Multiple DHCPs On WLC

[SiJian Bao]

Hi,

My 5508 WLC which runs version 7.4 is configured as a DHCP server for the clients and here's my problem:

One AP is attatched with an interface which has the vlan 10 and a ssid in AP groups

One AP is attatched with another interface which has the vlan 20 and the same ssid in AP groups

And there are two DHCP pools in WLC, one is for vlan 10 and one is for vlan 20.

For now, a PC accesses AP-1 and get an IP address from DHCP pool vlan 10, then I power off AP-1, then the client accesses AP-2 but still get the IP address from DHCP pool vlan10, i need to get the IP from DHCP VLAN20, what can i do now?

Thanks!

[Scott Fella]

This is normal because it is part of the seamless roaming.  In order for the client to get a new IP address because it was powered up and associated to AP-2, the client would have to be removed from the WLC. If you look at the client after you power the client off and before you power it back on, the client is still on the WLC and in the run state. The idle timer which is default at 300 seconds or 5 minutes has not expired. So you either need to manually remove the client from the WLC or wait 5 minutes. Now if the client starts out at AP-1 on vlan 10 and roams to AP-2, the client will keep its IP address unless the session timer expires, the idle timer expires or the client is manually removed from the WLC.

[SiJian Bao]

Hi Scott,

Thanks for your reply. so you mean that there is a cache in WLC which records the IP address and MAC of the client, and before the 5 minutes expire, my client will get the same IP address even when the client disconnects the SSID and reconnects( to AP-2) again? Can i change the expire time and i wanna know does my problem have something with Option 82?

Thanks a lot!

[Scott Fella]

The controller keeps the client in a RUN state until the idle timer expires, so it's like caching it I suppose. You can reduce the idle timer, but just monitor the wireless. Option 82 has nothing to do with what is happening. What is the purpose why you want to do what you mentioned.

[SiJian Bao]

Hi Scott,

Thanks for your reply. Here's my requirement:

Each floor of the building has one AP and each floor is assigned to a VLAN. Then when someone comes downstairs from Floor 10 to Floor 8, he needs to get the new IP of Floor 8 which should be the IP of vlan 80 as soon as possible.

Thanks!

[Scott Fella · Accepted solution]

That will not be possible. The only way to get that to work is to have two WLC's. you will not be able to accomplish what you want with one WLC as there is no configuration that you can do to force that. You can try to lower the idle timer to like 10 seconds but that might cause other issues.

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[SiJian Bao]

Thanks a lot for helping me with this. I learn a lot from you and i have got what I want, thanks.

[Scott Fella]

No problem. The one main reason it's not supported is due to seamless roaming. Clients that move from one floor to another might not be smart enough to request a new dhcp so that can be an issue. Other devices might also not requests a dhcp because it knows it already has one. The only thing you can do is setup two different SSID's that is similar like floor8 and floor10. This way the device will get an IP address when it associates to an ap on either floor. This should work for you since you are using ap groups anyways. Not like you will be adding another SSID to the floor, your just specifying what floor will have what SSID.

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