link aggregation - bundling 2 lagg interfaces together

Damien Fleuriot ml at
Fri Feb 4 12:35:34 UTC 2011

On 2/4/11 1:19 PM, Pete French wrote:
> If you want failover using lagg then your best bet is to get lagg between
> two ports on different switches. If you have a pair of switches which
> will present as a single device then you can use LACP to do this, else
> use simple failover. I do this for all our servers and it works very
> nicely.
> In your setup I am not sure why you are originally lagging the pairs of
> interfaces to the same switch. Is that to try and increase capacity ?
> If so then I have never found it to work - all the traffic goes over
> one interface for me.
> -pete.

Indeed, I'm lagg'ing the 2 interfaces together to increase capacity,
then I also wished to provide failover.

Sadly our switches are not stacked and will not allow me to get a
port-channel on 2 different switches.

For servers we use simple aggregation with 2 interfaces using failover,
on switches A and B.

For this project's firewalls, historically, we've had no layer 2
redundancy at all, just physical interfaces being given IPs, and the
firewalls sharing a CARP address.

Even if I can't concatenate my 2 lagg interfaces into a failover one
over the 2 switches, the new setup will still be an improvement.

Regarding your problem of not getting LACP to work correctly for you,
find below a snip from my firewall just now, with interface lagg0 as a
LACP on switch1

# ./ifstat -i em1,em2,lagg0,vlan16
       em1                 em2                lagg0               vlan16

 KB/s in  KB/s out   KB/s in  KB/s out   KB/s in  KB/s out   KB/s in
KB/s out

    0.00    129.87   4366.33      0.76   4366.33    130.63   4366.33
    0.00    323.96  11105.00      0.19  11105.00    324.15  11105.00
    0.00    301.76  10225.03      0.19  10225.03    301.94  10224.97
    0.00    202.13   6868.14      0.19   6868.14    202.32   6868.08

This is the traffic as I downloaded a MFS BSD iso from another server.

You will notice that inbound traffic is not shared amongst em1 and em2.

However, keep in mind that the LACP load balancing is done using a hash
combination of source/dest MACs and IPs as well as the vlan tag if any.

Perhaps you would get better results if you tried downloading a test
file from 2 different sources.

I'll have to try that.

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