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Occasional Contributor II
Posts: 21
Registered: ‎05-16-2011

VLAN Pools - what happens when a device roams

Dear all,

 

Just wondering if I do NOT use the "preserve-vlan" setting in the VAP profile, what will happen to the client when it roams in a Name VLAN Pools situation?

 

1, Named VLAN Pools - Hash type

when a client roams from one AP to another, will the client MAC address need to go through the hash algorithm everytime it roams?  But the output of hash algorithm will place the client into the same VLAN, so I guess it doesn't matter.  But I still want to know if the algorithm is run every time a client roams?

 

2, Named VLAN Pools - Even type

same as above, if roams from one AP to another, will the client MAC address need to go through the even algorithm everytime it roams?

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I'd imagine if a client walks away, disassociate and time out completely. Next time the client re-associates, it will go through the algorithm again, but not sure what happens when it just  roams from AP to another.

 

Thank you.

Guru Elite
Posts: 21,491
Registered: ‎03-29-2007

Re: VLAN Pools - what happens when a device roams

1 and 2, the client maintains the same VLAN during a roam.  The VLAN is typically only recalculated on a new client reassociation, or if the client roams into a new ap-group.

 

"Preserve VLAN" allows the controller to look up the client's VLAN in the bridge table, if a client is roaming between controllers to keep the VLAN.  It is not necessary in an operation where the client does not roam between controllers.



Colin Joseph
Aruba Customer Engineering

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Occasional Contributor II
Posts: 21
Registered: ‎05-16-2011

Re: VLAN Pools - what happens when a device roams

Thanks Colin.

 

I have a few more questions on "preserve VLAN".

 

This is from the User Guide:  "Preserve Client VLAN"

If you select this checkbox, clients retain their previous VLAN assignment if the client disassociates from an AP and then immediately re-associates either with same AP or another AP on the same switch.

 

I cannot see anywhere which says "roams between controllers"?

 

If a client is on a Controller A and is placed in VLAN x (by Even assignment type)  >>>> roams to >>> another AP which is on Controller B.

 

#1. I assume Controller A and Controller B will have the same Named VLAN Pools configuration.

#2. How can Controller B look up Controller A's bridge table? Do I need to configure something between them?

#3. What is the relationship of Controller A and B?  Are they both "local" controllers pointing to the same "Master"? Or what?

 

Kind regards

Kenneth

Guru Elite
Posts: 21,491
Registered: ‎03-29-2007

Re: VLAN Pools - what happens when a device roams

It is designed for roaming between controllers.  If controllers share the same VLAN and have the same ARP table, it will be able to tell the VLAN a user was on when it was on the previous controller.  If preserve VLAN is checked, it will look at its ARP table and put the user in the same VLAN if it appears there.

 

In the single controller model, if Preserve VLAN is enabled in the Virtual AP profile, it will look at the ARP table for assignment if the user is not in the user table.  So if the user has aged out of the user table (disconnected) and reconnects, and preserve VLAN is on, the controller will look at its ARP table to see if the client's mac address is there and assign the VLAN based on that.  That would only apply in the minority of circumstances:  Lets assume the controller's ARP table only holds mac addresses for 15 minutes before it ages out.  if a user disconnects and is still in the user for 5 minutes, the user would have 10 minutes before the ARP table is aged out, to reconnect and get back into the same VLAN.  That is the way it would work if you have "preserve vlan" enabled in a single controller environment.  It is DESIGNED for preserving the VLANs of users when they roam between controllers, that are bridging user traffic to a shared VLAN, NOT for a single controller deployment, really.



Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base

Occasional Contributor II
Posts: 21
Registered: ‎05-16-2011

Re: VLAN Pools - what happens when a device roams

Very good info indeed. Thank you.

 

If controllers share the same VLAN (<< yes, I can see how they can share the same VLAN)  and have the same ARP table, (<< but I'm really scratching my head and trying to figure out how can 2 controllers have the same ARP table??)

 

Could you elaborate, please?

 

Thank you. 

 

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