Re: Networking Idea/Question

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Burkhard,

I woke up with the same conclusion - LACP load balancing can break down
when the traffic traverses a router since the IP headers have the router as
the destination address and thus the Ethernet header has the same to MAC
addresses.

(I think that in a pure layer 2 fabric the MAC addresses vary enough to
produce reasonable - not perfect - LACP load balancing.)

Then add VXLAN, SDN, and other newer networking technologies and it all
gets even more confusing.

But I always come back to the starter cluster, likely a proof of concept
demonstration, that might be built with left-over parts.  Networking is
frequently an afterthought.  In this case node-level traffic management -
weighted fair queueing - could make all the difference.

-Dave

--
Dave Hall
Binghamton University
kdhall@xxxxxxxxxxxxxx

On Tue, Mar 16, 2021 at 4:20 AM Burkhard Linke <
Burkhard.Linke@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

> Hi,
>
> On 16.03.21 03:40, Dave Hall wrote:
> > Andrew,
> >
> > I agree that the choice of hash function is important for LACP. My
> > thinking has always been to stay down in layers 2 and 3.  With enough
> > hosts it seems likely that traffic would be split close to evenly.
> > Heads or tails - 50% of the time you're right.  TCP ports should also
> > be nearly equally split, but listening ports could introduce some
> > asymmetry.
>
>
> Just a comment on the hashing methods. LACP specs does not include
> layer3+4, so running it is somewhat outside of the spec.
>
> The main reason for it being present it the fact that LACP load
> balancing does not work well in case of routing. If all your clients are
> in a different network reachable via a gateway, all your traffic will be
> directed to the MAC address of the gateway. As a result all that traffic
> will use a single link only.
>
> Also keep in mind that these hashing methods only affect the traffic the
> originate from the corresponding system. In case of a ceph host only the
> traffic sent from the host is controlled by it; the traffic from the
> switch to the host uses the switch's hashing setting.
>
>
> We use layer 3+4 hashing on all baremetal hosts (including ceph hosts)
> and all switches, and traffic is roughly evenly distributed between the
> links.
>
>
> Regards,
>
> Burkhard
>
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