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TOPIC: some ip address are giving reply when we ping but we are not

Re: some ip address are giving reply when we ping but we are not 8 years 2 months ago #37167

  • Nevins
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If you are using static addresses update your network map, make sure that every pc has the IP that it is supposed to and make sure the addresses correspond to the proper vlan/subnet.


You are going to have issues if two nodes are assigned the same address.


Check to make sure that static devices are not in the same range as dhcp devices.


Bottom line however you can troubleshoot this problem by removing the offending devices and isolating them from the network.


1.) list IP's that are good/bad
2.) unplug devices that have bad ip's
3.) ping for bad ip's to check for duplications (while unpluged)
4.) check vlan's (make sure that the interfaces are in the right vlan)


Some possible causes:

IP duplication (possible rogue device)
Localized cable/port issue (check this by giving the ip out in another area)
DHCP reservation
In wrong subnet/vlan


At this point the big question is "Why can't I use the bad IP addresses?"

To me the key step in finding that answer is localizing the problem to the offending device or link. Turning things on or off and recording data will provide your answer.

Let me know how it goes.
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Re: some ip address are giving reply when we ping but we are not 8 years 2 months ago #37223

  • S0lo
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Try this, ping one of those (replying none existing IPs) from your PC. Then, immediately type the command arp -a to show the ARP table. You should find the MAC address of that IP listed. Copy that. Now, login to your switch CLI then type show mac dynamic. This should show you the switch's CAM table, you should find the same MAC address listed here, associated with one of the ports. Now you know the physical port connected to that replying IP. Obviously, trace that cable and see where it goes.

You have to do each step quickly, one after the other, otherwise, those ARP and CAM entries will expire and disappear from the tables.
Studying CCNP...

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