Class-Default bandwidth question

Unanswered Question
Mar 8th, 2007

Hi all,

Can someone point me to another source of documentation that confirms the actual bandwidth allocated to the Class-Default when no specific Bandwidth Percent command is given?

policy-map scheduler-lmc

class prec5

priority percent 10

class prec3

bandwidth percent 50

class class-default

fair-queue

Link Bandwidth: 4000

Link type: Ethernet

max-reserved-bandwidth is Default: 75%

Question: What percentage of link BW is allocated to the Class-Default?

My own argument would be:

75% (max-reserved-bandwidth) -/- 60 %(LLQ + 2nd Q) = 15%

I follow this reasoning because when I attempt to configure the Class-Default with the Bandwidth Percent command, I am only allowed to configure 15%.

HOWEVER: According to the infromation provided in the link below: Class-Default takes it?s bandwidth from the percentage that is not available for allocation to configured classes, read: 25% (100% of link BW -/- 75% max-reserved-bandwidth).

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt2/qcfconmg.htm#wp1003189

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mheusinger Thu, 03/08/2007 - 06:48

Hi,

First please understand, queueing config is only applied, if congestion exists. So class-default traffic could take 100% of your interface bandwidth, IF no other traffic is present.

Second, in most cases there is also traffic not accounted for in the class-maps, f.e. L2 keepalives, system packets, CDP, and so on. The router will prioritize some of this traffic. Therefore allocating 100% to user traffic is not feasible and thus the (conservative) approach with 75% of interface bandwidth.

Per se it is therefore also not simple to say X% of traffic is allocated to class-default.

By the way, the QoS 2.2 class covers this in detail.

Regards, Martin

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