Network Management

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Occasional Contributor I

DHCP, VLANs, HASHing, Oh My!

I have a question.

We have a vlan pool with 12 VLANs and our DHCP server has a /24 network for each one.  We have confirmed that this works.

 

We are currently operating in EVEN mode on the VLAN group.  Due to the fact that preserve VLAN only lasts 8 minutes (the age on the datapath bridge table) we are seeing a lot of duplicate IP leases as devices disconnect and reconnect before DHCP lease expiration (set to 1 hour).

 

I was thinking about using HASH mode on the VLAN group but am concerned that the controller's algorithm for VLAN placement will "overload" the VLAN with more devices than the respectice DHCP scope can handle.  If this a possibility or is the controller's intelligence such that it will not do this because it is aware of the network sizes based on it's configured interfaces?

Guru Elite

Re: DHCP, VLANs, HASHing, Oh My!

 Why use "preserve vlan"?  That is mainly for devices that are roaming from one controller to another to maintain the same VLAN, OR while you are adding a VLAN to the pool to keep users in the same vlan (typically they all get kicked off during the add).

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Answers and views expressed by me on this forum are my own and not necessarily the position of Aruba Networks or Hewlett Packard Enterprise.
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Occasional Contributor I

Re: DHCP, VLANs, HASHing, Oh My!

I want my devices to maintain their same VLAN for the DHCP lease duration essentially.  Otherwise, in EVEN mode, if the device disconnects for whatever reason and then gets back on wifi the device may potentially get kicked over to a different VLAN (because 8 minutes have passed and it's MAC is purged from the datapath bridge table) and get an IP off of that DHCP.  That is fine but that device's original lease would still be in use until it expired.

 

HASH mode would fix that for me, I just didn't know if the controller would overload a VLAN with more clients that it's respective DHCP scope could handle.

Guru Elite

Re: DHCP, VLANs, HASHing, Oh My!

If a device disconnects and it is still in the user table, it will appear back in the same VLAN.  If it is not in the user table, the controller will assign it another VLAN based on its algorithm, and how many users are in every VLAN, if you have EVEN configured.  You will have to reduce your leases based on the minimum time a user could connect, so that there is not any wasted DHCP space, regardless of your settings.

 

The best solution is to have a large contiguous VLAN so that you are not always juggling things.   If you have not, please see Knowledgebase Article here:  http://community.arubanetworks.com/t5/Controller-Based-WLANs/Best-Practices-to-implement-single-large-subnet-flat-network/ta-p/260770 or the Single VLAN VRD here:  http://community.arubanetworks.com/t5/Validated-Reference-Design/Single-VLAN-Architecture-for-WLAN/ta-p/257196

 

 

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Answers and views expressed by me on this forum are my own and not necessarily the position of Aruba Networks or Hewlett Packard Enterprise.
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