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QoS Implementation guide line in cisco 1841


I am Using Cisco 1841 router .

I have 2 mbps of LL Internet Line .

I want to implement Qos for my internet & Smtp traffic by givin 512 kbps to internet & 1 mbps to Smtp Traffic

I want to give prirotry to smtp traffice for In /out

Can any body will tell best solution.


Re: QoS Implementation guide line in cisco 1841

Below is the info. Please check it, there are lots of example that you can follow.

Do you mean the Internet is max. 512k or min. ?

Do you mean the SMTP traffic is min. 1Mbps ?

I propose to use CBWFQ.

Hope this helps.

New Member

Re: QoS Implementation guide line in cisco 1841

Hi Internet is 512 Max & SMTP min 1Mbps

Also I need to configure Priority for SMTP & mY SMTP PACKET SHOULD NOT DROP .

Check Below Config.

router(config)#class-map httppolicy

router(config-cmap)#match protocol http

''policy matches smtp protocol----------

router(config)#class-map smtppolicy

router(config-cmap)#match protocol smtp

''define policy map

router(config)#policy-map myBWpolicy

''match the class maps

router(config-pmap)# class httppolicy

router(config-pmap-cmap)#bandwidth 512

router(config-pmap)# class smtppolicy

router(config-pmap-cmap)#bandwidth 1024

apply the service policy outwards

router(config)#int s0/0

router(config-f)#service-policy output myBWpolicy


Re: QoS Implementation guide line in cisco 1841

Your setting enable the bandwidth limitation but not priority. Please refer to the last link to determine are you prefer to use CBWFQ ?

New Member

Re: QoS Implementation guide line in cisco 1841


I am ready to use if My perpose solve.

Can u ammend my config and send it to me .

Re: QoS Implementation guide line in cisco 1841

This policy sets minimum reserved bandwidth, but it doesn't specify upper limit for either of this.

If you really want to have priority (as opposed to 'dedicated bandwidth') for SMTP traffic, you'd have to either use 'priority' command or use priority queueing instead of CBWFQ. I'd strongly advise not to use priority. Because giving half of the link bandwidth will definetely starve other traffic to the point where other traffic won't get any bandwidth at all (even your link is actually not full). Luckily this won't be a problem, because you don't really need to have any priority for SMTP traffic in order to get SMTP working in timely fashion. Packets might be dropped if SMTP tries to go above 1Mbps, but TCP will make involved servers to slow down while providing guaranteed delivery.

Also, there is no good reason to limit maximum available bandwidth for HTTP - if your SMTP needs bandwidth it will get it, but putting HTTP into dedicated class with reserved bandwidth changes one interesting thing - within the class packets are handled in FIFO fashion and with only 512Kbps faster downloads may occupy whole 512K of bandwidth leaving nothing to slow downloads making them even slower. Fair-queue would be solution in this case, but unfortunately you can't specify both

'bandwidth' and 'fair-queue' at the same time. Moreover, fair-queue is only available for 'class-default'.

Your configuration is generally fine and will work for outgoing traffic. For traffic coming from the internet QoS would have to be configured on the upstream router too.

If SMTP is your primary concern, I'd suggest you removing class httppolicy and add class-default with only single command 'fair-queue'. Your users will have better experience using the network then with the config you initially posted.

New Member

Re: QoS Implementation guide line in cisco 1841


Thanks for good explaination regarding QoS.

if I use Fair Queue How it will work for my

Internet Traffic.

Will send me the best configuration .

Will help to work in better way .



Re: QoS Implementation guide line in cisco 1841

fair-queue in class-default will just ensure that traffic between different client-servers pairs gets fair share of the available link bandwidth. A bit oversimplifying it looks like this: for each client-server session create dedicated queue and place packets of this communication to this queue, scheduler will then just go round-robing through this queues picking up 1 packet a time from each queue. In contrast to this using FIFO will place all packets into single queue so if some client-server communication has a lot of traffic, this will occupy most of the FIFO leaving no space for other communications (except of course for your SMTP traffic which has dedicate queue anyway because it belongs to dedicated defined class smtppolicy).

Don't worry that Inet (more specifically web) traffic will occupy whole bandwidth - some bandwidth is reserved to SMTP, so as soon as there is SMTP traffic it will get transmitted regardless how much web traffic is out there.

But you need to remember that it's still only in outgoing direction from you towards your provider. You have no control over how traffic is delivered from your provider to you and will need to speak to them if you want they implement similar thing (most likely they will charge extra for this).

