Bandwidth Command

Answered Question
Aug 27th, 2009

Hi,

Applying bandwidth command on interface, will this restrict traffic according to the bandwidth defined.

int fa 0/0

bandwidth 2000

will send and receive be restricted to 2MB as above config.

I have this problem too.
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Correct Answer by yagnesh_tel about 7 years 4 months ago

No, it won't restrict your interface speed to 2MB. But configured bandwidth under interface will be used by routing protocols to calculate their metrics. TCP will also adjust its initial retransmission parameters based on the bandwidth configured on the interface. Also if QoS is used,it will use this as reference bandwidth.

So it is still important to configure interface with right bandwidth statement.

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Correct Answer
yagnesh_tel Thu, 08/27/2009 - 18:08

No, it won't restrict your interface speed to 2MB. But configured bandwidth under interface will be used by routing protocols to calculate their metrics. TCP will also adjust its initial retransmission parameters based on the bandwidth configured on the interface. Also if QoS is used,it will use this as reference bandwidth.

So it is still important to configure interface with right bandwidth statement.

Joseph W. Doherty Fri, 08/28/2009 - 04:26

"TCP will also adjust its initial retransmission parameters based on the bandwidth configured on the interface."

I was unaware of this.

This is for TCP traffic sourced from the network device?

Can you provide any reference?

vinod rathore Wed, 03/27/2013 - 21:32

Joseph,

seems here Yognesh wants to say - this bandwidth command adjest the TCP window size in random manner.

Yognesh - is it right ????

good explanation of bandwidth command !!!!!!

Lucien Avramov Thu, 08/27/2009 - 19:29

You can limit BW with Qos, for example for VoIP traffic :

int f0/0

service-policy output ect_policy

policy-map ect_policy

class ect_class_voice

bandwidth 64

class-map match-all ect_class_voice

match protocol h323

Jerry Ye Thu, 08/27/2009 - 20:17

Hi Lucien,

From you message "You can limit BW with Qos".

Maybe you want to say something else, but bandwidth in the policy-map will not limit the BW. It will guarantee BW during congestion.

Regards,

jerry

Lucien Avramov Thu, 08/27/2009 - 23:33

I want to give an example on how you can have 2 megs of bw guaranteed for specific traffic.

There is no point of making a pipe smaller than what it is, so QoS is a way to segment it into pieces as you know and guarantee traffic.

Joseph W. Doherty Fri, 08/28/2009 - 04:37

"There is no point of making a pipe smaller than what it is . . ."

To clarify for others, that although this is normally true, there are exceptions.

For instance, you might have a physical interface much faster than the end-to-end path supports. In such cases, you might want to slow a faster interface to match available bandwidth further along the path. (E.g. Ethernet hand-off to some WAN technology.)

Another instance, for some policy (or other special) reason, you might want to provide all or some traffic less bandwidth than what's actually available. (E.g. using VPN, you don't want VPN traffic sent to a remote site to fully utilize all remote's Internet bandwidth; leaving some bandwidth for remote's native Internet traffic.)

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