Network Collisions

Posted on 2007-10-11
Last Modified: 2008-01-09
I am seeing multiple collisions on both ethernet interfaces in a router, I cleared the counters and kept monitoring. The collisions keep showing at a fast rate, I don't see any other errors on the interfaces other that the collisions. Is there a way to find out the device causing this collisions from the router or what other solution would you recommend on finding the device creating the problem? Both interfaces are connected to Cisco switches. If not from the router, can I trace it from the switch?

Question by:rh102801
    LVL 9

    Assisted Solution

    You aren't using a HUB anywhere are you? If so, replace it.

    on the switches,

    run a show interfaces from the Switch CLI and see if any of the Up/ups are down.

    My guess is that you have the duplex settings setup wrong. Make sure they are all set to Full.
    LVL 6

    Assisted Solution

    I bet my shirt that you have duplex mismatch(es).
    Check both sides (router ports and switch ports). speed and duplex must match.
    If they do not and they are in auto mode, you may have to set them.

    I hope this helps
    LVL 50

    Accepted Solution

    Collisions will ALWAYS happen whenever an interface is half duplex.

    If you don't have any hubs (or coax) then make the interfaces full duplex.
    LVL 79

    Assisted Solution

    If the router is connecting to a switch, then you should not see collisions. If the router is connecting to a hub, then collisions are part of Ethernet. Agree with above that you need to check the duplex setting on the switch and router interfaces. If you set one interface to say, 100/full-duplex and the other at auto, then the result is guaranteed duplex mismatch and poor performance.
    Router port and switch port must match exactly. Both auto, both 100/full, both 10/full, etc.

    Author Comment

    I changed the dupex on the switch to full speed 10. This is a 10/100 switch but the router is only 10 mb. Collisions on the switch have dissapeared on the port connected to the router but the router is still showing collisions. I tried changed the router to full duplex as well but I can't get it to take the command duplex full. This is what I get:
    router(config-if)#duplex full
    % Invalid input detected at '^' marker.
    This is a cisco 2600 with the following IOS:
    IOS (tm) C2600 Software (C2600-JS-M), Version 12.0(9), RELEASE SOFTWARE (fc1)g
    I have not been able to figure out a different command to change it. Any ideas?
    LVL 50

    Expert Comment

    by:Don Johnston
    Some of the old 10mbps don't support full duplex. In which case, you should set the switch side to half duplex and just live with the collisions.

    Author Comment

    ok, if I leave the switch on auto, then there is collisions on both devices, if I change it to full but leave the router alone, then there is only collisions on the router... Is this a problem since the switch wants to talk full duplex but the router doesn't?
    LVL 50

    Expert Comment

    by:Don Johnston
    Yes. Whatever one side is, the other side must be the same. Since the router can't do full duplex, you'll have to use half duplex on the router and the switch.

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