[Asterisk-Users] NIC irq load balancing

Rich Adamson
2005-01-07T06:09:35Z

> 2. NetGear Switch - I'm using FS-526T Switch, which
> has 24 10/100 ports and 2 Gb sorts.
> I want to know if this kind of general purpose switch
> is not suitable for voip. If so, could you recommand
> one?

I've been doing network performance assessments for corporate clients
in 40+ states since 1993, and we see an absolute ton of supposedly
knowledgable engineers deploying switches from every major manufacturer.
One item they just never address is making sure a server's interface
matches the switch's interface settings. Over 90% of the time they
tend to let the switch and serve auto-negotiate the speed and duplex
settings.

Most nic card vendors and most switch vendors get the negotiated speed
correct (that's an easy one to do), but about 50% of the time the
negotiated duplex setting is wrong. (Eg, the switch will negotiate
half duplex while the server thinks he's in full duplex.) Under any
reasonable load, the interface will cause damaged packets, dropped
packets, etc. We've actually tested many of these and seen 100 meg
interfaces maxing out at less then 1 meg throughput, for an absolute
fact.

Part of the negotiation problem is until recently there have been no
industry standards as to how duplex settings should be negotiated.
So, with every reboot and/or interface interruption, the negotiated
duplex settings will be wrong about 50% of the time. Very few tech's
actually have the skills/knowledge to "see" the mismatch.

The only reasonable way to solve that issue is to lock both interfaces
(the switch interface and the server nic) at full duplex.

Since the FS-526T is a managed switch, if you lock the interface (and
the server) to 100 meg full duplex it will work just fine. If you
don't lock both interfaces, your actual throughput (and voip quality)
is totally up for grabs. (Gig interfaces are always full duplex.)

It also seems the majority of sys admins don't have a clue how to
look at their systems to "see" what the nic interface has negotiated.
For RH systems, take a look at the output from dmesg. Different distros
will have different ways to look at (and set) the duplex setting.

The duplex mismatch will have an increasingly negative impact with
greater load/throughput. So, if your implementation is a home/soho
system, duplex will seldom be an issue; however, if your implementation
is within a larger corporate network, duplex will have a very serious
impact.