https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=3112
--- Comment #26 from Sake <sake@xxxxxxxxxx> 2009-01-05 14:32:10 PDT ---
(In reply to comment #22)
> (In reply to comment #21)
> I'm building a
> program right now that uses winpcap to send out some CDP packets so that I can
> manipulate the checksum before sending them out in an effort to see what is and
> isn't accepted.
An easy way of doing this is using a hex editor (I use hexcurse) to change the
checksum values and then replay the packets using bittwist (or tcpreplay).
> I get the distinctly funny feeling that switches prime the
> checksum with -1 (0xFFFF) and the routers use 0, and that both of these
> calculations are considered valid by both devices.
Nope, I patched my wireshark and edited the tracefile so that the values that
Wireshark calculates are now used as checksum, but when I fire them towards my
2950, it does not accept them (and the Chksum error counter is indeed
incremented by 2).
So it seems there is only one correct checksum for a certain CDP packet. But
somehow the method of calculating the CDP checksum is sending-device dependent
:(
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