[c-nsp] packet processing delay & LLQ

[Volodymyr Yakovenko]

Hello!

 I need some advice in LLQ tuning for delay-critical traffic.

 As far as I know the right way to provide minimal delay for certain type
 of traffic is to put it into priority queue.

 To test it out I am using 2M Serial connected 2610 (Serial0/0) with the 
 following LLQ configuration:  

ip access-list ext NOC
 permit ip any host 172.20.2.20
 permit ip host 172.20.2.20 any
class-map match-any IS-NOC
  match access-group name NOC
policy-map E2
  class IS-NOC
   priority 512
  class class-default
   fair-queue  
int Serial0/0
 service-policy output E2

 In clean channel case (no other traffic flowing on link) RTT (ping to 
 172.20.2.20) over Serial is about 3ms. When I am saturating link with 
 class-default traffic RTT jumps to about 90-110ms and remains almost stable. 
 Priority queue works (class-default traffic RTT is 200ms and more), but
 delay value is not applicable.

 I have tried to adjust interface output hold-queue size, but visible delay 
 decrease happening only with values like 16 and less. I am not quite sure
 how IOS apply 'hold-queue 16 out' to LLQ and whould be grateful for any 
 technical explanation. I also noticed visible increase in a packet drops 
 count so I am not sure that hold-queue size adjustment is a good idea.

 To conclude - I will be very grateful for any technical references or 
 explanations how to optimize certain LLQ class for minimal delay.  

-- 
Regards,
Volodymyr.