On Mon, 12 Apr 2021 14:46:11 +0200 Tobias Waldekranz <tob...@waldekranz.com> wrote:
> I agree. Unless you only have a few really wideband flows, a LAG will > typically do a great job with balancing. This will happen without the > user having to do any configuration at all. It would also perform well > in "router-on-a-stick"-setups where the incoming and outgoing port is > the same. TLDR: The problem with LAGs how they are currently implemented is that for Turris Omnia, basically in 1/16 of configurations the traffic would go via one CPU port anyway. One potencial problem that I see with using LAGs for aggregating CPU ports on mv88e6xxx is how these switches determine the port for a packet: only the src and dst MAC address is used for the hash that chooses the port. The most common scenario for Turris Omnia, for example, where we have 2 CPU ports and 5 user ports, is that into these 5 user ports the user plugs 5 simple devices (no switches, so only one peer MAC address for port). So we have only 5 pairs of src + dst MAC addresses. If we simply fill the LAG table as it is done now, then there is 2 * 0.5^5 = 1/16 chance that all packets would go through one CPU port. In order to have real load balancing in this scenario, we would either have to recompute the LAG mask table depending on the MAC addresses, or rewrite the LAG mask table somewhat randomly periodically. (This could be in theory offloaded onto the Z80 internal CPU for some of the switches of the mv88e6xxx family, but not for Omnia.) Marek
