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What is relation between speed and bandwidth when it is under interface mode?

Posted on 2016-09-22
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Hi In the interface mode, we have two choices to configure, speed and bandwidth. What is relation between the speed and bandwidth? Thank you
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Question by:eemoon
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by:John Hurst
ID: 41811258
They are sometimes used interchangeably.

Speed is normally the native speed of the device or circuit. E.g. a 1 Gbit/second network card.

Bandwidth is normally the speed of supply. Your internet may be supplying 20 Bits/second even though your router is capable of more.

Configure bandwidth in your example above for the highest available unless you are trying to constrain it in some way.
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by:Jan Springer
ID: 41811270
In Cisco kit, "bandwidth" is an identifier to higher level protocols.  It can be used to de-prefer or prefer one interface over another.  It has no real impact on throughput.

Speed identifies the negotiation of the ethernet interface.

If you ever desire to restrict bandwidth on an interface, you want to use QoS: class-maps, policy-maps, access-lists, etc and apply the policy to the interface.
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Author Comment

by:eemoon
ID: 41811355
Thank you so much for your reply.

Speed identifies the negotiation of the ethernet interface

Can you explain it?

If we use "speed 100" under interface, or "speed 1000", what is difference between the two?
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by:Jan Springer
ID: 41811363
100M vs 1000M (or 100M and 1G).

If you use "speed 100" on a gigabit interface and both end negotiate properly, you have throttled the max throughput to 100M.
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by:John Hurst
ID: 41811447
That is what I was suggesting when I said you could set speed/bandwidth to constrain overall bandwidth used.
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by:Jan Springer
ID: 41811470
Not with the "bandwidth" command.  It's used for route preference.  

Only "speed" (which is limited) or QoS can change input and output throughput.
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by:John Hurst
ID: 41811473
That is for Cisco I assume
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by:Jan Springer
ID: 41811475
Yes.  That's why I mentioned it "with Cisco kit ... ".

But, Brocade doesn't have the "bandwidth" command and I believe Juniper does not either.  

I can't recall Adtran.
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by:Craig Beck
ID: 41812555
Bandwidth is better considered as capacity instead of speed.

Imagine a road.  With 1 lane your car can travel at a maximum speed of 100Mph.  If you add another lane you don't make the car go faster, but instead you add capacity.

In a circuit, if you have a 200Mbps connection you'd usually have a 1Gbps bearer, so the speed is 1Gbps but the bandwidth is 200Mbps.
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by:Jan Springer
ID: 41812584
No, Craig.

The author didn't ask for a definition of "bandwidth".  He asked how the "bandwidth" command is used within an interface configuration.
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by:eemoon
ID: 41812717
"bandwidth" is capacity concept, while "speed" is close to real traffic concept, but we can setup the "speed" to limit the traffic flow. and in the same physical interface, "bandwidth" is greater than "speed" am i right?
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by:Jan Springer
ID: 41812737
There is no relationship.  You cannot compare the two commands.

"bandwidth" is not relative to speed or capacity but is used to influence your interior routing.  

It has no impact on the throughput of the interface.
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Author Comment

by:eemoon
ID: 41812781
"bandwidth" is not relative to speed or capacity but is used to influence your interior routing.  

What does "bandwidth" impact on? Thank you
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Accepted Solution

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Jan Springer earned 500 total points
ID: 41812796
Interior routing protocols -- such as OSPF -- will use the bandwidth configured on an interface to determine cost.  

Cost is a routing metric that determines which path is more preferred over another.

When you configure it, you *do not change the maximum throughput based upon the negotiated speed/duplex on the interface*

So:

interface Gi1/0/1
  bandwidth 100
  speed 1000
  duplex full

You are telling OSPF to not prefer this interface where another interface has "bandwidth 1000" (or whatever the actual syntax is).  This does not prevent the downstream device from consuming 1G.  It can.

If your other interface is 100M, then, everything else being equal, both paths will be equally preferred.

Some tools will also use that in configuring thresholds (i.e., MRTG).
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by:Craig Beck
ID: 41812925
Jan is spot on.  Just to add, the bandwidth command doesn't affect the interface at all; only protocols that use the interface.  The speed command directly affects the interface, not protocols that use the interface.
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Author Comment

by:eemoon
ID: 41813275
Thank you Jan,  and Craig and john. Jan is right and gave very clear explanation.
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