A few years ago, I made the mistake of trying to fly home the weekend before Christmas. It was one of those travel days that made my road warrior senses go into high alert, but what was I to do? Christmas was just around the corner and I wanted to get home. Unfortunately, my senses were correct and just like dominos, the flight cancelation announcements starting blaring from the public announcement system one by one…hey, that was my flight they just announced.
If you have never been in this predicament before, you have only one option – beat your fellow travelers to the customer service counter. After all, the odds are in your favor to get your flight reassigned when you are in first in line. At this point, some travelers toss aside their civilized behavior and make a mad dash for the customer service counter. By the time I made it to the customer service counter, I was near the end of the line and even from here I could hear the customer service agent make comments like “I am sorry but the only available flights to your destination will depart here on Christmas Eve.”
My road warrior senses told me it was time for plan “B” to go into action. So, while I stood in line, I called rental car company after rental car company to see if I they had any cars available. If they did, I would gear up for a road trip and drive home. No such luck, every rental car company was out of cars due to all of the cancellations.
All I can do now is patiently wait in line and hope the customer service agent could find me a flight home. Finally, after 30 minutes, it was my turn to talk to the customer service agent. She said there was one more flight available scheduled to depart that evening. The only problem was that the last two seats belonged to two passengers who had not checked in, so the agent put me, along with everyone else, on the standby list.
To my surprise, the agent informed me that I was moved to the top of the standby list due to my elite status. At this point a glimmer of hope started to set in because the only things preventing me from getting home would be if both passengers checked in or other travelers with a higher elite status were put on the standby list. I waited on pins and needles for two hours before the agent announced the names of the two lucky standby passengers. Thanks to my elite priority status, I was able to take that flight home. It was a bitter sweet victory because I knew someone else got bumped from that flight because they were a lower priority on the standby list.
When a controller fails, the access points are engineered to search for a secondary, tertiary, master or least loaded controller. This works perfectly as long as you have enough access point capacity on the remaining controllers. However, as your wireless networks grows access point by access point, you run the risk of having inadequate access point capacity on those remaining controllers. This will cause an unpredictable wireless coverage outage since you can not guarantee which access points will join a controller and which ones will not.
This condition is caused by the way the controller manages the additional access point association requests in a first come first served basis. Just like the airline travelers, all the access points rush to find a controller, just to find out they are at the mercy of where they are in the controller’s association request queue. This can be detrimental to a mission critical wireless network, so in a controller based wireless network, excessive access point capacity is the only way to guarantee that access points located in strategic locations will have a controller to join.
Or, at least, that’s the way it was before Cisco added a controller feature called failover priority.
Now, the access points can have a priority value assigned to them that guarantees the access points will have priority over lower priority access points. Failover priority is like the airline’s platinum, gold, silver, and bronze priority club. Cisco’s controller based access points can be assigned a “priority club” status of critical (4), high (3), medium (2), or low (1). The premium priority is critical (4), but the default value for all access point is low (1).
The failover priority feature has been available since controller firmware version 5.1 and it is disabled by default. In my opinion, it makes sense to enable failover priority because it does not affect your regular day-to-day wireless network. Failover priority only engages when the controller sees additional access point association requests over and over again after a controller failure. These excessive association requests mean that the access points are failing to find a controller to join because there is not enough access point capacity on the remaining controllers.
If the requesting access points have a higher priority than the default of low (1), then the controller will allow that access point to cut in line and join the controller before a lower priority access point. Failover priority will even go as far as removing a lower priority access point that has joined the controller in order to give the higher priority access point a controller to join. Sort of like getting your airline seat taken away because a traveler with a higher priority status is on the standby list.
Next time I’ll show you how you can configure this feature on your system.
- Cisco Wireless LAN Controller Configuration Guide, Software Release 6.0, November 2009, Page 507
Guest Author: Dennis Nofsinger, Senior Wireless Training Specialist, Gigawave Technologies