Hi everybody,
http://lostintransit.se/category/qos/
According to above congestion is detected when:
QoS is only active during congestion. This is well known but it’s not as well known how congestion is detected. the TX ring is used to hold packets before they get transmitted out on an interface. This is a hardware FIFO queue and when the queue gets filled, the interface is congested.
I decided to lab it up:
R1--199.199.199.0--R2 f0/1--200.200.200.0--R3 ( Lo:3.3.3.3)
Above R1 will generate "interesting traffic" by pinging 3.3.3.3 , on R2 we define a class LEE for all traffic sourced from 199.199.199.1( R1) and destined to 3.3.3.3 ( R3)
This class is given 75% of f0/0' bandwidth.
R2#show policy-map interface f0/1
FastEthernet0/1
Service-policy output: LEE
Class-map: LEE (match-all)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 101
police:
rate 75 %
rate 75000000 bps, burst 2343750 bytes
conformed 0 packets, 0 bytes; actions:
transmit
exceeded 0 packets, 0 bytes; actions:
drop
conformed 0 bps, exceed 0 bps
Class-map: class-default (match-any)
3 packets, 214 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
Next , we are going to generate traffic from R1 to 3.3.3.3
R1#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/19/24 ms
R2#show policy-map interface f0/1
FastEthernet0/1
Service-policy output: LEE
Class-map: LEE (match-all)
5 packets, 570 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 101
police:
rate 75 %
rate 75000000 bps, burst 2343750 bytes
conformed 5 packets, 570 bytes; actions:
transmit
exceeded 0 packets, 0 bytes; actions:
drop
conformed 0 bps, exceed 0 bps
Class-map: class-default (match-any)
26 packets, 2282 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: any
########################
1) How do we interpret " conformed 5 packets" above? Should we say there is congestion on R2's f0/1 based on this ? If yes , how , because there is very little traffic exiting f0/1 , just 5 pings ?
2) Are those 5 packets queued in class LEE? If yes, then why, because there is no congestion on f0/1 on R2?
On the other hand if I modify Class LEE as priority, I see following:
R2(config)#policy-map LEE
R2(config-pmap)#no class LEE
R2(config-pmap)#class LEE
R2(config-pmap-c)#priority percent 75
R2#clear counters
Next we generate pings to 3.3.3.3 from R1
R1#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5),
R2#show policy-map interface f0/1
FastEthernet0/1
Service-policy output: LEE
Class-map: LEE (match-all)
5 packets, 570 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: access-group 101
Queueing
Strict Priority
Output Queue: Conversation 264
Bandwidth 75 (%)
Bandwidth 75000 (kbps) Burst 1875000 (Bytes)
(pkts matched/bytes matched) 0/0
(total drops/bytes drops) 0/0
Above we see none of the counters increment . To put it simply, should " conformed packets" counter be interpreted as indication of congestion or a just a counter showing conforming packets not congestion necessarily ?
Thanks and have a nice weekend.
