lagg(4) question

Frank Leonhardt frank2 at
Fri Nov 15 15:19:53 UTC 2013

On 15/11/2013 14:49, Brett Glass wrote:
> I'm interested in using lagg(4) to provide failover from a main 
> wireless link to a backup link. Unfortunately, one of the common 
> failure modes, when a link fails, is for the Ethernet port on the 
> radio to stay up but not pass any traffic. Does lagg(4) test the link, 
> by actually trying to send packets, to determine if the link has 
> failed? Or does it rely upon the Ethernet link losing "carrier"?
> --Brett Glass

I'm pretty sure you're right. lagg (aka truck) determines if an 
interface is working by checking its "up" flag in the link_state flags 
field; it also checks the firewall's idea. I stand to be corrected, but 
I think it's only the interface driver's responsibility to check for a 
carrier, not to "down" itself it's its not getting data through when the 
hardware layer looks healthy.

If we're right in our assumption, you could always have a cron job ping 
through the main wireless link, and if it fails to get a response you 
could fail it (down it with ifconfig), triggering lagg. Or you could 
just as easily ignore lagg and reconfigure the interfaces yourself.

Regards, Frank.

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