Unanswered Question
Apr 4th, 2007

I am preparing for my CCIP and the QoS exam seems to be the most tough one.

Say i design a LLQ technique like the below

class-map match-all voip

match ip DSCP EF

policy-map VOIP

class voip

priority 1024

class class-default


int fa 0/0

bandwidth 5120

service-policy ouput VOIP

If i do a sh int fa 0/0, it shows the available bandwidth as (5120-1024=4096K) which is expected.

What i would like to know is whether the 1024K is permanently reserved for EF traffic or this can be used by the class-default when no EF traffic is received


I have this problem too.
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spremkumar Wed, 04/04/2007 - 00:31

Hi Naryan

From my personal experience when u dont have any traffic in priority queue which u define using the key word priority your other classes will be able to burst the available bandwidth..


John Patrick Lopez Wed, 04/04/2007 - 01:44

You can only reserve 75% of the total bandwidth. So bandwidth - 25% = reservable bandwidth.

So in your case, the total reservable bandwidth is 3840 - 1024 = 2816kbps

2816 is the total bandwidth for class-default. Use show policy-map to verify configuration.


royalblues Wed, 04/04/2007 - 02:02

Prem Even i feel the same.

But is there any way to identify what traffic is actually flowing through the priority queue?

Say i generate a 512K traffic for priority queue will the other 512K be utilized for class default? if the traffic increases in priority queue will the class-default traffic be pusdhed to another queue?

John, i understand that the traffic will be reserved for only 75% but this is a default.

I have the max-reserve-bandwidth 100 command on the interface (not shown)


John Patrick Lopez Wed, 04/04/2007 - 02:08

Actually, i read two articles which is contradicting.

The first one said that LLQ is a strict priority. Even if the link is not congested. But the second article said that LLQ will just take effect if the link is congested.

Those are from Ciscopress books and both of them are CCIE authors. I don't know which one to believe. =)


spremkumar Wed, 04/04/2007 - 02:17

Hi Narayan

I still remember the tests which we carried in our backbone 2 years back with voice/ftp/icmp traffic to check the qos config behaviors.

We did see the behavior as normal and as mentioned in my first mail it doesn't strictly hold the bandwidth you configure in the priority queue..


royalblues Wed, 04/04/2007 - 03:59

Thanks Prem,

Is it possible to test whether i am getting the proper qos behavior for the strict priority?


spremkumar Wed, 04/04/2007 - 04:32

hi Narayan

Try connecting your laptop or pc onto the ethernet port of the routers on both the side and try to generate different traffic patterns like FTP/ICMP Ping.

You can classify icmp as prioritized traffic and make ftp as class default.

try to initiate different icmp sessions with different byte size and congest the link.

also try to do FTP between the PC so that you have both sort of traffic on the link.

By doing so you can verify the qos config behavior on your router.

also you can try to do a scenario of icmp only and ftp only.



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