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New Member

Cisco 7921: Connects to AP on floor above

Hello,  We have two different entities we support and both are set up as autonomouse wireless groups.  The SSID is the same though so they can use their 7921's between floors and buildings.  The problem we have had is that the phones on the first floor were getting the IP's of the organization on the second floor.  When you view the AP's on the phone the ap directly next to it on the first floor (the one its suppose to talk to) is listed first, The strange thing is that shows it is connected to the right AP (the first floor one). The AP on the second floor is also listed but its about third down.  After the phone sits it will actually change IP's but not all of them.  Their were a few phones that took about a half a day to get the correct IP. 

My question is how do these phones grab their ip's?  I know how DHCP works but I am needing to know specifically how the radio works on the 7921's.  What is the boot up process? It appears that it must cycle through each channel starting with the highest and loop through a process in descending order: compare ssid's and then authenticate if possible.  I am assuming it is seeing the second floor first because initially they all get the second floor ip.  The second floor is on channel 60 and the one it should connect to is on channel 36 (We are using 802.11a).  That's how I came up with my descending channel theory.  Let me also add that my theory's are usually only good to help me sleep until I find out what was really happening.

Does anyone know the process it goes through at bootup to find a wireless signal?  Is there any way to make them look first at a specific channel?

We don't want to change the SSID's as this would totally defeat what we were trying to do. 




Cisco 7921: Connects to AP on floor above

Hi, My name is Eric Moyers. I am a Network Support Engineer in the Cisco Small Business Support Center. Thank you for using the Cisco Community Post Forums.

What is the overall goal here?

I am not picturing the the topology that you are using here. Can you post  a diagram of what you have? You can use this site "" to easily create one.

My gut reaction is that it would be better to go with two SSID's on all AP's. But I would like a little more info.


Eric Moyers

Cisco Network Support Engineer

SBSC Wireless and Surveillance SME

CCNA, CCNA-Wireless


New Member

Cisco 7921: Connects to AP on floor above

Thanks Eric,  here is what we are doing.  We want the 1st and 2nd floor's voice to be on separate vlans.  They need to be different subnet for CER to know what floor the caller is on in case of an emergency.  We want the SSID to be the same so users can use their phones between floors. 


Cisco 7921: Connects to AP on floor above

Normally, you can't control which phones connect to which APs.  They'll search for the strongest signal.  Sometimes, based on AP density, that signal comes from an AP on a different floor.

Here's the other information you were asking about:

A wifi phone boots and attempts to connect to it's configured SSID.  In your case, ACMEvoWLAN.

The controller has a wlan - interface configuration (wlans - general tab). 

The interface provides the L3 subnet.

Based on your diagram, I'm assuming you're using AP groups and you have AP's broadcase specific wlans.

I've never used CER, but a quick at the Admin Guide says it only supports tracking to the subnet.

It goes on to say it can track to the switchport of the AP that the phone is using, but cannot track them to the location.

Open up the Admin Guide and search for this:

Configuring IP Subnet-based ERLs


Ven Taylor
New Member

Re: Cisco 7921: Connects to AP on floor above

Yes Ven we are using AP groups and the respective AP's have the same SSID but they are dishing out separate subnets so CER can know who is on what floor.  I realize that they should find the strongest signal and connect to that AP.  The problem is that this is not whats happening.  When the phone initially boots up it must be connecting to the AP on the floor above.  I go to phone settings, diagnostics, WLAN on the 7921 and shows me that the AP it is connecting to is not the strongest signal.  The AP on the second floor (I am doing this from the first floor) is about the third AP down as it is listed by signal strength.  So initially when it boots up it connects to the wrong AP but after a while it changes IP addresses and uses the IP of the AP on the first floor.  So it is not connecting to the strongest AP signal.  Not initially.

That is why I was wondering if anyone knew any specifics about the boot up process.  Why would it pull an ip from an AP with a weaker signal? My guess is that the phone cycles through the channels starting with the highest number first and descends, taking the first one it sees.  The phones work fine after a little while, getting the correct IP address but initially, when they boot up; they pull an ip address from the AP on the floor above which is a far weaker signal.  The phones even indicate that they see the second floor AP as a weaker signal but the IP address is from the AP on the floor above. 


Re: Cisco 7921: Connects to AP on floor above

You're close.  The phone boots up, then scans all channels.  It determines the strongest channel and attaches to it.

Now here's a question.  Most people deploy their APs in kind of a triangle format.  If you're between two APs on the same floor, the strongest AP may be the floor above you or below you.  That happens a lot.

I'd also like to add my 2 cents about the way you're deploying wireless for CER.

If your goal is to have the phones re-address themselves when they change floors, you run the risk of losing a call when changing floors... For that matter, you run the risk of losing calls any time you roam between APs.  Since the APs are handing out different wlans, this is a definite possibility.

I believe CER also has the ability to track you down to the switchport of the AP you're connected to.  This is probably as close as you're going to get with wireless phones without risking call loss while roaming.

If I were you, I'd re-visit this design.


Ven Taylor
New Member

Re: Cisco 7921: Connects to AP on floor above

Thanks Ven.  Our stairways and elevators lose the connection every time so it is okay that they lose connection between floors.  This also allows them to get a new IP for CER.  I am going to look into tracking down to the interface level as you specified.  This could isolate 911 callers much more precisely since we have expansive buildings.  I am not sure how that would work on downstream switches but I am definitely going to look into it. 

I think we may have isolated the problem.  It just so happens the AP's on the second floor were in the default group which appears to be the same vlan that is being used for the management vlan for trunking.  When the controller was configured traffic on the controller side was not tagged but was just left as "0" for the management vlan.  This is likely not tagging traffic over the management vlan.  I am assuming the "0" is equivelant to vlan 1 on a switch but I need to look into it a little more.  If the traffic on that vlan is untagged then that would explain the vlan hopping we are seeing and the wrong network addresses we are seeing occasionally on the phones.  I was confused because the switch connected to the controller has a native vlan specified which just happens to be the network address we are seeing.  I thought there wouldn't be a way for this network to get out if it was tagged but like I said when Prof. Services set up the thing they left the controller side untagged.  Or atleast I am assuming 0 is untagged.  I will let you know how it goes. 

Hopefully it clears up the problem and this forum will help someone else with similar anomolies.  I was going down the wrong track totally, but I was operating under the assumption that it was set up properly to begin with. 


Re: Cisco 7921: Connects to AP on floor above

Good job! Sounds like you've nailed it down.  If you're using AP groups, make sure they're all in a group.

The CER to the AP port is pretty cool.  It doesn't matter if the switch is daisy-chained or anything.  Considering the fact that your horizontal run from the switch port to the AP is less than 100M and your AP cell, depending on overlap, should be around 10-20M, it shouldn't be too hard to find someone.


Ven Taylor
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