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Would Half-Duplex factor into EIGRP route selection?

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  • Would Half-Duplex factor into EIGRP route selection?

    I had an issue where EIGRP was choosing to route all traffic over a T1 instead of a 10 Mbit circuit. When I changed the router's ethernet interface connected to the 10 Mbit circuit from Half Duplex to Full Duplex the routes changed to use the 10 Mbit circuit. I looked through the EIGRP metrics and I don't see where that would factor into the calculation of choosing a route. The T1 route and 10 Mbit route are both going to the same router, same number of hops, same delay, only the T1 is much slower than the 10 Mbit circuit and yet EIGRP on the router chose the T1. Would the interface being set to Half Duplex cause EIGRP to choose a T1 over a 10 Mbit circuit? I tested this on equipment at home and EIGRP always chooses the route with the fastest configured bandwidth even if the interface is set to Half Duplex. Would Half Duplex cause this in some situations? Does anyone have any clues as to why the above might occur?

    Thanks in advance.

  • #2
    Re: Would Half-Duplex factor into EIGRP route selection?

    Duplex settings do not effect the DUAL algorithm so the Passive route should have been via the 10Mb interface and not the T1 interface. EIGRP uses bandwidth/delay to calculate the cost with additional K values of load, MTU and reliability which are rarely ever used.

    The only reason I could think this would happen is that the 10Mb interface was not "up" because of a duplex mismatch error hence the only operating interface was the T1. When you changed to FD the 10Mb interface came "up" and DUAL recalulated the cost selecting this as the best route.

    Did you configure the "bandwidth" on the interfaces?
    Last edited by BigDeesDad; 9th November 2009, 10:35.
    Infrastructure Architect
    CCNA, CCNA Security, MCSE, JNCIS


    • #3
      Re: Would Half-Duplex factor into EIGRP route selection?

      Thanks. Interesting. The 10Mbit interface was up/up but EIGRP chose the T1 route so the 10Mbit interface wasn't being used. The T1 interface bandwidth was left at default speed and the 10Mbit interface was configured to 10Mbit.

      The routes are working like we want now so I haven't changed anything back to test any further theories but along the lines of your input I wonder if duplex mismatch errors though they didn't cause the interface to go down caused the interface to perform so poorly when the circuit was first installed and the interface first brought up that the original T1 remained the EIGRP chosen route.