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Today at one of my customersĀ I ran into a strange issue after they changed their network subnet from 24 to 23 bits.

When troubleshooting this issue we found out that mostly the first ping from a xp/2003 system to the 2008 systems got a reply and afterwards some did some didn't. Especially hosts in the extended part of the subnet were unable to reply.

looking at the arp table on both sides showed us something strange on the 2008 systems.

the 2008 servers showed us a arp table like below: 00-21-55-f8-db-e4 dynamic 00-21-55-f8-db-e4 dynamic 00-19-bb-d6-b1-18 dynamic 64-31-50-1d-2a-a5 dynamic 00-21-55-f8-db-e4 dynamic 00-21-55-f8-db-e4 dynamic 5c-26-0a-02-42-81 dynamic 00-21-55-f8-db-e4 dynamic 00-1f-29-83-51-2d dynamic

When searching for the manufacturer of this mac address we found cisco as the source.

So to pin down the issue, the customer luckily had only 1 cisco device, we checked the device and found out it was the source of that mac address.

specific to this device was that it was not managed by the customer, but by a telecom provider. so this device wasn't updated with the new subnetmask.

so it looks like the arp request from 2008 is formulated differently than the xp/2003 arp request. which causes the Cisco device to reply with it's own mac as it is a router device that only knows about the first part of the subnet and not about the new one.

the solution was to disconnect the cisco until the provider has changed the subnet mask.

to make everything work quickly we cleared the arp table at all servers using arp -d.

HopeĀ I can help anyone running into this issue with this post.

Posted on Friday, December 16, 2011 12:30 PM | Back to top

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