Cisco WLC Uplinks to ASR

I thought I’d share some example configuration for a scenario where you may have Cisco WLCs connecting to either one or two Advanced Services Routers (ASR) for the Layer 3 core as opposed to a Layer 3 switch.  If you’ve ever configured an ASR (in this case an ASR920) you’ll know that the configuration of VLAN interfaces and 802.1Q trunking is much different to an ISR or Catalyst switch.

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Cisco Dynamic Channel Assignment (DCA) Intervals

Some of you may already be aware of this pain point running WLC code 8.2 or later; this is when Cisco introduced the Flexible Radio Assignment (FRA) feature.

If you’re running the default Dynamic Channel Assignment (DCA) interval, which is 10 minutes, you really should change it to suit your wireless network – there are wireless clients that do not behave well (i.e. lose connectivity) when experiencing a channel change, you especially don’t want this with Voice over Wi-Fi.  However if you’ve never changed the update interval, or need to change it after upgrading to 8.2 code, you will run into a supposed issue trying to set the update interval longer than one hour.

The reasoning for this is easy to miss in the above link: “Set the FRA run interval (Valid Choices 1-24 hours must meet or exceed DCA interval)”, especially if you have never deployed any FRA-capable APs and don’t have FRA enabled.

If you try to set the DCA interval above 1 hour in the GUI or CLI you will get the following error:

(WLC-P001) >config advanced 802.11a channel dca interval 24

DCA Interval cannot be greater than FRA Interval.

You might go to double-check that FRA isn’t enabled, but you have to set the FRA interval to greater than the DCA interval:

(WLC-P001) >config advanced fra interval 24

(WLC-P001) >config advanced 802.11a channel dca interval 24

(WLC-P001) >config advanced 802.11b channel dca interval 24

I’ve not had a chance to play around with FRA on 2800 or 3800 APs yet, however the crew over at the No Strings Attached show have produced a white paper based on some testing, and the Borderless CCIE blog gives a good summary of what it is here.

If you want to debug WLC dynamic channel assignment updates you can use the following command: debug airewave-director channel enable

I hope this saves some people some headaches!