04-15-2009 05:45 AM - edited 03-04-2019 04:22 AM
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
08-30-2009 08:46 AM
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%
08-30-2009 09:20 AM
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.
08-30-2009 11:04 AM
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
08-31-2009 04:11 AM
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.
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