Incomplete ARP's on a Class C Subnet

Posted on 2008-11-05
Last Modified: 2012-06-27
From this morning we've had around 6 users complain that they are not getting a network connection. On further investigation the device is getting an IP address and can be pinged locally but is unable to get off the site.

On checking the ARP cache on the 3750 the devices not getting off the network all have Incomplete next to there addresses, I've flushed the cache a number of times and it still comes back incomplete.

The devices plug into a 2950 which is connected to a 3750 that does all the routing off the LAN and its from the 3750 that we cannot see the devices in question.

Any help would be much appreciated.

Question by:jonathanchill

    Expert Comment

    if you can ping the local address that mean the interface it's good, but i suggest you ping the ethernet router, I maean the default gateway, if that work fine then the problem it's on the routing protocol..
    try if you ping another computer from the same site the the problem in on the router..
    but if you still wan to check the swicth , you can do that thru a console port or thru vlan. but to make it thru vlan you have yo had set the vlan1 password (that´s for administrative propose), and then check the port. to make sure it's no closed or blocked.
    hope it can help
    LVL 79

    Assisted Solution

    Can you ping the devices from the 3750?
    Is the DHCP server also connected to the 2950 switch with the clients?
    Sounds like a problem with the link between the two switches. Cycle power on both and see if they come up better, or check the uplink ports for error counters.


    Accepted Solution

    Are they on the same "block" on the switch?  The 2950 is modular, and while extremely rare, the individual modules can fail (very very rare).  I agree that a power cycle might fix it right up.  If not, look for layer 2 issues like collisions, etc.  Also make sure that the devices, if they are computers (not other things like routers, switches, etc) are set to auto-detect link speed on both the switch and the device.  If they are, try hard setting the link duplex and speed on both the device and the switch and see if that helps.  This could be an auto-negotiation issue.  

    Hope this helps.

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