I fixed the same problem a week ago.
The message is caused by packets which are OK on layer 2 but their IP header checksum is bad.
If you try sh count sup command you can see increasing counter of "IP pkts with cksum error". Every 30 minutes the message you mentioned is saved into the syslog.
The solution is to find a PC in your network which is sending these corrupted packets. This is a tought problem in a large LAN. The best way is to check is anybody is having connectivity problem. If not, use protocol analyzer and span ports to it. Be careful, the source of corrupted packets might not be connected directly to your Cat6509, the corrupted packet might come from trunk.
This meesage is a pretty new feature of CatOS ver. 7, these packets have been dropped with no notice in previous CatOS versions. The only Cat6000 platform is detecting them, so even if the souce of corrupted packets is connected to other switch (Cat3500, 4000) you will not see the error message on it.
In my case the cause of corrupted packets was PGP program v. 7.0.3 configured to "secure" network adapter which was beaking multicasts (not unicasts, so the PC had no problem with network connectivity).
HTH,
Milan
