Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
New Member

Burst value for Policers in a 3750


I am trying to create a Policy-Map to use it at the ingress of a Fastethernet interface to be able to enforce bandwidth utilization and marking for incoming packets.

One of my queues is used for VOICE. My objective for this queue, is to be able to guarantee around 6Mbps. I started using the below configuration, but after using 3rd party testing software (WAN Killer and Qcheck) I realized that the enforcing wasn't working because I was reaching speeds of 90Mbs for that particular class.

The first thing I thought about, was queueing at the egress.

As you can see, at a queueing level, I am using shaping for the PQ with a 10% (10 0 0 0), so I assume all traffic should be dropped after using 10Mbps (assuming I use a 100Mbps port).

After not being able to explain why my shape wasn't working the way I thought, I focused on the policer.

Looking at the Burst value, I tried to modify it, but the results of my testing didn't make any sense *.

I started testing with pairs of Bandwidth/Burst and the average speed reached just didn't make any sense.

I would like to be able to predict the max. speed based on my Policers (Bandwidth/burst). Is there any way to do it?

Thanks in advance for the help.

mls qos

mls qos map policed-dscp 24 26 to 0

mls qos map cos-dscp 0 8 24 26 34 46 48 56

mls qos srr-queue output dscp-map queue 1 threshold 2 34

mls qos srr-queue output dscp-map queue 1 threshold 3 46

mls qos srr-queue output dscp-map queue 2 threshold 2 24

mls qos srr-queue output dscp-map queue 2 threshold 3 26

mls qos srr-queue output dscp-map queue 3 threshold 3 0

mls qos srr-queue output dscp-map queue 4 threshold 1 8

mls qos queue-set output 1 buffers 5 10 84 1



ip access-list extended MAPI

deny ip any any

ip access-list extended SCAVENGER

deny ip any any

ip access-list extended VOICE-SIGNALING

permit tcp any any range 2000 2002

permit tcp any range 2000 2002 any

ip access-list extended VIDEO

deny ip any any

ip access-list extended VOICE

permit udp any any range 16384 32767


class-map match-all MAPI

match access-group name MAPI

class-map match-all VOICE-SIGNALING

match access-group name VOICE-SIGNALING

class-map match-all VIDEO

match access-group name VIDEO

class-map match-all VOICE

match access-group name VOICE

class-map match-all SCAVENGER

match access-group name SCAVENGER


policy-map QOS

class VOICE

police 6000000 450000 exceed-action drop

set dscp 46

class VIDEO

police 4000000 300000 exceed-action drop

set dscp 34


police 1000000 75000 exceed-action policed-dscp-transmit

set dscp 26

class MAPI

police 3000000 225000 exceed-action policed-dscp-transmit

set dscp 24


police 1000000 75000 exceed-action drop

set dscp 8

class class-default

police 85000000 1000000 exceed-action drop

set dscp 0


! Access Interfaces:



interface range FastEthernet0/1 - 48

switchport trunk encapsulation dot1q

switchport mode trunk

switchport voice vlan 800

spanning-tree portfast

srr-queue bandwidth share 1 5 94 1

srr-queue bandwidth shape 10 0 0 0

priority-queue out

service-policy input QOS



* I tried testing different sets of values and the Bandwidth I was able to reach wasn't congruent at least with the way I understand it.

Here are some sets of values I used and the average reached speed:

Bandwidth/Burst -> Reached-Bandwidth

10000/8000 -> About 1Mbps

20000/8000 -> About 2Mbps

20000/20000 -> About 2Mbps

30000/8000 -> About 0.5Mbps

30000/15000 -> About 0.5Mbps

30000/500000 -> About 0.5Mbps

320000/8000 -> About 320Kbps -> This is an actual example from the SRND.

4000000/8000 -> About 3Mbps

5000000/8000 -> About 5Mbps

6000000/8000 -> More than 15Mbps

5500000/8000 -> Around 6Mbps (I had a few instances where the max. speed reached 25Mbps)


Re: Burst value for Policers in a 3750


I agree ur point , that if you shape 10 0 0 0 , then queue1 would be policed to 10M , but in ur case , queue 1 would be used by default dscp value 40-47 .

I have a query wheter the third party application what you use , is it set to some dscp value in that range or it falls in the default class . If there is no traffic in the other queues , default class can utilise the entire bandwidth.

Pls clarify on the same to discuss


vanesh k

New Member

Re: Burst value for Policers in a 3750

Thanks for your answer Vanesh,

The third party app does send the data with a DSCP=0 (I confirmed that with ethereal) however, in my input Policer, I do specify "set dscp EF" for that particular class. I am able confirm that the data is being marked EF by the Policer using Ethereal.

