New device with old IP doesnt ping

Unanswered Question
Apr 29th, 2009

Im having an issue that is causing my head to spin.

Switches: 6500 & 3750 series

Problem: We are currently in the process of replacing old printers with new ones. We are utilizing the same static IP's for the new devices as the old ones were using. When the old device is removed and the new device plugged in, the address does not ping, however the MAC table on the switch updates showing the new mac address. If the device is then changed to a different static IP it works immediatly, but when switched back to the original IP it stops working.

Any thoughts?

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Overall Rating: 4 (2 ratings)
Giuseppe Larosa Wed, 04/29/2009 - 04:19

Hello Derek,

you need to clear the arp entry on the L3 device or it will try to answer to old device MAC address

clear arp ?

A.B.C.D IP address

interface Clear the entire ARP cache on the interface

vrf Clear entries for a VPN Routing/Forwarding instance

Hope to help


stratmann83 Wed, 04/29/2009 - 06:35

I have the same Problem with 2950 and 2924 Switches. A Solution would be very nice. I don`t have any Idea. The port status goes up when i switch on the printer, but ping doesn`t work. Any other static IP works.

Richard Burts Wed, 04/29/2009 - 07:35


It was clear in the description of the issue given by Derek that the issue was that the ARP table had the old MAC address (and would continue to have the old MAC address until it timed out). So the suggestion from Giuseppe was correct that clearing the ARP table was the solution. It is a little less clear in your description whether you have the same issue as Derek or not.

Perhaps it would help everyone understand this issue a little better if we clarify the difference between the switching table (frequently referred to as the MAC address table and cometimes referred to as the CAM) and the ARP table. The switching table is maintained by the switch and is used to make forwarding decisions at layer 2. The switching table updates quickly to indicate the new MAC address used by the new equipment connected to the switch port. The ARP table is maintained by the layer 3 device that is doing routing for the subnet and is used to relate an IP address to the corresponding MAC address for IP forwarding. Once an entry is learned that relates an IP address to a MAC address and put into the ARP table, then this entry stays in the ARP table for 4 hours. This produces the symptom that the new device has the same IP, has a different MAC, the new MAC shows up in the switch table, but the new device does not respond to ping - because the ARP table still has the old MAC. So clearing the ARP table is the effective way to get the new MAC into the ARP table as well as into the switching table.



derek.leonard Thu, 04/30/2009 - 06:35

Thanks for the quick replies! I cleared out the arp cache for that port on the local switch and removed the mac entries from the core switches. That seemed to have done the trick as the printer then started working.

I am wondering if the issue is not with these devices themselves. We replace plenty of other Lexmark/HP printers using the same IP's and have never had this issue. It has only been with this new brand that we just got in.

Giuseppe Larosa Thu, 04/30/2009 - 06:50

Hello Derek,

the only thing the new device can do is to send a gratuitos ARP telling "this ip address is reachable via this MAC address".

if the device does this and the router is willing to accept it the manual clear of the ARP entry is not needed as the entry is updated by this message.

note: the switch can filter gratuitous ARP if Dynamic ARP inspection is configured.

if the new device doesn't send the unsolicited ARP only the clear arp works.

Hope to help



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