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I have a small cluster consisting of 3 servers. Each has two 10Gbe SFP+ optical network cards. There are two separate 10Gbe switches. On all servers one NIC is connected to switch 1, second NIC is connected to switch 2 to provide fault tolerance.

Physical interfaces are bonded on server level using LACP.

All servers can ping each other, but on one there is small (4%) packet loss (over bonded interface, which looks suspicious to me)

When I check with iperf3 transfer rates between two good servers, they show about 9.8Gbit/s transfer rates in both directions.

Those two good servers can also download from problematic one also about 9.8 Gbit/s

Iperf3 show strange thing when run as client on problematic server. It starts with a few hundred megabit in first turn. Later speed drops to 0 bit/s (while still running ICMP ping with ~96% success rate). Only in one direction. When other servers download from this, they get full speed.

It's all running on a same hardware even firmware version is the same (Dell R620 servers, Mellanox ConnextX-3-EN NIC's, Opton SPF+ modules, Mikrotik CRS309-1G-8S switches). Also OS is the same latest stable Debian with all updates and exact installed packages.

There is no firewall, all iptables rules are cleared on all servers

On problematic server i check interfaces, both NIC's show UP and running at 10Gbit full duplex

Also cat /proc/net/bonding/bond0 show both interfaces UP, active, no physical link errors

I checked/replaced SFP+ modules, used different fiber patch cords, tried different switch ports and nothing changes, still this one problematic server get poor download speed from others and small packet loss (over bonded interface!).

I also tried patch cord combinations with: (both on, first on second off, first off second on). Also no change

Any ideas how can I diagnose it better?

  • How are your switches connected and co-configured? The right way to do LACP is to have a shared MAC/IP table between switches - if you don't have this then you're likely to get 'MAC-flapping' between them which can lead to slower performance than expected and lost packets. – Chopper3 Feb 28 at 12:43
  • Ah - just looked at the specs for your switches, they can't be configured in a way that allows for dual-switch-LACP sorry. Go back to active/standby config on your NICs and you'll be fine ok. – Chopper3 Feb 28 at 12:49
  • @Chopper3 thanks for advice, I will check if other bonding mode will help and post updates here – Mateusz Bartczak Feb 28 at 14:22
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Unless the switches support stacking and support LACP across chassis, LACP cannot work that way. In fact, static LAG trunking won't work either.

Generally, link aggregation only works with a single opposite switch (or a stack acting like it).

With simple L2 redundancy, you can only run the NICs in active/passive pairs with failover. Using multiple L3 links with appropriate load balancing and IP migration on failover or monitoring by an external load balancer will also work in your scenario.

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Please see my answer here (don't forget to hit thumbs up if it will be useful in your situation):

Why am I only achieving 2.5Gbps over a 10Gbe direct connection between 2 machines?

It is most probably related to LRO GRO with stands for RECEIVE OFFLOAD, that can be easily disabled. There is also a nice explanation on why this happens. Here: https://lwn.net/Articles/358910/

Tuning 10G network interfaces is a huge topic.

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