Unanswered Question
mounir.mohamed Wed, 07/16/2008 - 08:43


If you need to allocate more than 75 percent for CBWFQ and IP RTP Priority, you can use the this command, because the sum of all bandwidth allocation on an interface should not exceed 75 percent of the available bandwidth on an interface. The remaining 25 percent of bandwidth is used for overhead, including Layer 2 overhead, control traffic, and best-effort traffic.

Best Regards,

Mounir Mohamed

mounir.mohamed Wed, 07/16/2008 - 09:16


Here is an example, below we have 3 different classes (HTTP/FTP/ICMP)in addition to the default class, i will allocate 500k for each class and only 44k for the default class,

since CBWFQ can not allocate more than 75% of the interface bandwidth the above bandwidth allocation doesn't comply with default bandwidth of serial interfaces which is 1544 (1544x25/100=386 so 1544-386=1158) so 1158 is the maximum available bandwidth for allocation on this interface, so the router will never accept policy-map with bandwidth over than 1158 unless we use max-reserved-bandwidth command.

PE1#conf t

PE1(config)#class-map http

PE1(config-cmap)#match protocol http

PE1(config-cmap)#class-map ftp

PE1(config-cmap)#match protocol ftp

PE1(config-cmap)#class-map icmp

PE1(config-cmap)#match protocol icmp

PE1(config)#policy-map qos

PE1(config-pmap)#class http

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class ftp

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class icmp

PE1(config-pmap-c)#bandwidth 500

PE1(config-pmap-c)#class class-default

PE1(config-pmap-c)#bandwidth 44

PE1(config-pmap)#int s2/1

PE1(config-if)#service-policy output qos

I/f Serial2/1 class icmp requested bandwidth 500 (kbps), available only 158 (kbps)

PE1#show queueing int s2/1 | inc Bandwidth

Available Bandwidth 1158 kilobits/sec

PE1(config-if)#max-reserved-bandwidth 100

PE1(config-if)#do show queueing int s2/1 | inc Bandwidth

Available Bandwidth 1544 kilobits/sec

PE1(config-if)#service-policy output qos

PE1(config-if)#do show queueing int s2/1 | inc Bandwidth

Available Bandwidth 0 kilobits/sec

Best Regards,

Mounir Mohamed

Giuseppe Larosa Wed, 07/16/2008 - 12:04


the maximum reserved bandwidth refers to the sum of resources used by CBWFQ w/o LLQ not to a single class.

To be noticed that not all Cisco router platforms really use this concept (have an implicit system queue for routing protocols traffic etc)

For example on C7500 and GSR 12000 I remenber we had to allocate resources = BW for the routing protocols traffic and there isn't a control at 75% percent of bandwith.

So the command can be used to leave BW to user traffic on a high speed links where 25% of BW is too much for overhead.

Hope to help


Joseph W. Doherty Wed, 07/16/2008 - 16:16

Haven't had much need for it when later IOS versions supported percentages and/or remaining percentages.

Joseph W. Doherty Tue, 07/22/2008 - 14:52

Before class percentages was a feature, you had to remember not to allow the combined explicit class bandwidths to exceed 75% of the link's bandwidth, unless you modified max reserve. With percentages, a bit easier since you only have to keep from exceeding 75% without referencing the link's bandwidth. With remaining percentages, you don't even need to account for reserve percentage, although it's still there.


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