Extreme Networks Summit WM Technical Reference Manual page 94

WDS – Wireless Distribution Service
AP can take the parent role in multiple WM-ADs. However, it will take the child role in at most one
WM-AD, even if it is configured as child in multiple WM-ADs.
The WDS links are very secure. They are always encrypted with AES (WPAv2-PSK), with each link
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using a distinct pair-wise master key.
The WDS solution implements several mechanisms that protect the WDS links against configuration
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errors — see details below. This prevents APs from becoming stranded due to configuration errors.
WDS, radio settings, and DFS
While scanning for a parent, the WDS child uses its own country settings to determine which
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channels to scan. To ensure that a WDS child scans all the channels on which the parent may
operate, the WDS child must be configured with the same country as its parent. Configuring a
different country may prevent the WDS child to find its parent.
NOTE
The channel of the parent may change due to many reasons, such as radar detection, user intervention, ACS,
etc. When this happens, the child may become stranded if configured with a different country than the parent.
An AP that operates as the WDS child must operate on the same channel as its parent. When an AP
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operates as a WDS child, it ignores any change in its country and channel settings and continues to
operate with the same country and channel it used when it connected to its parent. This prevents a
shut-down of the WDS link by a configuration error.
When an AP operates as a WDS child, it ignores any change in Tx Power settings and continues to
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operate with the same Tx Power it used when it connected to its parent. This prevents a shut-down
of the WDS link by a configuration error.
The formation of the WDS tree is considerably slower in a DFS channel taking an additional 60
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seconds for each level in the tree. This happens because each AP must perform 60 seconds of
channel evaluation before starting service on such a channel. Therefore, it is recommended to avoid
DFS channels for the WDS service.
Another reason to avoid DFS channels for WDS service is that when one of the APs in a WDS tree
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detects radar, it causes the whole tree to be destroyed and re-constructed on a different channel. The
AP that detects radar informs its parent before shutting down its own service. A repeater AP
processes the notification from one of its children as it has detected radar himself. For example, it
informs its parent before shutting down its own service. The process propagates to the root AP,
which tears down the entire tree by stopping its service and restarts the service on a new channel.
Again, it is important that all APs have been configured with the same country to guarantee they
will find their parents after a radar detection event.
WDS and availability
The availability feature works with WDS APs in the same manner as it works with non-WDS APs.
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However, in order to operate properly, it is expected that under normal conditions all APs in a WDS
tree will be registered with the same home controller. If the home controller fails, all APs in the WDS
tree will switch to the back-up controller.
APs that are using a WDS WM-AD as their uplink connection to the network infrastructure will not
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allow the back-up controller to delete or change that WDS WM-AD. This protects the WDS uplink
against configuration errors.
If link persistence is enabled on an AP operating as WDS parent and the AP loses its communication
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to the controller, the AP will preserve the existing children but it will not accept new children until it
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Summit WM Technical Reference Guide, Software Version 5.1