New Member

Re: QoS Implementation guide line in cisco 1841


As My current config it is showing some drops on smtp policy.

As per ur suggestion should I remove my HTTP Policy and use defualt class with fair queue .

I also I like now regrading Traffic which is commig from out side.

Around 40 users re pop users & 10 users r VPN users .

My vpn users r coonecting throug ssl .

I also want to reserve bandwitdth for them from out to in this users r using avaya softphone so along with data, voice traffic will be comming in .

Pls gudie me for best solution

Re: QoS Implementation guide line in cisco 1841

You can see reason why traffic is dropped by using 'show service-policy interface '.

For incoming traffic you cannot do anything on your side - traffic has already went through the line, it's too late to do anything about it. Only your provider can help you here.


Re: QoS Implementation guide line in cisco 1841

Thanks for Ilya to propose a simplier and better solution. I used to configure the service for CBWFQ, so I propose it. :p

The the packet drop, the bandwidth command guarantee to 1M, so if there is congestion and the SMTP above 1M, there may be drop.

I agreed to remove the http group and let it be the best-effort class.

Due to we cannot control incoming traffic, so how much and the priority of the traffic from the provider that cannot be defined. You need to ask the provide to enable QoS(if there is) at their side.

For the multimedia traffic, we normally color it and let the provider able to oberve the class and transmit it via their QoS policy. And the policy is assigned at output.

Hope this helps.

New Member

Re: QoS Implementation guide line in cisco 1841


To avoid such kind of issue wht will be the best way .

Pls help me out ....

should i increase my bw for smtp &

and remove http policy & use class default with fair queue.

By changing this will this isssue will resolve.

New Member

Re: QoS Implementation guide line in cisco 1841


I have faced problem today due to my policy.

One user has sent email of to 400 people with attachment of 4 Mb each.

Due to this My user unable to brows internet .

I have check my policy

In SMTP POLICY IT WAS SHOWING TOTAL DROP:2500 Bps . & current out put showing 121220 bps

In httpy policy it is showing

out put :3500 ,drop = 0 bps

But mY default class

out put showing 15000 bps , drop = 0 bps

To avoid this issue tell me the best way.

Should I change ....

Help me out to avoid this issue will not occure in future.




Re: QoS Implementation guide line in cisco 1841

In this case, if your user will send these large volume of mails. I suggest to limit the SMTP packet instead garantee it to 1Mbps and leave the remaining 1Mbps for http and others.

You can use "police" command or "CAR" to control the max. throutput of SMTP to 1Mbps.

Check below link for the example of police. However, it will shape the SMTP and not garantee it. Please consider the way your want to implement. Otherwise, you can garatee the http and put the SMTP as best effor. i.e. remove SMTP class and keep http only.

Re: QoS Implementation guide line in cisco 1841

you could add traffic shaping to your SMTP class, this will attempt to limit SMTP traffic to 1Mbps but not as aggressive as policing. From all your descriptions it looks like following policy could be appropriate for you:


class-map match-all SMTP

match protocol smtp

policy-map MyInet

class SMTP

bandwidth 1000

shape average 1000000


class class-default



int Serial0

service-policy output MyInet


But I'd recommend you to reconsider whether 1Mbps for SMTP is good idea. After all SMTP is not interactive traffic, so 1 minute sooner, one minute later - it won't make much difference unless your business relies on real-time emails (unusual). Make an aproximation what mail volume you have per hour and re-calculate required bandwidth.


Re: QoS Implementation guide line in cisco 1841

Agreed on Ilya. And I believe there is an alternate solution is to garantee other applications with minimum bandwidth to avoid the SMTP to occupy all bandwidth. Therefore, I suggest to use bandwidth command but only for non-SMTP application, e.g. 256k. It means if there is congestion, the max. SMTP is 1.744Mbps but other applications garanteed w/ 256k.

I also suggest to try both solutions from ilya and me to obtain the most optimal solution. BTW, agreed w/ Ilya again, you need to calculate the average usage of the SMTP traffic then put the correct value.

Hope this helps.

Re: QoS Implementation guide line in cisco 1841

Jack, the problem with approach of giving guaranteed bandwidth to other apps is that they will all end up in single fifo queue. No problem for fast interfaces, but for sub-2Mbps links fair-queue is often desirable.


Re: QoS Implementation guide line in cisco 1841

Ilya, thanks for your notification. I still think it is worth to test if the one of solution still not make a good result then there is no harm to test another one. Right ?

Re: QoS Implementation guide line in cisco 1841

Indeed :-)

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