[c-nsp] Question about CBWFQ and PING times

[Peter Rathlev]

On Thu, 2009-03-26 at 11:04 +1100, Andy Saykao wrote:
> I tried to create a Heirarchical QoS policy on a spare 7606 we have here
> and no go. Tried to create a parent shaper and policer and neither
> worked when the service-policy was applied to the interface.

I would've thought the SIP-400 could do shaping. Data sheet says DTS is
supported, but I don't have one at hand to test it. The specific PA
might also set limitations. You may be out of luck with those
interfaces.

I assume it is a SPA in the SIP-400 you add the service-policy to,
right? Interface on LAN cards can't do it this way.

> You wrote - "You need to tell the router that it only has 200 mbps and
> not the full 1 Gbps. Otherwise it will allocate ~50 mbps (your
> 5%) for priority traffic and ~950 mbps for class-default."
> 
> This statement may be true but when I do a "show policy-map interface"
> command, it seems to allocate the percentage of bandwidth correctly as
> to what I've specified with the "bandwidth" interface command (ie:
> bandwidth 5% (10000 kbps)). I read somewhere that the QoS policy takes
> into account what you set the "bandwidth" interface command to. This
> seems to be true when I do a "show policy-map interface" because it's
> using the "bandwidth" interface command to allocate the bandwidth as
> shown below.

The "bandwidth" command doesn't do anything by itself, other than
letting e.g. routing protocols know what bandwidth is available on this
link. EIGRP and RSVP could use this. The command does not in itself help
with shaping/policing.

It's correct that the policy-map "percent" parameter looks at exactly
this parameter, but this is just configuration short-hand. Disregarding
everything but your priority queue, these four methods all reserve 100
mbps on a Gigabit-interface:

- No "bandwidth" parameter (default), "priority percent 10"
- No "bandwidth" parameter (default), "priority 100000"
- Specify "bandwidth 200000", "priority percent 50"
- Specify "bandwidth 200000", "priority 100000"

Consider the "bandwidth" parameter strictly informational.

You would have to find out what features your interface supports. DTS
and hierarchical QoS should let you use a parent shaper. Some LAN cards
support SRR which could give you a crude way of shaping.

We use shaping on 7200s with no problems, but I have never used DTS on
the switch platforms (7600/6500) so I may make some wrong conclusions.
And my SRR experience has so far been limited to lab tests.

Regards,
Peter