From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sun Apr 27 2008 - 17:53:53 ART
Hi Huan,
I think the doc cd is giving a simplistic view of the behaviour. My
understanding of the mechanics involved is that the priority command
traffic metering is only activated by packets which are handled by the
processor. Perhaps someone with a better understanding of the low level
processes will be able to add some insight to this (or correct any
mistakes).
Since transit packets are normally cef switched, they do not hit the
meter unless there is interface congestion. If transit traffic is
instead fast switched, only the first packet hits the processor so fast
switched traffic is also (practically) unaffected, unless there is
interface congestion.
I think you will find that 'packets matched/bytes matched' increases for
every packet you source locally, but it will not increase for cef
switched traffic unless there is interface congestion.
In your case you have the policy applied to the router where you are
sourcing the packets, so all the packets are handled by the processor
and hit the inbuilt policer. If at the same time there was also transit
ICMP traffic passing through the router, then the class could still
exceed its allocated priority bandwidth reservation unless that transit
traffic was process switched.
Paul.
Huan Pham wrote:
> Hi friends,
>
> Both DOC CD links below indicate that the priority traffic can burst above
> the configured priority bandwidth when there is no congestion. It is only
> policed at the configured bandwidth when there's congestion. However, my
> simple test below seems to prove the other way. It seems that the LLQ drops
> priority traffic even though that there's no congestion. The bandwidth
> configured with the "priority" command seems to be the hard limit.
>
> Can anybody please explain why I get pings (priority traffic) dropped, when
> the QOS policy applied, and do not get any drops when there's no QOS policy.
>
>
> The relevant links and my configuration below
>
> Low Latency Queueing
> http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfconmg.ht
> ml#wp1001291
>
> "Priority traffic metering has the following qualities:
>
> It is much like the rate-limiting feature of CAR, except that priority
> traffic metering is only performed under congestion conditions. When the
> device is not congested, the priority class traffic is allowed to exceed its
> allocated bandwidth. When the device is congested, the priority class
> traffic above the allocated bandwidth is discarded."
>
>
> Comparing the bandwidth and priority Commands of a QoS Service Policy
> http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080
> 103eae.shtml
>
>
>
> R1 ------- R3
>
>
> R1#
> ip access-list extended RT
> permit icmp any any
> !
> class-map match-all RT
> match access-group name RT
> !
> !
> policy-map WAN_QOS
> class RT
> priority 64 10000
> !
> interface Serial1/1
> ip address 13.0.0.1 255.0.0.0
> load-interval 30
> service-policy output WAN_QOS
>
>
>
> R3#
> interface Serial1/2
> ip address 13.0.0.3 255.0.0.0
>
>
>
>
> R1#ping 13.0.0.3 rep 100 size 500
>
> Type escape sequence to abort.
> Sending 100, 500-byte ICMP Echos to 13.0.0.3, timeout is 2 seconds:
> !!!!!!!!!!!!!!!!!!!!!!.!!!!!!!!!!!!!!!.!!!!!!!!!!!!!!!!.!!!!!!!!!!!!!!
> !!!!.!!!!!!!!!!!!!!!!!!.!!!!!!
> Success rate is 95 percent (95/100), round-trip min/avg/max = 12/55/236 ms
>
> R1#sh policy-map int s1/1
> Serial1/1
>
> Service-policy output: WAN_QOS
>
> Class-map: RT (match-all)
> 8925 packets, 4288600 bytes
> 30 second offered rate 50000 bps, drop rate 0 bps
> Match: access-group name RT
> Queueing
> Strict Priority
> Output Queue: Conversation 264
> Bandwidth 64 (kbps) Burst 10000 (Bytes)
> (pkts matched/bytes matched) 8925/4288600
> (total drops/bytes drops) 299/172896
>
> Class-map: class-default (match-any)
> 120 packets, 7929 bytes
> 30 second offered rate 0 bps, drop rate 0 bps
> Match: any
>
>
> R1#conf t
> Enter configuration commands, one per line. End with CNTL/Z.
> R1(config)#int s1/1
> R1(config-if)#no service-policy output WAN_QOS
> R1(config-if)#
> R1#ping 13.0.0.3 rep 100 size 500
>
> Type escape sequence to abort.
> Sending 100, 500-byte ICMP Echos to 13.0.0.3, timeout is 2 seconds:
> !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> Success rate is 100 percent (100/100), round-trip min/avg/max = 1/40/232 ms
>
>
> Pass the CCIE in six weeks, Guaranteed!
> http://www.certscience.com/CCIE
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
Pass the CCIE in six weeks, Guaranteed!
http://www.certscience.com/CCIE
This archive was generated by hypermail 2.1.4 : Thu May 01 2008 - 08:25:52 ART