Current inventory: 2x 3400 controllers
Option1:
Replace 2x 3400 with 2x3600 controllers.
Deployment: Master -- Standby
You will have full redundancy in case the current master goes down.
Option2:
Buy 1x3400 controller and deploy as:
New inventory: 3x 3400 controllers
1 Master and 2 Local
In this deployment you do not get complete redundancy in case both the locals go down.
Option3:
Buy 2x 3400 controllers
New Inventory: 4x3400 controllers
Deployment:
Current Building: Master - Standby
Extension: Local1 and Local2 (split the APs between the two local controllers)
In this deployment you will still not get full redundancy in case both the locals go down.
Hope that helps!
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HT