Steven wrote:
I found this command if your Cisco switches support it:
"auto qos voip trust"
You set this on each interface.
It automatically prioritizes all SIP and skinny traffic, but not iax.
There is also "auto qos voip cisco-phone". This one can detect a Cisco phone
and prioritize it.
I just have to figure out how to verify that it is actually doing anything.
The auto qos function is a relatively new addition to the cisco routers
and switches (eg, last year or so). The parameter is added to an
individual interface (usually a serial interface), and it truly watches
for actual traffic on that interface until you shut it down. At that
point, auto qos "writes" the policy statements into the router config
needed to support that actual traffic.
To use it, you must:
- enable it on an individual interface,
- do not change the interface "bandwidth" statement while its running,
- cisco express forwarding must be enabled, and,
- all previously attached QoS policies must be removed from the
interface being sampled.
Its my understanding (although I've not actually done this) that auto
qos can be used to monitor all traffic and not just voip packets. For
example, some companies may wish to generate qos policies for Citrix, MS
Terminal Server traffic, etc, and may not have any voip implementation
at all. So, auto qos is not just for voip traffic and should be very
usable with iax.
Since you've specifically mentioned the "auto qos voip cisco-phone"
statement, that statement essentially says watch for voip traffic coming
from a cisco phone. Reading between the lines says: Cisco ships their
voip phones with QoS already preconfigured with signaling traffic in one
DSCP class and rtp traffic in another DSCP class. If your non-cisco
phones aren't set up with those exact same DSCP markings, auto qos won't
write the policy statements into your router's config. (E.g., cisco
tends to push their proprietary voip sutff, so guess what... "auto qos
voip cisco-phone" was oriented around those phones and not necessarily
the sip versions of that same cisco phone.) The simplest command is
"auto qos" applied to an individual interface without any other
qualifying parameters.
Keep in mind that auto qos is actually monitoring your traffic in real
time, which assumes you've got voip phones, asterisk box, etc, already
preconfigured to mark packets with TOS or DSCP bits. If that's not the
case, then your voip traffic appears as "default" non-qos traffic and no
policy will be written to the router's config.
For testing purposes, auto qos can be applied to an interface then
multiple voip test calls can be initiated manually. It would then write
the appropriate policy statements into your config based on those voip
test calls. In a large production world, one would apply auto qos to an
interface and let it be for some much longer period of time (eg, hours).
Then auto qos would write the config statements necessary to support the
actual traffic observed over that period of time.
There is no magic behind using auto qos; you can do the exact same thing
manually by configuring policies in the router and doing something like
"show policy-map interface s1". That display will tell you how much
bandwidth is consumed for each QoS class that has been configured in
your policy. The problem with doing that manually is that you have to
know when your peak traffic period is for voip traffic, and then run the
commands during that peak period to get it right.
There are technical white papers on the cisco web site (somewhere) that
describes how to use the auto qos function, but keep in mind the
function was only recently introduced so it is not yet implemented on
every product or in every IOS image.
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