3.5.1 ICMP Redirects
nWhen a source sends a packet to a gateway for forwarding, and that gateway is not the default next hop, then the gateway will forward the packet to the appropriate gateway. It then uses ICMP to send a message to the source giving it the gateway IP address that should be used next time in conjunction with that destination.
nIf one examines
the routing table after an ICMP redirect, we
see that a new entry has been added and
the flag “D” inserted to indicate its source.