Information Technology Reference
In-Depth Information
because of the asymmetric communication of Layer 3 roaming. Proactive key caching
(PKC) or Cisco Compatible Extensions (CCKM) Version 4 is recommended to speed up
and secure roaming.
This section covers controller redundancy design, radio frequency groups, site survey, and
WLAN design considerations.
Controller Redundancy Design: Deterministic vs. Dynamic
WLCs can be configured for dynamic or deterministic redundancy. For deterministic re-
dundancy, the AP is configured with a primary, secondary, and tertiary controller. This re-
quires more upfront planning but allows better predictability and faster failover times.
Deterministic redundancy is the recommended best practice. N+1, N+N, and N+N+1 are
examples of deterministic redundancy. Advantages of deterministic redundancy include
Key
To p i c
Predictability
■
Network stability
■
Flexible and powerful redundancy design options
■
Faster failover times
■
Fallback option in case of failover
■
The disadvantage of deterministic controller redundancy is that it requires more upfront
planning and configuration.
Dynamic controller redundancy uses CAPWAP to load balance APs across WLCs. CAP-
WA P p o p u l a t e s A P s w i t h a b a c k u p W L C . T h i s s o l u t i o n w o r k s b e t t e r w h e n W L C s a r e i n a
centralized cluster. The disadvantages are longer failover times and unpredictable opera-
tion. An example is adjacent APs registering with differing WLCs.
Advantages of Dynamic controller redundancy include
Easy to deploy and configure
■
Access Points dynamically load balance
■
Disadvantages include longer failover times, unpredictable operation, more intercontroller
roaming, and no fallback option in the event of controller failure.
N+1 WLC Redundancy
With N+1 redundancy, shown in Figure 5-13, a single WLC acts as the backup of multiple
WLCs. The backup WLC is configured as the secondary WLC on each AP. One design
constraint is that the backup WLC might become oversubscribed if there are too many
APs and it is normally placed in the data center.
