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Forum Replies

  1. Hi Rene,

    in multipoint:

    Hub(config)#interface serial 0/0
    Hub(config-if)#bandwidth 256

    Do we also have to set the 4 spokes with a CIR of 64 kbps each?

    Does it mean that PVC 3: CIR 256 kbps will become 64 kbps and will only be using a maximum of 32 kbps for EIGRP traffic?

    Thanks

  2. Hi James,

    First of all, keep in mind that the bandwidth command doesn’t affect the actual bandwidth of the interface. It is only used as the input for things like QoS or routing protocols like EIGRP to calculate the metric. We also use it in this example to figure out how much bandwidth EIGRP can use. In a scenario like this, you want to make sure that EIGRP doesn’t use more than X% of the bandwidth that you have.

    With 6 spoke routers, you would have 256 kbps / 6 = 42 kbps per PVC. The actual PVC offers 64 kbps.

    If we want to configure EIGRP so that it can still use 50% of the bandwidth, then we can do it like this:

    32 kbps / 42 kbps * 100 = 76%

    So when we configure it as 76%, EIGRP will be able to use up to 32 kbps.

    Hub(config)#interface serial 0/0
    Hub(config-if)#bandwidth 256
    
    Hub(config)#interface serial 0/0.1 point-to-point  
    Hub(config-subif)#ip bandwidth-percent eigrp 1 76
    
    Hub(config)#interface serial 0/0.2 point-to-point  
    Hub(config-subif)#ip bandwidth-percent eigrp 1 76
    
    Hub(config)#interface serial 0/0.3 point-to-point  
    Hub(config-subif)#ip bandwidth-percent eigrp 1 76
    

    Does this help?

    Rene

  3. Hi Rene,
    I am confused with multipoint scenario.

    1. “When you run EIGRP over a frame-relay multipoint network then EIGRP will divide the bandwidth of the physical interface by the number of EIGRP neighbors.” Let’s assume that we have hub router with physical bandwith of 1544kbps and 4 neighbors, so 1544/4 =386. Is this correct that maximum CIR on our PVCs can’t be greater then this value?

    2. In your scenario you configured only a hub router with bandwidth 256 subint command. That’s how I see it: Spoke 3 with CIR 256kbps on PVC3 still will be sending EIGRP traffic up to 128kbps(256/2), but on the hub router there will be a queue with eigrp traffic, since it can only send 256/4 * 50%=32kbps. Am I correct?

  4. 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

    ,

  5. Thanks a lot, Laz! Now it’s clear to me.

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