[c-nsp] etherchannel load-balancing and unpredictability

[Keegan Holley]

2011/7/19 Steven Pfister <SPfister at dps.k12.oh.us>

> I have a question regarding etherchannel load balancing. I've got a
> 4507R switch connected to a 3560 switch by means of two content filters
> which are acting as transparent bridges. The two ports on each side that
> the content filters are connected to are set up as access ports and are
> in an etherchannel. The load balancing method on each switch is set to
> src-dst-ip. I was under the impression that each pair of source and
> destination ip address would select exactly one content filter no matter
> which direction.
>
> this is exactly what is unpredictable about it.  The operation is a hash so
different src/dst pairs aren't always going to chose different links.  For
example (a very simple one)  if you have a subnet 192.168.100.0/24 and
station .1 talks to station .3 and .5 there's nothing to prevent both of
those conversations from using the same link.  It's based on a mathematical
formula that does not vary.  If for example you add .2 and it talks to .4
and .6 they may use the second link.  However, what if the "odd" numbered
conversations are just above 1G say 1.2G or so.  They will drop packets and
there will be no way to hash them to a different link other than changing
load-balancing algorithms.  The being said the other algorithms are just as
unpredictable for just the same reasons.  It depends completely on your
traffic patterns.  Adding TCP/UDP port may even this out a bit but I don't
believe it is supported on the 3560.