What could this icmp mean?

["Tomas" <s.tomas () gmail com>]

Hello list,

We have networks (10.30.0.0/24 and 10.30.1.0/24) connected trough VPN and
one internet line. The gateways for VPN are 10.30.0.1 from one side and
10.30.1.1 from the other, and 10.30.1.254 for internet (for both networks).

I've launched tcpdump today on my internet firewall's internal interface
(10.30.1.254) and I found this:

10.30.1.254 > 10.30.1.16: icmp: redirect 10.30.0.4 to host 10.30.1.1 for
10.30.1.16.445 > 10.30.0.4.1959: [|tcp] (DF) (ttl 127, id  7691, bad cksum
c76d! differs by 100) (ttl 255, id 23807)

I'm a bit confused, what could this icmp mean? First of all, I'm sure that
neither of these hosts (10.30.1.254, 10.30.1.16, 10.30.0.4) are sending any
icmp requests (I'm not sure about 10.30.1.1; it's not in my control). And
the second of all, why the checksum is bad?