[c-nsp] 3550 WRR/Priority Queue jitter

[Clinton Work]

I'm seeing maximum jitter reach 34ms when simulating a VOIP call between
Tester1 and Tester2 with 6Mbps of background TCP traffic. VOIP traffic
is using DSCP value 46 (EF) and the background traffic is using DSCP value
0.
I would have expected a maximum jitter for the VOIP call of around 4-5ms.
I looked through the 3550 configuration guide and I can't find a problem
with the QoS configuration on the Ethernet ports.

Network setup:
Tester1 <-->  3550a <-- 802.1q ---> 3550b <--- 802.1q---> 3550c <---> Tester
2

All links are 100/Full except for 3550b to 3550c and 3550c to Tester2 which
are 10/Full.
The 3550s are running IOS 12.1(19)EA1c EMI image.

QoS config is trusting DSCP values on input. WRR queueing is used on output
with
a priority queue for DSCP values 40 and above. The same QoS config is used
on all
3550 ports.

3550b# show run int f0/18
!
interface FastEthernet0/18
 desc 10/Full link to 3550c
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 15
 switchport trunk allowed vlan 15,200,201
 switchport mode trunk
 no ip address
 load-interval 30
 duplex full
 speed 10
 mls qos trust dscp
 mls qos monitor dscp 0 16 24 32 40 46
 wrr-queue bandwidth 3 12 85 255
 wrr-queue cos-map 1 0 1 2 3 4
 wrr-queue cos-map 4 5 6 7
 priority-queue out

router#show mls qos maps dscp-cos
   Dscp-cos map:
     d1 :  d2 0  1  2  3  4  5  6  7  8  9
     ---------------------------------------
      0 :    00 00 00 00 00 00 00 00 01 01
      1 :    01 01 01 01 01 01 02 02 02 02
      2 :    02 02 02 02 03 03 03 03 03 03
      3 :    03 03 04 04 04 04 04 04 04 04
      4 :    05 05 05 05 05 05 05 05 06 06
      5 :    06 06 06 06 06 06 07 07 07 07
      6 :    07 07 07 07



=========================================================================
Clinton Work                                        clinton at scripty.com
Calgary, Alberta