Trouble understanding default class queue drops, when circuit is not even 50% utilized

[jhouchins]

Hello,

I have been tasked with implementing a QoS policy not designed by me, and i do not have the freedom to just change it, but i would like to understand what/why my issue is going on, so that i am better informed when i try and relate what i am seeing back up the "chain".

I have been trying to read up on how Cisco routers do what they do in these situations, but i am getting a little wrapped around the axle, and it has not quite locked in yet... so, please bear with me.

The Reader's Digest scenario is:

GigE LAN, with dual DS3 WAN (MPLS) circuits (some sites has only a single DS3, but it happens at both):

The QoS policy looks something like this:

priority percent 10

shape average percent 27

bandwidth remaining x

bandwidth remaining xx

bandwidth remaining xx

bandwidth remaining x

bandwidth remaining 25

I am seeing bursts of output queue drops.  Deeper checks show that the largest amount of drops occur in the last class, the class-default.  There are small amounts in one other queue, but the ratio is nominal.

I probably sound like an idiot, but i am still shuffling through the ACL assignments, etc...

Thank you in advance for any time and assistance provided.

~ john

[Cisco Freak]

H John,

shape average percent 27

Why are shaping at 27% of the total BW? If you have a 45Mbps you are shaping at near to 12Mbps. Anything above that BW will be queued and therefore will have the chance to get dropped.

CF

[jhouchins]

Hello Cisco Freak,

I appreciate you taking the time to answer my question.  Unfortunately i have not been given the "why", only the "do"... and that has left me with dealing with the mess once Operations began noticing the issue.

The 12% you mention, would this be in addition to the 10% set aside for the priority percent?  (For a total of approximately 16mbps).

Your answer is a bit helpful, but the default queue... is this just because it is the last bucket of the queues?