What are the recommendations for deploying the Aruba Mobile Edge architecture across low-speed and high-latency links using ArubaOS 2.5?
07-09-2014 03:26 PM
Product and Software: This article applies to all Aruba controllers and ArubaOS 2.5.
Deploying APs over Low-Speed Links
Aruba recommends that controllers and APs be connected over a link with a capacity of 1 Mbps or greater, and requires a minimum link speed of 64 Kb/s per tunnel-mode SSID. LANs, many metro area networks, and most broadband DSL or cable connections today provide link speeds greater than 1Mbps. For low-speed links such as ISDN, T1/E1, and many frame relay WAN connections, specific design guidelines and Aruba configuration are necessary to avoid connectivity problems.
To achieve high reliability and fast failover in the event of a network or controller outage, APs and controllers maintain "heartbeat" or "keepalive" packets between themselves. Failure to receive these heartbeat packets, which are described in detail next, can cause APs to "rebootstrap", which means to go through a process of tunnel re-establishment with the controller. During the bootstrap process, the AP shuts off all radios for approximately 20 ms, and all clients will be required to reassociate.
Two types of heartbeat packets are sent: GRE tunnel keepalives and PAPI keepalives:
GRE Tunnel Keepalives
· Sent once each second.
· Bi-directional: Sent from AP to controller and from controller to AP.
· By default, AP rebootstraps after missing eight consecutive keepalive packets (which can be adjusted using the "bootstrapthreshold" parameter).
· By default, the controller removes an AP's tunnel after 12 seconds of inactivity(which can be adjusted using "stm apinactivity-timeout" in ArubaOS 2.x.)
· Does not apply to bridge mode SSIDs (remote AP).
• Sent once every minute and are retransmitted every 5 seconds when not acknowledged.
• By default, APs rebootstrap after communication with the controller is interrupted. In ArubaOS 2.5, the timeout occurs after 10 consecutive missed keepalives, which is approximately 50 seconds.
• Effectively applies only to bridge-mode SSIDs. For tunnel-mode SSIDs, GRE tunnel keepalive intervals time out long before PAPI keepalive intervals.