[c-nsp] Cisco 3550 QoS to limit FTP

[Daniel Hagerty]

nick at precisionmillworks.com writes:

> I'm trying to implement QoS on a 3550, here's the situation.  I've got
> about 400 hosts on my network.  One machine is used to ftp large
> files(2-4gig) to clients.  Currently we only transfer these files at night
> now, because it causes packet loss on our network.  I want to be able to
> give all of the FTP traffic coming from this machine a low priority,

    That's a very ill behaved FTP server, but anyway.

    Lots of knobs in QoS.  You're looking for what cisco calls
congestion management, congestion avoidance, and perhaps
classification.  Management for when the link is full, avoidance to
push back the inevitable, and classification to tune the management
and avoidance behavior.

    I've found the best thing for situations like yours (bulk
transfers interfering with interactive services) is weighted fair
queue (WFQ) w/ random early discard (RED) and some classification.
Sadly, I don't have a config in front of me.  Ask privately, and I can
probably dig one up.

    First, turn on WFQ on the congested interface if it isn't
already.  Second, turn on RED within the WFQ engine.

    This may be enough by itself, but probably isn't.  Classification
offers a bunch of differernt ways to change how WFQ manages the full
link; ToS, qos groups, and IP precedence values are all an option with
slightly different results (I *think* precedence is the only method
that preserves flow based WFQ).

    The upside of a config like this is that you can use the full link
bandwith the way you like, and the behavior is dynamic -- if the FTP
is the only thing going, it gets the whole link.  And the interactive
users will *love* you -- they generally only notice a very slight
latency tax during link full, rather than the huge latency you get
under FIFO queue.  The downside is that your configuration is more
complicated and CPU intensive.

    See:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt2/qcfwfq.htm

for WFQ and using RED under WFQ,

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt3/qcfwred.htm

for a little more on RED,

and

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt1/qcfcbmrk.htm

for packet classification.

Note that it's a good idea to clear the precedence bits at your
network ingress/egress points if you have queues heeding it in the
middle of your network.  You usually don't want randoms controlling
queueing behavior.