I would supposed that before the packet is sent to the Output interface, it is tagged with EF and consequently, sent to PQ1 at the egress, right?

I have tested with two scenarios, 1) Using one Cat3750 and two PCs and 2) Using two CAT3750 (Access) one Cat6500(Core/Dist) and the two PCs. Same result.

Again, I appreciate you answer.

Re: Burst value for Policers in a 3750


Thanx for your reply . Pls confirm me on this

1.You are matching the ports and setting a dscp value as 46 , can u confirm wheter your application uses that ports .

You are right if your packet is tagged with EF , then it would be sent via the PQ1 .

2.When you are using 6500 , pls check wheter you have given " trust dscp " in case if you have configured ip in the 6500 interface.

Hope this helps


vanesh k


Re: Burst value for Policers in a 3750

None of this is making sense to me. Please provide more details after considering the following:

1. The priority-queue on the c3750 is output only, so setting a 'shape' to 10% would only effect outgoing traffic from the port, not inbound traffic. You can configure one ingress queue as the priority queue by using the 'mls qos srr-queue input priority-queue bandwidth ' global configuration command (see the docs).

2. The 'shaping' that you are describing for the priority queue is queue shaping, not a traffic shaper as if you had a policy map with a 'shape average ' command. This 'queue shaping' is outbound only and will not impact inbound traffic to the port. This is different if this is a metro c3750 on the ES ports.

3. Why are all 48 ports configured as trunks?

4. Why are you configuring portfast on each port instead of globally 'spanning-tree portfast default'?

5. What is the egress port for this switch? A Gig port or just other copper ports? Where does this traffic go from these copper ports? What is the config on the uplink or downlink ports? If this is the only switch involved in your test than you are definitely creating an interesting test environment in which your policer is policing inbound traffic and each port is 'queue shaping' outbound traffic. I would suggest one or the other and configure what is appropriate for what you need rather than configuring what looks like everything and trying to find out what works. I've been down this road, you will never get it exactly the way you want it.

6. Why on earth are the burst parameters so high on your config example? In most cases you should just configure the average and have the switch determine the appropriate burst values.

7. Why are you using access-lists for matching voice traffic? Your ACL matching udp ports from 16384 to 32767 have two huge problems: first, no guarantee that the udp packet is voice (many worms use ports in that range), second RTP traffic only uses even port numbers, not odd, odd port numbers are SRTP signalling packets. I want to be on your network, if I configure my laptop to trunk and send my edonkey traffic on udp port 16999 I will have high priority! At least include the destination ip of the call manager and voice gateways in the ACL to be more restrictive.

8. Why aren't you trusting the end device like an ip phone rather than trying to re-write the IP DSCP value using an ACL? The best practice is that the switch ports be configured (using auto qos or not) to use CDP to allow access to the voice vlan and NOT to use a trunk (the ip phone will tag the voice traffic using dot1q for the voice vlan and the port will never 'trunk'.

9. There are significant restrictions on how you can apply QoS policies to the switch ports on the ASIC based Catalyst platforms, including policing granularity, number of TCAM entries required, number of match statements per class, number of classes, etc.

10. Last, can you provide the IOS version and switch model that you are using?

I will provide some more advise once I understand the above information.


New Member

Re: Burst value for Policers in a 3750

Thanks for your time Rick.

Here is the information you are requiring.

1. My main question is about input Policers, although, if the packet is being marked at the ingress (I can confirm this) why isn't this being limited to a 10% at the egress? Better, why isn't the traffic being limited to 6M at the ingress with the Policy-Map?

2. Agree.

3. This is copy and paste typo. Voice VLANs are only supported on access-ports.

4. The Port Fast feature is automatically enabled when voice VLAN is configured.

5. Cooper-to-Cooper. No uplinks involved at this time but they will once the Policers are tested and operational. Thanks for the suggestion but this is required for SLAs. We need to make the switch limit the ingress traffic at a certain rate.

6. You are asking me the question I originally asked. That's what I want to know, "What would be right combination of Bandwidth/Burst".

7. Thanks for the suggestion but involving the Call Manager and VG IP addresses in the access-list would exclude the traffic between the IP Phones themselves (no to mention that the protocol is different -SCCP). If any, I could include the Voice VLANs defined, which still doesn't cover me from your edonkey.

8. There will be more applications defined. If you trust at a port level, you cannot apply a port Policy Map. So, in order to support a Policy Map, I would have to trust within the voice class (RTP) the savings on configuration lines will be negligible, don't you think?

9. Yep, 256 Policers per ASIC and no more than 64 per port -Not the case here though. I believe we have 2 ASICs, so I think I am in range, you can use 10 policers per port MAX. Of course I won?t go this far but I?ll be aware.

10. 12.2(25)SEE.

CreatePlease to create content