Controller Based WLANs

How do I set up a redundant link for a RAP? When two links are configured for a RAP, which one takes priority? Is there preemption

Product and Software: This article applies to Aruba controllers running RN 3.1 and later when used with RAPs equipped with a USB port (that is, RAP-5WN, RAP-5, AP-70, and AP-2E).

Abstract
For remote APs (RAPs) equipped with a USB port (RAP-5WN, RAP-5, AP-70, and AP-2E), a USB cellular modem adaptor can be used as a primary or secondary link between the RAP and the Aruba controller running RN 3.1 and later over a public cellular packet network. For details on how to set this up and what USB modems and carriers/ISP combinations have been tested, refer to the RN documentation and the release notes. This article discusses how to control the behavior of these links, such as whether the cellular up-link is to be used as a primary or secondary (back-up) link and whether preemption (automatic fail-back) is enabled.

Solution Overview

  • Two up-links
  • One wired (enet0) link.
  • One link uses a cellular USB modem (3G/EVDO depending on country).
  • Either can be primary or back-up.
  • Select using link priority.
  • Preemption (fail-back) is automatic.
  • IPsec tunnel is encapsulated in the cellular network PPP tunnel.
  • GRE tunnels are encapsulated in the IPsec tunnel.

Note that future releases will include the ability to use more than one Ethernet port as up-links.

Configuration
Refer to documentation to configure a RAP to dial out using a cellular modem. Usually, the following minimum parameters are needed:

  • Device Type
  • Initialization String
  • Dial String

To control the relative priorities of the wired (using enet0) link and cellular uplink (using USB), use the following two parameters:

  • link-priority-cellular
  • link-priority-ethernet

The valid values are from 0 to 255. The higher the value, the more likely the link will be used first.

Operations
For example, if the link priorities are configured like this:

  • link-priority-cellular = 100
  • link-priority-ethernet = 110

The wired (enet0) is preferred. The RAP attempts to connect to the controller using Ethernet port 0, if it is available. If Ethernet connection fails, the RAP tries the cellular link.

Note:

  • If the Ethernet link is down or disconnected, the RAP will try the USB link immediately
  • If the Ethernet link is NOT down, but some intermediate (between RAP and controller) connectivity is broken (LMS not reachable), the RAP will try 2 times attempting the connection using the Ethernet port before switching to the cellular link.
  • Preemption is automatically enabled if two different priorities are defines.
  • The 'hold-down' period is set to 600 seconds. In the above example, when the RAP fails-over from the Ethernet link to the cellular link, if the Ethernet becomes active again, the RAP waits 600 seconds and switches back to using the Ethernet link as the connection.

Issue
In the current RN release, the local debugging console (http://rapconsole.arubanetworks.com) is not accessible if the cellular link is used as the primary link and when the link is down. This issue will be corrected in future releases.

Reference
For additional information on RAPs, refer to the following documents:

  • ArubaOS RN 3.1 User Guide
  • ArubaOS RN 3.1 Release Notes
  • Validated Reference Design Guide for Virtual Branch Networks
Version History
Revision #:
1 of 1
Last update:
‎06-30-2014 05:36 PM
Updated by:
 
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