Simple Policy map question - I hope.

[DialerString_2]

I have the below policy attached to s0/0/0 - (service-policy output ETMS) Is the 256 being set aside until use?

policy-map ETMS

class SILVER

set ip dscp af31

bandwidth 256

[davy.timmermans]

Hi,

I don't think that the 25% is used for the class-default. I think that class-default uses the left-overs of other classes if no bandwidth is specified.

I thought by default

75% of the bandwidth can be allocated

to LLQ/CBWFQ (including the default class)

I think that control traffic is automatically prioritorized by the software and is part of the 25%

[davy.timmermans]

can't edit my post anymore

I was wrong

from cisco:

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.

[Giuseppe Larosa]

Hello Davy,

class-default traffic should go with the 75% of BW.

I may be wrong but there is no sense to have best effort traffic to compete with routing protocol packets.

So I agree with your first post.

To be added:

not all cisco platforms have this concept of a system queue for control traffic.

For example on C7500, C7200 and GSR 12000 we had to explicitly provide a class to protect routing protocol messages like BGP or OSPF.

the 25% is a rule of thumb that can be reduced on high speed links to a lower value freeing BW to be left for user traffic.

Hope to help

Giuseppe

[Joseph W. Doherty]

Bandwidth reservation depends on the platform and IOS version. On many Cisco routers, CBWFQ won't allow you to explictly define more than 75% of the bandwidth unless the default reserved bandwidth reservation is changed on the interface.

The reason for this is to reserve bandwidth for other traffic, including, but not limited to, class-default (when bandwidth isn't defined for it).

Since only LLQ's bandwidth statement limits bandwidth, any class can use up to 100% of available bandwidth (this also assumes there isn't a defined shaper or policer within the class).

On many platforms CBWFQ, control traffic, unless explictly matched in a defined class, will fall within class-default. Assuming max reserved bandwidth hasn't been changed, and also assuming WFQ is active with class-default, control traffic is often not adversely impacted by other traffic.