Thursday, March 16, 2006

 

Bandwidth percent command for EIGRP. (Question #40)

Consider the following output when the help string for the EIGRP bandwidth-percent command is prompted with "?"

R1(config-if)#ip bandwidth-percent eigrp 22 ?
<1-999999> Maximum bandwidth percentage that EIGRP may use

It looks like one can configure percent values greater than 100. Is this a bug? Why would configuring bandwidth percent greater than 100 ever make sense?

Comments:
You can configure EIGRP to use more than 100% of "available" bandwidth, because you can lie to it about how much bandwidth you have. You could, for example, tell EIGRP that your T1 is a 128Kbps connection and that you want routing updates to consume no more than 400% of bandwitdh. In this case, EIGRP will allow itself to send up to 512Kbps of traffic to the neighbor on that interface.

The only time I can think of this as being actually useful is in a point to multipoint FR situation, where you have a variety of CIRs and have to set the interface bandwidth based on the lowest one. Then you might want to bump up EIGRP's limit to make sure it converges quickly.
 
That's right geko29. I too had the FR situation that you describe in mind.
 
I've had to do this on many occasions. Not only in FR situations as mentioned here, but at times when the bandwidth statement may need to be "tweaked" to force routing down a particular link (since the bandwidth statement is mostly just used for the routing algorithm).

Obviously there are other methods to "force" routing, but I find tweaking the bandwidth statement to be my personal preference.

When the bandwidth statement is modified to reflect something that is not "real", then the percent needs to be adjusted accordingly.
 
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