ICMP redirects aren't covered in depth in the CCNA track, yet they seem to be common place on more advanced networks.

It would be nice to see a real world explaination of when ICMP redirects are used, when they shouldn't be used and what impact they have on network performance and security.

Should an ICMP redirect be a one time message that stays in the host routing table or is it okay to see them continually? Does it make more sense to change the default gateway of a host rather than to have ICMP messages flying around constantly?

How easy is it for an attacker to leverage redirects to become a MITM?

Obviously the answers to these questions will vary based on the topology but that I think that would be a good starting point.