Hello Artem
First of all, in this lesson, we are talking ONLY about the traffic that EIGRP generates in order to maintain routing and topology tables on the routers participating in EIGRP. That is, hello, update, query request and reply packets. We are not talking about the traffic that is generated by users on the network and their applications.
As stated in the lesson, EIGRP will use up to 50% of the bandwidth of a link. The bandwidth that EIGRP always uses to gauge “how much is 50%” is the bandwidth parameter of the physical interface, NOT the actual physical throughput available. In a point to point situation, if the interface is configured with a bandwidth parameter of 1.544Mbps for example, then up to half of that (772Kbps) will be available to be used by EIGRP at any time. In a multipoint topology however, the question is how do we calculate 50%? 50% of what?
When stating that “EIGRP will divide the bandwidth of the physical interface by the number of EIGRP neighbours” we are essentially saying that whatever is configured as the bandwidth parameter on the physical interface, we take that and divide it by the number of spokes in the topology. So if we have 1544 Kbps as you stated, the resulting number is 386Kbps. This means that EIGRP will use up to 386Kbps/2 = 193Kbps for each spoke for EIGRP traffic.
Now, what Rene is saying in the lesson is that this is often still too large for EIGRP traffic going from spoke to spoke, especially if you have varying CIRs on each spoke. So in order to make sure that the EIGRP traffic will not overwhelm your links, it is a good idea to limit all EIGRP traffic to half of the smallest CIR available on your spokes. This way, no link will be overwhelmed.
In order to do this, take the smallest CIR and multiply by the number of spokes. That is, 64Kbps*4 = 256Kbps. Configure this as the bandwidth parameter of your physical interface on your hub. The result will be that EIGRP will not use more than half of each spoke’s available bandwidth.
How much is that? Well we start off with the physical bandwidth configured on the hub which is 256Kbps. Divide that by the number of spokes (4) and we get 64Kbps. EIGRP will not use more than 50% of this value, that is 32Kbps, so none of the four spokes nor the hub link will be overwhelmed with this setup.
I hope this has been helpful!
Laz
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