Interface Speed , transmission rate, bandwidth

[NDP]

Hi All,

Good day.

We have a LAN interface which is 100 Mbps ( FastEthernet ) connected to our Service provider. The circuit capacity is 2 Mbps. We have hard coded the speed to 10 Mbps to bring the circuit into production. 

I believe that the speed value is negotiated by both devices and transmit and receives the frames at agreed rate. 

In case if we are planning for circuit upgrade to 20 Mbps , should we change the speed value to 100 Mbps.

if the speed remains at 10 Mbps and circuit upgrade is performed to 20 Mbps, the circuit works at 10 Mbps? 

please help me to understand. Thanks 

[Mark Malone]

Hey Yes it will still work at 20mbps even though bandwidth is set to 10mb , the bandwidth statement is really irrespective of the actual physical bandwidth and it does not limit or physically adjust bandwidth in reality

The reason you would use  the command bandwidth 20000 is so your IGPs such as EIGRP and OSPF etc can calculate the correct metrics and best paths for your traffic .Always recommended to use the bandwidth statement's or you could also end up getting false outputs and you could have issues with tcp too

[NDP]

Hi Mark,

Thanks for the reply.

Cisco documents say that Speed command under the interface command defines the rate at which interface transmit and receive the frames .

so.. have a query , if the traffic which is more than 10 Mbps  is still permitted when the interface  remains at 10 Mbps .

Thanks 

[Mark Malone]

Am I right that your saying if you were to send 10mb or more of traffic will it go through ?

If that's what you mean then if the interface is currently maxed at 10mb,  traffic will be held in the interface buffer/output queue until it can be sent , if the output queue fills up then you will start to see drops, depending on what type of traffic it is it can retransmit but if its real time like rtp that's not good , that's why most networks would use some form of qos to prioritize the traffic

[Joseph W. Doherty · Accepted solution]

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Posting

Service provider links might have a physical bandwidth capacity higher than a logical bandwidth restriction, that they enforce.  So, from what you've described, the provider's link is physically capable of 100 Mbps, but if during some time interval, your usage, if it exceeds 2 Mbps, will likely be discarded.

If the physical link supports Ethernet, as you describe your does, as long as both sides allow multiple speeds, you can physically step down your speed.  In your case from 100 Mbps to 10 Mbps.  As this is still faster than your logical cap of 2 Mbps, there's no real advantage in doing this.

If you get your logical cap increased to 20 Mbps, you would be unable to take advantage of the additional bandwidth you're paying for unless you changed the physical speed to allow 100 Mbps.

What I would recommend, is have your logical bandwidth cap match your physical link capacity or shape for your bandwidth cap.  For example, run your link physically at 10 Mbps with a 10 Mbps cap, or run your link physically at 100 Mbps with a 20 Mbps cap, but shape for 20 Mbps.

[NDP]

Hi Joseph,

Nice explanation. clear information. I will have the speed changed 100 Mbps to upgrade the circuit. to 20 Mb. 

Thank you So much. 

Regards