CCNP Wireless CUWSS 642-732 Summary Notes 21 : Infrastructure requirements Part 2

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802.11n APs with 100% duty cycle:

  • the 2,4 band when oversubscribed with clients communicating at 144 Mbps may consume up to 70 – 75 Mbps on wired side.
  • the 5 GHz band when oversubscribed and using 40 MHz channels with clients communicating at 300 Mbps may consume up to 160 – 170 Mbps on wired side.
  • Both bands on the Ethernet Side will consume (75 + 170) = 245 Mbps per AP

Planning:

  • Limit number of APs per WLC to make sure that overall traffic is less than port bandwidth
  • use a 20:1 oversubscription ( 20 APs to 1 Controller)
  • At RF level, the distance between the APs should be such that if one goes down, RRM can increase power of surrounding APs and cover the hole.
  • Controller redundancy should be in place so that if one controller goes down, APs can join another. Redundancy models:
    • N+1:
      • one controller backs up n controllers
      • Only 1 additional controller
      • Cheap redundancy
      • only 1 WLC can fail at a time. the redundant controller could become oversubscribed with access points if there are multiple primary controller failures,
      • When a controller reaches the maximum number of joined access points, it accepts no more Lightweight Access Point Protocol (LWAPP) join requests.
      • Each access point is configured with a primary controller and all access points point to the next single redundant controller as secondary.
    • N+N:
      • N controllers back up N controllers
      • Each WLC loaded to half its license capacity
      • More expensive model
      • If network fails, all APs  move to the other WLC
    • N+N+1:
      • N controllers back up N controllers as secondary, and
        one controller backs up all N controllers as tertiary.
      • Intermediate but more common model
      • In this design, it is important to load-balance the access point capacity across both controllers. WLC are loaded to a variable perecntage of their licence (eg 66%).
      • to logically group access points on controllers to minimize inter-controller roaming events.
      • Additional backup is available in NOC
      • When one or several WLCs fail, the APs can join the backup or other controllers in the network
      • There should be enough access point and bandwidth capacity on each controller to handle a failover situation.
      • the plus one controller at the NOC or data center to no longer have to be a member of the same mobility group. It can now be referenced by its IP address versus being part of the same mobility group.
      • Most redundant but most expensive
  • AP deployment Model
    • Standard deployment (not advised)
      • All APs connect to same controller
    • Salt and Paper Model
      • Neighbouring APs connect to different controllers
      • In one WLC gets disconnected, only half have to fail over
      • Better redundancy than standard model
      • Downside: when all controllers are available, interAP roaming leads to intercontroller roaming leading to added delays when roaming
      • Recommended only if applicatiosn are not time sensitive
      • Determine the roaming paths during the walkthrough visit. deploy APs along the path so that roaming in that path occurs on the same controller
      • If WLCs do not connect to same switched network, their WLAN – VLAN mapping may be different which means that we will have local to foreign roaming that increase wired bandwidth consumption
  • Controller location
    • Distributed model
      • WLCs in each building close to the APs.
      • Negatively affects roaming efficiency. Intercontroller roaming even represents 20 ms delay when both controllers are in same subnet and 30 ms when in different subnets
    • Centralized model
      • All WLCs in same location
      • Capacity management is simpler
      • Simpler to create high availability controller topology
      • All packets must be sent to the controller

 

References:

  1. CCNP Wireless  (642-732 CUWSS) Quick Reference Guide by Jerome Henry

  2. IPexpert’s CCNP CUWSS Wireless Voice on Demand (642-731)

  3. CUWSS Student Guide 1.0

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