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Why link latency is important for Cisco lightweight access points.

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Link latency, it’s a convenient little check box you turn on individually (or globally) on lightweight access points (Operating in FlexConnect/H-REAP or OfficeExtend) to see the latency details to the controller, while this is one of those ‘yea that’s nice to know’ type things it can be a key resource when troubleshooting certain WLAN issues.

First lets start with why knowing the latency is important, well Cisco has published the requirements for CAPWAP latency from the LAP to the controller and that requirement is no more than 300 ms of latency. Now while most enterprise’s do not have to worry about those latency requirements too much (due to a typical MPLS, VPLS, or metro backbones) other types of companies that lets’s say rely on a pretty common DMVPN over the Wild West (the Internet) may have to keep in mind these latency requirements. If you do not meet these latency requirements you might be seeing CAPWAP packets drop in transit or your Flexconnect/H-REAP LAPs flapping between ‘Connected’ mode and ‘Standalone’ mode which depending on your setup can cause a host of issues.

Link latency will monitor the following:

  1. The round trip time of the CAPWAP heartbeat, it does this by comparing the timestamps of when the request is sent to when the reply is received. Now, by default this CAPWAP heartbeat occurs every 30 seconds and this CAPWAP latency is different from normal network latency, as the CAPWAP heartbeat also has a dependency concerning how quickly the controller can process the request and send the reply back out.

Something to keep in mind about the link latency stats, is the fact they do not reset unless the LAP reboots or they are manually cleared so if you decide to turn on link latency come back 6 months later and review the states you are not going to get any useful information.

Configuring and viewing Link Latency

Configuring and viewing Link Latency

As you can see from the above screen shot link latency will provide you a quick glance of the current latency, minimum latency, and the maximum latency to the controller. (From when link latency was first enabled or last cleared)

Written by Stephen J. Occhiogrosso

June 21, 2013 at 5:20 PM

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