lagg(4) and failover

Pete French petefrench at
Tue Aug 12 11:40:13 UTC 2008

> As far as I can tell, not especially well :-(.  It doesn't seem to detect
> much short of layer 1 failure.  In particular, shutting down the switch
> port will not trigger a failover.

Are you using bce devices as your phsyical interfaces ? Take a look at
the thread from last week about ifconfig - with the patch posted a port
shutdown now *does* trigger a failover quite happily. If you are using
e devices then I suggest you try it.

> With lacp, all the physical interfaces must be connected to a single
> switch.  With failover, the physical interfaces will normally be
> connected to different switches (so a failure in one switch will not
> cause the loss of all connectivity.

This is true - with the caveat that certain pairs of switches can be made
to appear as a single phsyical device for the purposes of LACP, in which
case it works fine for failover.

We have two farms here - an old one using a pair of Cisco 3560s and
a new one using a pair of 3750-Es. The 3750s will act as a single
device and we use LACP on the machines connected to those, but the 3560s
appear as a pair of devices, so for those we use failover mode. LACP
failover always worked fine, and with the bce patch from last week
the normal failover now also works.

Nore that you can enable LACP on the 3560,s and it does appear to negotiate
and work, but the switches keep changing their idea of which port to use every
few seconds. So the connection works, but with high rates of packet loss
as a few go missing every time the switch pair flip-flops.


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