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I've been looking for information regarding LAG/LACP configuration and the impact on QoS (such as packet delay, out of order delivery, etc.). I could only find statements such as "LAG/LACP guarantees same 'flow' QoS" - what exactly is flow? Which packets are part of the same flow and which packets aren't? Is "flow" guaranteed to be transmitted via the same physical link of the LAG?

Should I consider employing a specific hash method for a specific service? For example, would I use src & dst MAC for some services and src & dst IP for others?

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Feb 19, 2018 at 4:02

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Flow is a chain of packets combined by some attribute. For different protocols attributes are different (ports, seq numbers, etc). Load balancing in LAG may differ, but usually per-flow load balancing is being used. When per-flow balancing is used, flow is guaranteed to be transmitted via the same link.

On the other hand it is possible to balance based on packet (per-packet balancing). It's not usually the case because of issues, such as out-of-order packets.

You should consider using different hash methods based on your traffic profile. If you have big diversity of MAC addresses (e. g. L2 switched domain), use MAC for hash. If you have big diversity of IP addresses (e. g. small L2 domain + routing), use IP for hash.

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  • "Flow is guaranteed to be transmitted via the same link" - is a flow unidirectional or bidirectional? In addition, how LAG is used for link protection? Is it a good practice to use LAG for link protection?
    – MJPeter
    Commented May 25, 2017 at 11:57
  • Egress flow from device perspective. Ingress is being balanced by peer device. Whether it is good practice or not depends on your task and topology. It may be good solution, but not the best for certain cases. Protection is the simple part: if one link fails, other links continue to work. Though device must be configured properly in order to be allowed to use reduced number of links. E. g. if minimum of 2 links configured, link goes down when less than 2 links operational. Default is usually minimum of 1 link.
    – ar_
    Commented May 25, 2017 at 12:45

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