Cisco 851 creates TCP packets with incorrect checksum

anyarcadmin · ‎10-10-2007

The cisco router we are using sends TCP syn packets with incorrect checksum. The error occurs when the packet is both SNATed and DNATed on the router. Has someone seen this before? Is there an option to force checksum recalculation?

The problem is that the next server refuses to accept packets with checksum errors.

wdrootz · ‎10-16-2007

During a TCP flow connection, Local Director model may identify a bad TCP checksum and drop the received packets. This incorrect identification of a bad TCP checksum typically occurs in any LocalDirector with Rev 8 Network Interface Card . The packets, however, do not have a bad TCP checksum; the checksum is computed incorrectly. To check the revision of the installed Network Interface Card in the LocalDirector, use the show interface CLI command.

anyarcadmin · ‎10-16-2007

I used the command but I failed to find any version info, I got only the interface overview/statistics (see attachment)

Apart from that, the device does not receive packets with invalid checksum, it sends them when a valid packet was NATed on the device.