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  1. Junior Member
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    Sep 2017
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    #1

    Default Home lab troubleshooting

    I thought I'd open a separate thread because I didn't want to derail the post your lab pics one with my own issues.

    Here's my first problem I encountered:

    I keep getting this CDP log message over and over on the crossover cable connection between two of my routers. What would cause a duplex mismatch issue like this between routers?

    *Jun 1 22:22:33.344: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on GigabitEthernet0/1 (not full duplex), with R1 Ethernet0/1 (full duplex).

    The Gigabit Ethernet side has auto duplex while the Ethernet has full duplex. Wouldn't the auto-negotiate mean that becomes full duplex too?

    I suppose I could manually set the Gi0/1 interface to full duplex but I wanted to ask about it first. Can the problem be because one is Gigabit and the other regular Ethernet? Do I have to tweak the speeds? I thought it just goes as fast as the slowest type of connection when they are not the same on both sides of the link.

    From what I've seen from a search, many others just choose to turn off the logging for this or turn off CDP (it usually pertains to connections to switches), but I'm interested in trying to fix it first or at least understanding why it may be happening.
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  3. Member awitt11's Avatar
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    #2
    Did you set the R1 Ethernet to Full or Auto? GE0/1 can't negotiate with a device that isn't negotiating. Leave them both Auto or set both Full.
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  4. Junior Member
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    Sep 2017
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    #3
    Ah that's right, thanks, so the same kind of negotiating logic applies as to trunking and Etherchannel then?

    As in having it "on" or nonegotiate on one end and negotiate the other will cause problems. Duplex settings don't have "desirable" modes, unlike trunking though.

    EDIT- I set both to full and that made the link go down (as in the interfaces were up but line protocol was down). I then tried setting them both to half duplex and they went back up, and I can ping across them now.

    Why would half work while full doesn't in this situation? Is it due to just being Ethernet and not Fast Ethernet?

    Also when doing the sh int e0/1 command, the duplex settings don't even come up. While doing sh int gi0/1 on the other side of the link does display it.
    Last edited by the_dude7; 06-04-2018 at 07:52 PM.
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