14 blinks: portfast on Cisco switch helped

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  • #1388

    I too was suffering from the dreaded 14 blinks but solved it. Documenting for the forum.

    After some debugging I found the plant keeps the Ethernet down and only activates it when it wants to Tweet.

    This is a good power saving model, but with a (Cisco) ethernet switch the issue is that a port blocks Ethernet traffic for about 10-30 seconds when a device comes online to find out if it is a normal endpoint or a connection to another switch potentially creating a loop. This is normal ethernet switch spanning tree behaviour.

    Problem is that by the time the ethernet port went into forwarding mode and the plant could actually get a DHCP lease, it had given up and was giving me 14 blinks.

    On a Cisco switch you can solve this by adding a command like

    interface FastEthernet0/1

    switchport mode access

    spanning-tree portfast

    which will disable spanning tree and make the plant happy again.

    I would recommend to review the plant’s DHCP behaviour though: when the initial lease request fails, wait 30-60 seconds and try again.

    I agree it’s unlikely a consumer is using an enterprise ethernet switch, but on the other hand no normal consumer has a twittering plant :-)

    Anyhoo – problem solved for me. Off to find a plant to monitor. My wife just decided to move the one I had planned to use as it was nice and close to an Ethernet outlet. Sigh.

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