talking about client impact to CPU/battery/etc, I agree. If you are talking about airtime, the sum of the broadcast traffic is the same. Stopping broadcast over the air is the scalable way to solve Thanks Jake Snyder Sent from my iPhone On Jul 26, 2016, at 6:00 AM, Osborne, Bruce W (Network Services) <bosbo...@liberty.edu<mailto:bosbo...@liberty.edu>> wrote: Actually, you reduce the broadcast traffic with smaller subnets. Remember that all clients on the subnet *must* respond to a broadcast. Smaller subnets generally mean fewer clients responding to a given broadcast. This leaves more airtime for productive Wi-Fi traffic. Bruce Osborne Wireless Engineer IT Network Services - Wireless (434) 592-4229 LIBERTY UNIVERSITY Training Champions for Christ since 1971 From: Jake Snyder [mailto:jsnyde...@gmail.com] Sent: Monday, July 25, 2016 1:28 PM Subject: Re: How big are your wireless segments? One thing to remember is that over the air you have the same amount of broadcast whether it is one vlan or a pool of 4. For Example: If you have 4 client segments that are a /24, and each AP has a client on one of the 4 subnets, you still send the sum of 4x /24 network broadcast over the air. Meaning only on lightly loaded APs where you don't have all 4 subuets do you get a net gain of airtime. Same applies for link-local multicast. Smaller subnets in pools don't really gain you much without the suppression techniques, and with the suppression techniques, you don't need the smaller subnets. The place where pools/groups of vlans are attractive is where you may be using public IPs and don't have a large contiguous block of IPs in which to place clients. So picking 4 non-contiguous /24 networks is easier to do than picking a full class B. On Mon, Jul 25, 2016 at 11:04 AM, Tim Tyler <ty...@beloit.edu<mailto:ty...@beloit.edu>> wrote: Brian, We have pools of /22 /23/ and /24. We separate our pools from students vs fac/staff (still on the same ssid). It may be ok to do /16. I know that Aruba does a lot to prevent broadcast storms, but I feared the overhead of one large segment might have on it. We also give students a different ip pool depending whether they are in a residential building vs an academic/admin building. This allows us to shape traffic differently. But this will become less of an issue as we acquire more bandwidth (hopefully). I am curious of those using /16, does that resolve your layer 2 issues? Aruba does a good job of bridging many layer 2 solutions anyways, but having one /16 vlan does seem enticing and perhaps unnecessary for bridging protocols. However, I am curious about other overhead efficiency issues. Tim From: The EDUCAUSE Wireless Issues Constituent Group Listserv [mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>] On Behalf Of Brian Helman Sent: Monday, July 25, 2016 10:22 AM To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU> Subject: [WIRELESS-LAN] How big are your wireless segments? We are in the process of moving from a controllerless vendor to Aruba. Our current design is very segmented, to keep wireless device broadcasts from overwhelming the network and AP’s (we had this problem back in 11g days). Presently, we’ve limited segments to /23’s (give or take). In your controller-based environments, how large have you let these segments go? Is a /21, /20 … viable? -Brian ____________________________________ Brian Helman, M.Ed | Director, ITS/Networking Services | •: 978.542.7272<tel:978.542.7272> Salem State University, 352 Lafayette St., Salem Massachusetts 01970 GPS: 42.502129, -70.894779 ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/groups/.
