[c-nsp] QoS - questions

[Oliver Boehmer (oboehmer)]

varaillon <> wrote on Tuesday, August 21, 2007 9:20 AM:

> I have few questions about queuing on Cisco.
> 
> 7200----HDLC/ISIS/TDP----26xx
> 
> I have set-up QoS between the two Cisco routers mainly use to carry
> voice traffic, as follow:
> 
> Queue1: Voice Bearer    - LLQ        - priority bandwidth 55%
> Queue2: Voice Signaling - CBWFQ      - bandwidth 10%
> Queue3: Any IP traffic  - CBWFQ WRED - bandwidth 10%
> Queue4: default
> 
> * If my LLQ queue is full, can it borrow bandwidth from any other non
> full queues including the default one?

No, "priority" implicitly configures a policer, and excess packets are
dropped.

> * If any of my CBWFQ queues is full, can it borrow bandwidth from any
> other non full queues including the default one?

Yes.

> If I use the command "max-reserved-bandwidth 90" on the serial
> outgoing interface of both routers, and if I have the following
> queues: 
> 
> Queue1: Voice Bearer    - LLQ        - priority bandwidth 70%
> Queue2: Voice Signaling - CBWFQ      - bandwidth 10%
> Queue3: Any IP traffic  - CBWFQ WRED - bandwidth 10%
> Queue4: default

Usually you don't want to provision more than 50% of your BW for LLQ.
Experience has shown that this can introduce some delay..

What would you put into the default class? See also comment below.

> What would happen in case of congestion:
> 
> * Since I know that I have very little IP traffic, Queue 3,
> guaranties that the TDP session won't go down, right?

well, since you enabled WRED, we could also drop TDP packets..

> * Since the default queue has the remaining 10% of the bandwidth,
> HDLC and ISIS won't go down, right?

well, first of all you also want to reserve BW there, i.e. add
"bandwidth percent 10" as well. Then check out
http://www.cisco.com/warp/public/105/rtgupdates.html to see how your
platform treats those packets.
 
> * So, in this case and despite Cisco advices on that matter, is it
> safe to use the "max-reserved-bandwidth 90" command?

I would think so. 

> * Do I risk to lose the serial link due to a lack of bandwidth?
> 
> * Do I risk anything else?

No, looks good with the above changes..

	oli