QoS on Cisco 1841 router

ravisambaji · ‎04-09-2008

Dear Friends,

i have configured the qos on my cisco 1841 router which is connected to the MPLS network. I have allocated the bandwidth as below

Critical App - 50%

transactional app - 15%

Management traffic - 10%

all- default - 25%

here is my policy-map configuration

policy-map EGRESS_MARKING

class CBS_TRAFFIC_SERIAL

bandwidth percent 50

class RTGS_ATM_TRAFFIC_SERIAL

bandwidth percent 15

class MGMT_TRAFFIC_SERIAL

bandwidth percent 10

class class-default

bandwidth percent 25

random-detect

when i apply this to the interface

int ser0/1/0

service-policy output EGRESS_MARKING

i get the following error message when i apply the policy map to the interface and the command doesn't take

I/f Serial0/1/0 class class-default requested bandwidth 25%, available only 1%

iam not able to understand why is it so... please help me..

Regards

Ravi

Jon Marshall · ‎04-09-2008

Ravi

It is because the default maximum bandwidth you can allocate is 75% not 100%. The router reservers 25% for itself and this is not allocatable. Not sure why it is saying 1% available actually.

You can increase the default maximum if you want but Cisco generally recommend against it.

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfconmg_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1001225

Jon

Joseph W. Doherty · ‎04-10-2008

From Jon's reference "The sum of all bandwidth allocation on an interface cannot exceed 75 percent of the total available interface bandwidth. The remaining 25 percent is used for other overhead, including Layer 2 overhead, routing traffic, and best-effort traffic. Bandwidth for the CBWFQ class-default class, for instance, is taken from the remaining 25 percent."

What's not clear is how an explicitly defined class-default interacts with the reserved 25%. I've believed the class-default had always shared its bandwidth with both user traffic and overhead traffic. (For instance, your usage of RED might show counts for IP Prec 6/7, if you're doing routing across the interface.) What might be the issue here is your explicitly defined 25% within class-default.

Try class-default without the explicit bandwidth setting. If you can load down the interface with traffic, examine the queues and see if the queue weights correspond to expected ratios (e.g. class-default weight being half of class CBS_TRAFFIC_SERIAL weight).