router interface bandwidth assignment

Answered Question
Oct 21st, 2013
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Dear, If I have 10Mbps internet bandwidth for company used and I would like to assign bandwidth to both fastethernet and Tunnel, which of the below setup is correct to assign bandwidth


(If I have 10M, does it limited and not excess 10M like below?  7M + 3M = 10M)
interface FastEthernet0/1
description - Internet connection
bandwidth 7000

interface Tunnel0
description - tunnel to headquarter
bandwidth 3000


or


(If I have 10M, Can I assign bandwidth to interface like below?  10M for Fastethernet0/1 and 3M for tunnel0?)
interface FastEthernet0/1
description - Internet connection
bandwidth 10000

interface Tunnel0
description - tunnel to headquarter
bandwidth 3000

Correct Answer by Richard Burts about 3 years 10 months ago

1) When you configure a value for bandwidth for FastEthernet that value will be used by routing protocols such as OSPF and EIGRP as part of their calculation of the metric to use for routes. That value will also be used in the calculation of how busy the interface is which may be used by various network monitoring software applications and may also be used in the operation of QOS.

2) If the tunel is configured with a bandwidth of 1544 then this is the value that EIGRP will use to calculate the metric and not the 3000.

3) show interface reports the configured bandwidth and not the physical capacity of the interface. That is the way that the command was programmed to work.


HTH


Rick

Correct Answer by Richard Burts about 3 years 10 months ago

In addition to the points already made that the interface bandwidth is a software implementation and does not actually control the physical bandwidth, I would like to make the point that your first alternative is certainly not the right approach. You would not add together the FastEthernet and Tunnel interfaces to get the bandwidth. The FastEthernet is a physical interface and you need to configure a value that represents the total of traffic going through the interface. The tunnel is a vurtual interface and probably runs through the FastEthernet and uses part of the bandwidth of the physical interface.


HTH


Rick

Correct Answer by Sandeep Choudhary about 3 years 10 months ago

adding to John Post:


The bandwidth command is only used to communicate the current bandwidth value to higher level protocols.  Therefore, the configured bandwidth value should accurately reflect the actual bandwidth of the link.  For example, routing protocols (such as EIGRP) use this value to compute the route metric.  If the value is incorrectly configured, it will badly affect the metric calculation process and can even cause suboptimal routing.


It is also used for Qos but it doesn't represent  the througput of the interface.


calculate the composite metric as


metric=[k1*bandwidth+(k2*bandwidth)/(256-load)+k3*delay]


Default K values are k1=1,k2=0,k3=1,k4=0,k5=0


Hope it helps.

Regards

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JohnTylerPearce Mon, 10/21/2013 - 03:56
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Remember, the 'bandwidth' command, is just a soft value, that QoS, Routing Protocols, and among other things, use to calculate cost, and QoS.


If you go to fa0/1, which is 100Mbps, if you put 10000, you'r not configuring it for 10Mbps, you're just configuring a soft value.

alan-wong Mon, 10/21/2013 - 04:04
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so, may i know, how bandwidth affecting QoS, Routing Protocols and cost calculation?

Correct Answer
Sandeep Choudhary Mon, 10/21/2013 - 04:45
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adding to John Post:


The bandwidth command is only used to communicate the current bandwidth value to higher level protocols.  Therefore, the configured bandwidth value should accurately reflect the actual bandwidth of the link.  For example, routing protocols (such as EIGRP) use this value to compute the route metric.  If the value is incorrectly configured, it will badly affect the metric calculation process and can even cause suboptimal routing.


It is also used for Qos but it doesn't represent  the througput of the interface.


calculate the composite metric as


metric=[k1*bandwidth+(k2*bandwidth)/(256-load)+k3*delay]


Default K values are k1=1,k2=0,k3=1,k4=0,k5=0


Hope it helps.

Regards

Correct Answer
Richard Burts Mon, 10/21/2013 - 06:02
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In addition to the points already made that the interface bandwidth is a software implementation and does not actually control the physical bandwidth, I would like to make the point that your first alternative is certainly not the right approach. You would not add together the FastEthernet and Tunnel interfaces to get the bandwidth. The FastEthernet is a physical interface and you need to configure a value that represents the total of traffic going through the interface. The tunnel is a vurtual interface and probably runs through the FastEthernet and uses part of the bandwidth of the physical interface.


HTH


Rick

alan-wong Mon, 10/21/2013 - 06:28
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Thank you so so so so much all.  You are correct.  My tunnel is using for EIGRP. 


Actually, below is my company setup.  We have 10M Internet line in office.  I am quite confuse.


Interface FastEthernet0/1

description - Internet connection

bandwidth 3000


interface Tunnel0

description - tunnel to headquarter

bandwidth 1544


1) If Fastethernet0/1 bandwidth 3000 does not affect the actual physical connection.  Why we need to set "bandwidth 3000" in Fastethernet0/1?

2) the tunnel0 is going through FastEthernet0/1, so for EIGRP.  it will using bandwidht 1544 or 3000 to calculate the EIGRP metric?

3) If bandwidth does not affect actual physical of FasEthernet 0/1 port.  Why when I "sh int" I saw "MTU 1500 bytes, BW 3000 Kbit, DLY 100 usec,"

Correct Answer
Richard Burts Mon, 10/21/2013 - 07:15
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1) When you configure a value for bandwidth for FastEthernet that value will be used by routing protocols such as OSPF and EIGRP as part of their calculation of the metric to use for routes. That value will also be used in the calculation of how busy the interface is which may be used by various network monitoring software applications and may also be used in the operation of QOS.

2) If the tunel is configured with a bandwidth of 1544 then this is the value that EIGRP will use to calculate the metric and not the 3000.

3) show interface reports the configured bandwidth and not the physical capacity of the interface. That is the way that the command was programmed to work.


HTH


Rick

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