ICMP Redirect
ICMP Redirect messages allow a host's default gateway router to inform local hosts of a better router to use to reach certain destinations. To do so, a router sends an ICMP Redirect to the host to tell it the IP address of the better alternative router. For example, in Figure 5-2, the PC uses RouterB as its default router, but RouterA's route to subnet 10.1.4.0/24 is a better route. Following the steps in Figure 5-2:
1. The PC sends a packet, destined for subnet 10.1.4.0/24, to RouterB.
2. RouterB forwards the packet based on its own routing table.
3. RouterB sends the ICMP Redirect message to the PC, telling it to forward future packets destined for 10.1.4.0/24 to RouterA instead.
4. The PC sends future packets to that host directly to RouterA.
Figure 5-2 Example of an ICMP Redirect
KEY POINT
ICMP redirect —use Router A
Host
ICMP redirect —use Router A
Host
Ironically, the host can ignore the redirect and keep sending the packets to RouterB.
Continue reading here: ARP and Proxy ARP
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