Dec 13, 2009

When Are ICMP Redirects Sent?

How ICMP Redirect Messages Work

ICMP redirect messages are used by routers to notify the hosts on the data link that a better route is available for a particular destination.
For example, the two routers R1 and R2 are connected to the same Ethernet segment as Host H. The default gateway for Host H is configured to use router R1.Host H sends a packet to router R1 to reach the destination on Remote Branch office Host 10.1.1.1.
Router R1, after it consults its routing table, finds that the next-hop to reach Host 10.1.1.1 is router R2.
Now router R1 must forward the packet out the same Ethernet interface on which it was received. Router R1 forwards the packet to router R2 and also sends an ICMP redirect message to Host H.
This informs the host that the best route to reach Host 10.1.1.1 is by way of router R2.
Host H then forwards all the subsequent packets destined for Host 10.1.1.1 to router R2.
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