Router QoS setting

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Mar 17th, 2008
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Dear all,

I have some question about QoS setting and bandwidth allocation.

Case 1:

Can i set overall QoS percentage exceed 100% like as below?

If can, how can it allocate the BW?

If the percentage set cannot exceed 100%, then what is the difference/purpose to set "priority" and "bandwidth", since they all use to reserve BW?

policy-map policy1

class voice1

priority percentage 40

class voice2

priority percentage 40

class data

bandwidth percentage 30

Case 2:

In case that the BW allocation is not 100%, how will the traffic drop if BW congest or non-congest?

policy-map policy1

class voice1

priority percentage 40

class data

bandwidth percentage 30

Thanks all,

Eric HUI

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shiva_ial Mon, 03/17/2008 - 22:39
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hi Eric,

The link connected through routers

has some IGP (ospf )traffic apart from data if you reserve the 100 % of total

bandwidth of the link, the ospf traffic will

not get priority so probelm may in convergence.

ospf changes,routing table will not be proper

so CISCO suggests to reserve only 75% of total traffic.

in the second case,with WRED enabled it tries to drops packets with low priority value

hope my post clarifies



eric.huihk Tue, 03/18/2008 - 07:55
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For case 1, is the configuration workable if the sum of percentage bandwidth has exceeded 100%? Also, how will the bandwidth be allocated?

For case 2,

If low priority value will be drop firstly, how is the packet identified as low priority?



Joseph W. Doherty Tue, 03/18/2008 - 11:58
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I recall most routers won't accept CBWFQ of more than 100%, but can't recall whether that's while defining the policy map or when policy map is applied to interface.

There's a major difference between "priority" % and bandwidth %. The former sets a cap that the class can not exceed. The latter sets a minimum, but more may be used if available. The latter also sets ratios between different classes when they both want more bandwidth and it's available.


class llq

priority % 10

class a

bandwidth % 10

class b

bandwidth % 20

class c

bandwidth % 30

If there's only traffic in classes a and c, and they both want all, they'll get 25% and 75%.

With regard to congestion drops, again "priority" will drop whenever its allocation is exceeded. The other classes, usually have a FIFO queue, which drops with queue packet overflow.


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