Cisco WLAN Device Disconnects

Problem statement

The issue I’m facing in an 802.1x Cisco Controller based Wireless Network is that 802.1x Wireless Clients are either A. whilst completely static, and sitting at desks, devices are disconnecting and I’m seeing a connectivity drop with DHCP renewal (according to Cisco AnyConnect supplicants) or B. roaming devices are hanging on to AP’s which are nowhere near the closest AP to the client when moving about the building.

Cisco Wireless LAN Controller

In Cisco WLC Release 8.0, Cisco included Optimized Roaming into their Controller code.

Cisco states the following on Optimized Roaming:-

Information About Optimized Roaming

“Optimized roaming resolves the problem of sticky clients that remain associated to access points that are far away and outbound clients that attempt to connect to a Wi-Fi network without having a stable connection. This feature disassociates clients based on the RSSI of the client data packets and data rate. The client is disassociated if the RSSI alarm condition is met and the current data rate of the client is lower than the optimized roaming data rate threshold. You can disable the data rate option so that only RSSI is used for disassociating clients.

Optimized roaming also prevents client association when the client’s RSSI is low. This feature checks the RSSI of the incoming client against the RSSI threshold. This check prevents the clients from connecting to a Wi-Fi network unless the client has a viable connection. In many scenarios, even though clients can hear beacons and connect to a Wi-Fi network, the signal might not be strong enough to support a stable connection.

You can also configure the client coverage reporting interval for a radio by using optimized roaming. The client coverage statistics include data packet RSSIs, Coverage Hole Detection and Mitigation (CHDM) pre-alarm failures, retransmission requests, and current data rates.”

I’m also very interested in getting log events into my SIEM platform to be able to see when disconnect events are happening. I’m currently trying to get clarity from the advanced logging options in the WLC as I’d like quite just those specific events to come through. I’ll update this section when I’m there.

Now, my final configuration will probably not fit your environment, in that we’re dealing with physics here (radio). My environment has it’s own unique physical characteristics. But I hope to share the journey I took to get to the correct balance for my organisation.

To gain an understanding of my particular environment, here’s some detail.

Building

The building in question is 72m x 29m in size. The build is mostly wood, glass and concrete and 70% open plan with two floors and a large atrium space.
The two floors are not discrete in that there’s clear air in two directions from any upper floor seat to the lower floor and vice versa.

Radios

Within the building, there’s 22 internal 3500 series APs which are the focus of our discussion and 8 external 2600 series APs which contribute to the RF characteristics but aren’t a major player for this discussion.
The radio spectrum in question is a 20Mhz wide 5Ghz WLAN based in the UK using all available indoor UK channels:
36,40,44,48,52,56,60,64,100,104,108,112,116,132,136,140

WiFi Nigel does an exceptional job of explaining the constraints of 5Ghz in the UK, here.

Devices

Day to day, there’s anything up to 400 devices across 5 WLANs within the building. The devices we’re focusing on number up to 270 on a single 5Ghz 802.1x enabled WLAN. They are the managed laptop devices.

The devices in my organisation are loaded with Intel® Centrino® Advanced-N 6235 Wi-Fi adapters which have several options for client side ‘Roaming Aggressiveness’.

centrino

A colleague of mine has already changed a couple devices to ‘5’ for Roaming Aggressiveness with no detrimental feedback, so we assume that is safe – for now – on those devices whilst we pay attention to the WLC.

Taking a note of the configuration of both the Laptops and the WLC configuration, I’m starting with the WLC Optimized Roaming configuration and leaving the Laptops on ‘4’ for Roaming Aggressiveness.

The strategy for the changes pan out as follows:-

  1. Enable Optimized Roaming on the WLC without RSSI – 10 sec interval
  2. Review Optimized Roaming change after 2 weeks and consider interval period.
  3. *Repeats* Add/Increase RSSI thresholds

Cisco states “If you configure a low value for the reporting interval, the network can get overloaded with coverage report messages”. I don’t understand how data, sent every 20secs from 30 APs would overload the network which is Gigabit access ports and 10Gig uplinks to the Core.
My with that long suggested timer interval is that if Optimized Roaming only executed disconnects every 1.5mins, that’s a long time for a device to be hanging around on a sub-optimal AP before it re-connects to something useful.

In the interest of only turning one knob at a time, I’m changing the WLC to enable Optimized Roaming allowing RSSI as it’s only metric and ignoring data rates for the time being.

Step 1. Configuring Optimized Roaming without RSSI from the WLC CLI.

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming enable
config advanced 802.11b optimized-roaming enable
config advanced 802.11a optimized-roaming interval 20
config advanced 802.11b optimized-roaming interval 20
config advanced 802.11a optimized-roaming datarate 0
config advanced 802.11b optimized-roaming datarate 0

config 802.11a enable network
config 802.11b enable network

show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats

Unless there’s any immediate negative consequence from enabling these settings, it’s only fair that the configuration is left alone for a reasonable amount of time before moving on with the RSSI modifications.

Two weeks seems like a good start, it enables you to carefully investigate any issues that aren’t global within the environment and confirm if they were real issues or emotional responses to the change.

Step 2. Configure RSSI as a part of Optimized Roaming

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming datarate 12
config advanced 802.11b optimized-roaming datarate 12
config 802.11a enable network
config 802.11b enable network
show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats
 

Step 3. Increase RSSI thresholds in Optimized Roaming

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming datarate 24
config advanced 802.11b optimized-roaming datarate 24
config 802.11a enable network
config 802.11b enable network
show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats
 

Step 4. Increase RSSI thresholds in Optimized Roaming

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming datarate 36
config advanced 802.11b optimized-roaming datarate 36
config 802.11a enable network
config 802.11b enable network
show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats
 

Step 5. Increase RSSI thresholds in Optimized Roaming

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming datarate 48
config advanced 802.11b optimized-roaming datarate 48
config 802.11a enable network
config 802.11b enable network
show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats
 

Step 6. Increase RSSI thresholds in Optimized Roaming

*You will need to disable your radios to complete this work!*

config 802.11a disable network
config 802.11b disable network
config advanced 802.11a optimized-roaming datarate 54
config advanced 802.11b optimized-roaming datarate 54
config 802.11a enable network
config 802.11b enable network
show advanced 802.11a optimized-roaming
show advanced 802.11a optimized-roaming stats

As of 25/11/15 I’m executing Step 1. on the 27/11/15.
I’ll continue to update this post as the process develops.