Multi-building campus WiFi and VLAN IDs

[andrewcisco1]

Hi all.  We've a customer who has a large campus site with ~1,200 users (all who could use WiFi) and using flexconnect local switching.  It had been set up by a previous supplier as user VLAN per building, which has been giving issues with users who move between buildings (they don't release their DHCP allocation, and suddenly it won't work as their IP is the wrong VLAN).  The obvious solution is to use the same VLAN across the whole campus, but I'm worried about broadcasts with possibly 1,000 users on a single VLAN.  Has anyone any experience of this scenario?

For reasons of traffic flow, latency etc central switching wouldn't be desirable.

[Leo Laohoo · Accepted solution]

Break up into, for example, /24 subnets. 

Use VLAN Groups to "bundle" the VLANs together.  

vlan 101
 name BLAH-101
!
vlan 102
 name BLAH-102
!
vlan 103
 name BLAH-103
!
interface vlan 101
 ip address 10.0.1.1 255.255.255.0
 ip helper address 10.10.10.10
 no shutdown
!
interface vlan 102
 ip address 10.0.2.1 255.255.255.0
 ip helper address 10.10.10.10
 no shutdown
!
interface vlan 103
 ip address 10.0.3.1 255.255.255.0
 ip helper address 10.10.10.10
 no shutdown
!
vlan group BLAH-BLAH-BLAH vlan-list 101 - 103

[andrewcisco1]

The limitation with that is that it's not supported with APs in flexconnect mode, which these all would be.  Local mode APs work fine, but then we'd need double the number of APs. Very frustrating limitation as that sounds exactly what's needed here.

I'm currently looking at Proxy Mobile IP, which sounds promising but only appears in old docs.

[Rich R]

What clients are you seeing that problem with?
In my own testing previously I found Android, iOS and Windows clients all transitioned pretty quickly between VLANs and subnets with that type of scenario.  Real-time voice or video would see a brief interruption but streaming video wouldn't even notice it most of the time because of buffering.

Also what model of WLC, what AP models and what version of software?
Does the WLAN have DHCP required set?
Are the APs for each building in separate AP and flexconnect groups/tags/profiles?

[andrewcisco1]

Thanks for the reply  As it's only windows client which connect (corporate 802.1x with flexconnect local), I've only seen it with Windows.
The WLCs are 5520 running 8.10.190.0 (about to be replaced next year or so with 9800), so I assume flexconnect tags and profiles aren't relevant.  There's a single flexconnect group (all APs using local vlan assignments), though having just looked at it I wish I'd seen this before!  It's a constant problem of people forgetting to setup the VLANs per AP.

DHCP isn't set as required, I'd read bad reports about it so avoided.

It feels like one of things were neither microsoft or cisco are doing anything wrong, it's a odd scenario where the buildings are large but close so you don't lose WiFi signal when walking between them.  Is there some mechanism in windows to detect that the device needs to request a new DHCP IP?  I'd kind of expect it not to if signal isn't lost.

[Rich R]

The testing I did was with 2 separate sets of APs on different subnets literally next to each other so walking down a passage the user would roam from one to the other and that still worked ok but that was always with DHCP required.  We use that setting almost everywhere and never seen it cause us any problems.  As far as I could tell Windows seemed to be checking/renewing DHCP automatically every time it roamed but I didn't actually do a packet capture to confirm it.  That meant as soon as it transitioned it would get a NACK to the renewal and then know that it needed to complete DORA again.

The other thing to check is that the OS and WiFi drivers are fully up to date - in particular Intel as they had some major bugs in earlier versions (but same applies to all vendors):  https://www.intel.com/content/www/us/en/download/19351/windows-10-and-windows-11-wi-fi-drivers-for-intel-wireless-adapters.html