From: Huan Pham (huan.pham@valuenet.com.au)
Date: Sat Apr 26 2008 - 23:49:46 ART
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
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