Understanding hold-queues and policing on an interface

Unanswered Question
Nov 15th, 2009

Hi All,

I'm a bit lost as to why the output drops start increasing reapidly when I start policing the interface.

For example we look after several schools and we police theses schools to 2M, 4M, etc...

If I do not apply the service-policy to the interface, I do not see any output drops. When I apply the service-policy to the interface, the output drops start increasing rapidly.

interface GigabitEthernet0/1

description School Interconnection

mtu 9216

bandwidth 600000

ip address 203.17.98.x 255.255.255.252

ip flow ingress

load-interval 30

media-type rj45

speed 1000

duplex full

no negotiation auto

no clns route-cache

service-policy output POLICE-SCHOOLS

router#sh int g0/1

GigabitEthernet0/1 is up, line protocol is up

Hardware is MV64460 Internal MAC, address is 0023.33bf.e01a (bia 0023.33bf.e01a)

Description: School Interconnection

Internet address is 203.17.98.x/30

MTU 9216 bytes, BW 600000 Kbit, DLY 10 usec,

reliability 255/255, txload 81/255, rxload 7/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full Duplex, 1000Mbps, RJ45, media type is RJ45

output flow-control is XON, input flow-control is XON

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters 00:04:11

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1594

Queueing strategy: fifo

Output queue: 0/40 (size/max)

30 second input rate 18571000 bits/sec, 16421 packets/sec

30 second output rate 192800000 bits/sec, 22236 packets/sec

4271641 packets input, 621574470 bytes, 0 no buffer

Received 4 broadcasts (0 IP multicasts)

0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 126 multicast, 0 pause input

5911195 packets output, 2295301227 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier, 0 PAUSE output

0 output buffer failures, 0 output buffers swapped out

I've tried increasing the hold-queue to 1024 and 2048 but this hasn't stopped the output drops.

The policing looks like this with some schools removed for brevity

policy-map POLICE-SCHOOLS

class CLASS-TCS-200002-CURRIC

police 2000000 375000 750000 conform-action transmit exceed-action transmit violate-action drop

class CLASS-TCS-200002-ADMIN

police 500000 93750 187500 conform-action transmit exceed-action transmit violate-action drop

class CLASS-TCS-200176-ADMIN

police 500000 93750 187500 conform-action transmit exceed-action transmit violate-action drop

class CLASS-TCS-200176-CURRIC

police 2000000 375000 750000 conform-action transmit exceed-action transmit violate-action drop

class CLASS-TCS-200176-DMZ

police 2000000 375000 750000 conform-action transmit exceed-action transmit violate-action drop

.

.

.

.

class class-default

Any ideas as to why this happens and if it's bad or normal behaviour????

Thanks.

Andy

I have this problem too.
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Nagendra Kumar ... Sun, 11/15/2009 - 22:02

Andy,

This is an expected behavior.

When you configure policing, it will regulate the traffic flow at certain rate and will drop excess traffic.

Say, If you have configured policing at 3 Mbps and if you have 4 Mbps of traffic flowing, 1 Mbps of traffic will be dropped.

If you don't want the excess traffic to be dropped, you need shaping to be configured.

Shaping will buffer the excess traffic and will forward at next interval and avoid excess traffic getting dropped to some extend.

If you are an end customer, it is advisable to configure shaping rather than dropping your own traffic.

HTH,

Nagendra

Joseph W. Doherty Mon, 11/16/2009 - 17:21

Ditto Nagendra's post, but might add: the interface's "Total output drops:" will count policy drops. Showing interface policy map output stats should provide additional details on the policed drops.

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