Cisco Wireless 802.1X Authentication per WLAN?

Hi Guys,

We have a wireless network with a centralize Cisco WLC and AP's in flex connect setup. The WLC is located in different site and AP's in site A connects via a private line.

At site A we have a WLAN with 802.1X Authentication enabled and the radius server is located also in different site. Now, We recently noticed that the volume of traffic increases and sometimes cause congestion and with that, I think it affects the issue with Client wireless connection since they're saying that they encountered or have issue with wireless connection.

It is still under investigation whether this Congestion affect the currently client connected to WLAN or it's just the new client that want to connect to WLAN.

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Sample topology:

WLC & Authentication server <--------- Private line ---------> Site A AP's and Client

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Note: That we have multiple site connected to WLC and Authentication server but only site a having this kind of issue although the issue is not consistent.

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Question:

1. Does the authentication between client and the authentication server happens only during the time that client connect to the ssid? or this authentication still being performed even though client is already authenticated/connected?
2. Does the congestion affects the connected client connection to the Wireless not work or specific ssid?
3. The only port being used by client and authentication server is 1812(udp)?

Thank you